Propeptides: precursor proteins, processing and biological roles

A propeptide is a segment associated with the maturation of a precursor protein that is usually removed during a specific enzymatic process. However, before it is removed, it can perform important biological functions. Depending on the protein, it can help with proper folding, prevent premature biological activity, influence the assembly of molecular structures, or regulate the moment and location at which the mature product becomes accessible.

This means that propeptides should not be understood simply as another name for a small peptide. It is also not automatically an active hormone, a signaling peptide, or a cosmetic ingredient. This term describes the relationship between a specific sequence and the maturation process of a larger precursor molecule.

This distinction is particularly important because the propeptide, proprotein, prohormone, prepropeptide, signaling peptide, and prodomain describe related but different biological concepts. At the same time, in cosmetics and commercial products, terms such as „pro-peptide” are sometimes used that do not necessarily refer to a biological precursor.

Understanding the way precursor proteins are formed and processed therefore makes it easier to interpret both the scientific terminology and the names used in commercial products.

In this article, the process of processing precursors is presented through examples involving the production of hormones, the maturation of enzymes, the regulation of growth factors, and collagen biology. These examples demonstrate specific biological mechanisms under specific experimental conditions. They do not prove that a cosmetic or other product produces the same effect simply because the term „pro-peptide” appears in its name.

What is a propeptide?

Segment involved in protein maturation

Many proteins are not initially synthesized in exactly the molecular form in which they ultimately fulfill their function.

Cells can first produce a precursor molecule containing additional amino acid sequences. During maturation, some of these sequences may be removed, chemically modified, reorganized, or subjected to other processes before the final functional product is formed.

Propeptide usually refers to the segment of such a precursor associated with its maturation, whereas proalbumin refers more broadly to the larger precursor molecule containing this region.

This distinction is useful, although the biological terminology is not entirely uniform across all protein families.

In some publications, the term „propeptide” is used for a relatively large structural region, and „propeptide” for the corresponding segment. Other authors use the term propeptide in a somewhat broader sense when discussing precursor molecules.

Therefore, the most reliable approach is not to interpret the prefix itself. Rather, it is necessary to analyze the boundaries of the sequence, the structure of the molecule, the stages of processing, and the terminology used by researchers studying the given protein.

The prefix „pro” in this biological context indicates a connection with the precursor or the maturation process.

This does not mean that the molecule is professionally formulated, stronger, more advanced, more effective, or necessarily more beneficial when added to a cosmetic product.

Propeptides are not just unnecessary „packaging”

It is easy to imagine the propeptide as an unnecessary molecular package that is simply removed and discarded during the formation of mature protein.

However, biology is often more complex.

The removed segment can perform important functions before it disappears from the mature molecule.

Studies involving the bacterial enzyme subtilisin E provide a particularly clear example. Scientists have shown that a synthetic propeptide can help the denatured enzyme regain its active three-dimensional structure. This same propeptide could also bind to the mature enzyme and inhibit its activity.[1]

These observations show two distinct biological functions.

First of all, a propeptide can participate in protein folding.

Secondly, it may help to limit enzymatic activity to the appropriate stage of maturation.

This type of folding assistance is referred to as an intracellular protective protein function. It is part of the precursor molecule that helps another fragment of the same molecule take on the correct structure.

The example of subtilisin thus shows why the absence of a given segment in the final active form of the enzyme does not mean that it was biologically insignificant.

At the same time, the results of these studies should not be generalized excessively.

Subtilinase is a bacterial enzyme. Demonstrating a specific function related to folding and inhibition in this system does not mean that every human propeptide performs the same functions.

Later studies of pro-subtilin linked several stages of maturation together, including folding, self-assembly, release of propeptide, and activation of the enzyme.[2]

These experiments demonstrate another important principle: the maturation of a precursor can constitute a coordinated sequence of molecular events, rather than a single cleavage that immediately leads to full biological activity.

The moment of release of the propeptide can therefore be as important as the initial cutting itself.

How to distinguish related concepts?

Several related terms appear regularly in discussions about precursor proteins, but they should not be treated as a single concept.

These categories overlap to some extent because they describe different aspects of the biology of precursors, but they are not synonymous.

A prohormone, for example, is a specific type of precursor associated with the formation of a hormone or related peptides.

A propeptide, however, can only represent one segment of a much larger precursor molecule.

This distinction is important when interpreting studies, because the biological function depends on the actual molecule and the pathway through which it is processed.

Pre-testing and processing of precursors

The „pre” sequence is responsible for directing the protein.

In the case of many secretory proteins, the earliest precursor form contains an initial amino acid sequence that helps direct the newly synthesized molecule to the appropriate intracellular pathway.

This sequence is commonly called a signaling peptide or, in the terminology of preprobes, the „pre” part.

Its role differs from that of the propeptide that later participates in folding, inhibition, assembly, or activation.

Structural studies of the signal-sensing complex have examined the way in which the intracellular mechanism of protein signaling recognizes the emerging signaling sequence even during protein synthesis on the ribosome and helps coordinate its transport towards the membrane. [3]

This is an intracellular mechanism for directing proteins.

It should not be interpreted as evidence that the peptide used externally in a cosmetic product automatically penetrates into skin cells or follows the same pathway.

The removal of the signaling sequence and the subsequent processing of the precursor are therefore separate biological events.

Cells can occur in various places, involve different enzymes, and perform different functions.

In simple terms, this route can be described as follows:

pre-test sample → test sample → mature product

However, this is only a didactic model. Individual proteins can undergo additional modifications, intermediate stages of cleavage, folding, transport, and assembly.

Not every precursor matures according to the same chronological pattern.

Example of insulin

Insulin is one of the best-known examples of a hormone that is produced by the processing of a precursor.

The earliest precursor, preproinsulin, contains a guiding sequence.

After the removal of this signaling sequence, proinsulin remains.

Then proinsulin undergoes further processing, during which the connecting region is separated from the fragments that ultimately form the mature chains of insulin.

This example shows why preproprotein, proprotein, and mature protein are not interchangeable concepts. They describe the different molecular stages of the same biosynthesis pathway.

In one study on the processing of proinsulin, scientists expressed proinsulin along with potential processing enzymes in cultured cells. Different cleavage preferences were observed for PC2 and PC3, which confirmed their involvement in insulin maturation.[4]

In later terminology, PC3 is commonly referred to as PC1/3.

The most important principle is that the processing of precursors is selective.

The processing enzymes do not randomly cleave the precursor proteins into smaller peptides.

Recognizing the specific environment of the sequence and the cutting sites, however, helps determine which mature molecular products will be produced.

The experiment with proinsulin also shows why such details as the identity of the enzyme, the type of substrate, and the experimental model should be included in scientific descriptions.

These factors can significantly influence the interpretation of the results.

One precursor can lead to the creation of several products

The precursor molecule does not have to lead to the formation of only one active product.

Some precursor proteins contain several regions from which different biologically active peptides can ultimately be formed.

An important example is proopiomelanocortin, or POMC.

POMC contains many regions that can be processed into various peptide products. What products ultimately accumulate depends partly on the enzymatic machinery present in the cells in question.

In a 1991 study, researchers jointly expressed POMC from PC1 or PC2 in several cell models. Different cleavage patterns were observed for individual enzymes.[5]

The experiment therefore provided direct evidence that different processing enzymes can generate different maturation products from the same precursor.

This explains why knowing the sequence of the precursor does not necessarily tell researchers exactly which mature peptides will appear in each tissue.

The amino acid sequence determines the molecular material that can potentially be processed.

The enzymatic environment and intracellular conditions, on the other hand, help determine what will actually be processed and produced under the biological conditions being studied.

Two cells producing the same precursor can therefore produce different mixtures of mature peptides if their processing machinery differs.

Processing is more than just cutting a chain

Proteolytic cleavage is only one of the elements of protein maturation.

Depending on the molecule, maturation can also involve protein folding, the formation of disulfide bridges, intracellular transport, binding to other molecules, and chemical modifications.

These processes can influence each other.

The cleavage site may be present in the amino acid sequence, but remain inaccessible to the processing enzyme due to the three-dimensional structure of the protein.

Similarly, the correct folding may depend on the previous processing stages.

Therefore, when evaluating research on precursors, three questions must be distinguished:

Has the precursor been produced?

Has it been properly processed into an identified mature product?

Has the product produced shown adequate biological activity?

These questions are related, but they are not equivalent.

Measuring the amount of the precursor can answer the first question, but it does not provide an answer to the second and third questions at the same time.

For example, a test that detects both the precursor and several fragments of it simultaneously can indicate a significant total amount of protein, providing little information about the actual amount of mature, active product.

If the biological interpretation depends on the maturation process, analytical methods are therefore needed to distinguish the precursor, intermediate products, and mature forms of the molecule.

Processing can occur outside the cells

Not every significant step in the processing of the precursor occurs within secretory granules or other cellular compartments.

An important example of the extracellular process is collagen biology.

Procollagen molecules contain the final propeptides. Removing these regions is involved in the maturation and formation of collagen fibers.

In a 1996 study, bone morphogenetic protein 1, or BMP-1, was identified as C-procollagen type I proteinase.[6]

The study linked a specific enzyme with the removal of C-terminal propeptides from type I procollagen.

This example is useful for several reasons.

First, it shows that precursor processing can be an important component of the maturation of the extracellular matrix, and not merely a hormonal signaling process.

Secondly, it shows why protein names should not be interpreted solely based on known terminology. Despite its name, BMP-1 should not be automatically considered a protein that acts the same as all other proteins referred to as bone morphogenetic proteins.

Thirdly, this example shows why a propeptide is not automatically a hormone.

In the case of procolagen, the biological context concerns the formation and maturation of the extracellular matrix, rather than the hormonal signaling directed to a distant organ.

Activation and cutting

Cutting changes the molecular identity

Proteolytic cleavage involves breaking the selected peptide bonds within the precursor.

Depending on the specific molecule and the site of cleavage, this process can release a mature peptide, remove the inhibitory region, reveal the receptor binding site, enable the assembly of the molecular structure, create an intermediate form, or otherwise alter the biological properties of the molecule.

Cutting and activation are not, however, synonymous terms.

Cutting describes a chemical event.

Activation describes its functional consequence.

Cutting can activate the molecule, but it can also inactivate it, produce an intermediate form, or lead to the formation of fragments whose biological functions remain uncertain.

This distinction prevents an excessively simplified interpretation of experiments concerning the processing of precursors.

The demonstration that the precursor has been cut does not in itself prove that the expected activity of the mature product has increased.

In order to demonstrate activation, it is usually also necessary to evaluate the biological function of the resulting product.

Cutting does not always mean immediate release

Even after the covalent bond is broken, the removed segment can still remain physically bound to its partner through noncovalent interactions.

This means that the cleavage does not have to lead to the immediate release of the fully accessible active molecule.

A good example is the structural studies of the secreted transforming growth factor beta, or TGF-β.

The studies showed how prodomene can mask the growth factor from being recognized by the receptor and linked the activation to structural changes in the hidden complex, including processes dependent on force in the studied system.[7]

This example shows that regulation can go beyond cutting itself.

The precursor can already be proteolytically processed, while the biologically active component remains limited by interactions with prodomene or other molecular partners.

This does not mean that every propeptide behaves like the prodomain of TGF-β.

The broader principle is that activation should be demonstrated for a specific biological system, and not derived from the assumption that „cut means active.”.

Cutting sites and enzymes must match each other

Many hormone precursors contain basic amino acids near important processing sites.

The presence of a particular pair or arrangement of amino acids does not, however, mean that each such site will be cut in the same way.

The processing depends, among other things, on the specificity of the enzyme, the local context of the sequence, the protein conformation, the cellular localization, and the availability of the appropriate enzymatic apparatus.

Experiments involving proinsulin and POMC demonstrate this principle through the example of various precursors and convertases [4][5].

Their results confirm selective enzymatic processing, rather than the universal rule that the short sequence motif itself determines the further course of the process.

The predicted cutting point is therefore best treated as a hypothesis that can be verified.

Evidence becomes stronger when researchers identify the actual processing product and link its formation to the proposed enzyme and the appropriate biological conditions.

What experiments confirm the processing of the precursor?

A reliable statement regarding processing is usually based on several complementary types of evidence, rather than on a single isolated observation.

Researchers can identify the resulting fragments using mass spectrometry or sequencing. They can compare the amount of precursor and mature product over time, change the proposed cutting site, or restrict the activity or expression of the potential processing enzyme.

Functional experiments can then determine whether the resulting product exhibits the biological activity attributed to its mature form.

Each of these approaches has limitations.

The change in apparent molecular mass in the gel can suggest processing, but it does not identify the exact point of cutting.

An enzyme inhibitor can affect more than one biological pathway.

Changing the proposed cutting site can disrupt both protein folding and the recognition by the enzyme.

Mass spectrometry can identify molecular products, but it does not itself confirm their biological activity.

Therefore, the consistency of the results obtained using independent analytical and functional methods is generally more convincing than a single measurement.

These principles serve to interpret the research. They should not be treated as a laboratory protocol or as a statement that every study cited here has performed all possible experiments.

The type of evidence required depends on whether the research question concerns molecular identity, processing mechanism, enzyme specificity, or biological function.

The released fragments can exhibit their own activity

A fragment released from the propeptide region or identified within it can sometimes become the subject of study on its own.

An appropriate example comes from research on collagen.

Scientists identified the pentapeptide KTTKS in the propeptide region of type I collagen and investigated its biological activity in cultured mesenchymal cells. An effect on collagen and fibronection production was noted.[8]

This result should be described at the level that was actually demonstrated.

A specific peptide fragment changed the measured extracellular matrix-related parameters in a cell culture experiment.

This does not mean that a complete propeptide of collagen, the isolated KTTKS sequence, chemically modified KTTKS derivatives, and any cosmetic containing a similar ingredient are equivalent.

Each of these cases represents a different experimental or conceptual context.

The result also does not prove that the use of the cosmetic on human skin replicates the proper intracellular or extracellular process of collagen processing.

Biological origin can provide an interesting scientific justification for the study of a given peptide sequence.

However, it does not eliminate questions regarding formulation, concentration, stability, penetration, exposure, the effects of the finished product, and the results of human studies.

In the context of cosmetics, it is also worth separating this mechanism from the broader issue. signaling peptides in skin care, Because the sequence of the precursor does not automatically mean a defined action of the finished cosmetic product.

Propeptides and the cosmetic term „pro-peptide”

The term on the label does not identify the sequence

The term „pro-peptide” on a product for the mouth, serum, cream, or other cosmetic does not itself identify a specific molecule.

It may refer to the brand's proprietary ingredient complex, the manufacturer's own terminology, the marketing concept, or a general declaration about the product.

Determining what is actually contained in the product requires an analysis of the list of ingredients and — if available — the relevant technical documents.

Scientifically useful molecular identification should specify the component and, in appropriate cases, its amino acid sequence, modification, or chemical form.

Even an unequivocal determination of identity only answers part of the question.

It does not specify how much of a given ingredient is present in the finished formula, whether it remains stable during storage, whether it reaches the appropriate biological site, or whether the entire formulation provides a measurable effect in humans.

The presence or absence of a connector in the name does not create a scientific guarantee.

The terms „propeptide”, „pro-peptide” and „propeptide” may lead to partially overlapping search results, but the appearance of any of them in the product name should not be automatically interpreted according to the definition of a propeptide used in molecular biology.

The biology of precursors does not confirm cosmetic claims

The study showing that the propeptide helps in the folding of the enzyme provides information about this enzyme and the specific experimental system.

It does not prove that a lip balm improves hydration.

Similarly, the study on the processing of prokolagen is not in itself evidence that the finished cream reduces the visibility of wrinkles.

These are different scientific questions that require different types of evidence.

A useful cosmetic study should specify the finished formulation, the method of its application, the population being studied, a comparator, the duration, and the evaluated result.

If the declaration concerns humidity, an appropriate method of humidity assessment must be used.

If wrinkles are concerned, the study should use appropriate end-points related to appearance and take into account the effect of the moisturizing base of the product.

If the product specifically claims an effect on the processing of precursors, the evidence would have to demonstrate an appropriate biological event, rather than merely referring to a general publication on the biology of precursors.

If the supporting studies for the declaration only include purified proteins, computational models, or cultured cells, the wording should maintain these limitations.

„Studied in a cell model” is not equivalent to the statement „demonstrated in people using this ready-made formulation”.

Three questions facilitate the assessment of the declaration

Three simple questions can facilitate the evaluation of both biological and commercial claims.

The first one reads:

What exactly is present?

This allows to determine whether the discussion concerns a biological propeptide, a defined synthetic sequence, a modified peptide, a mixture, or only commercial terminology.

The second question is:

What experiment was actually conducted?

It allows to determine whether the evidence comes from studies of purified proteins, cells, animals, tests of the component, or studies of the finished product in humans.

The third question is:

Does the measured result actually correspond to the declared effect?

Molecular effects are not automatically evidence of wrinkle reduction. Increased expression of a laboratory marker is not automatically evidence of a visible improvement in skin appearance.

These questions help prevent a situation where scientific terminology replaces the missing evidence.

The cosmetic product can very well moisturize the skin without containing biological propionate.

On the other hand, the presence of a component related to a biological precursor does not guarantee a visible effect.

The finished product can therefore be evaluated based on its own evidence, without the need to create an unconfirmed explanation based on the activation of a precursor.

Such a distinction allows for the precise discussion of both basic biology and cosmetic evidence, without the mistaken assumption that the prefix „pro” represents a measure of effectiveness.

Frequently asked questions

1. Are propeptides active hormones?

Not necessarily. Many propeptides are segments associated with precursors that participate in protein folding, inhibiting activity, assembling structures, or maturation, rather than active hormones. Some precursor proteins ultimately lead to the formation of hormones, but others mature into enzymes, structural proteins, growth factors, or other biological molecules. The processing of procollagen and the folding of subtilisin demonstrate very different functions of propeptides.[6][1] Therefore, whether a particular propeptide exhibits hormonal activity must be determined for that particular molecule, rather than inferred from the term itself.

2. Does the term „pro-peptide” on the product label identify a specific molecule?

No. The mere term „pro-peptide” does not provide unambiguous molecular identification. It may mean a commercial concept, a branded complex, a marketing term, or a proprietary term used by the manufacturer for the component, rather than a biologically defined precursor segment. To determine which substance is actually present, a list of ingredients and appropriate technical documentation are required. Even after identifying the substance, evidence regarding the finished product is necessary to determine whether it provides the claimed effect. The product name cannot replace either molecular identification or evidence of efficacy.

3. How is a propeptide different from a probiotic?

Proalbumin is usually a larger precursor molecule, whereas propeptide usually means a maturation-related segment found within that precursor or released during its processing. The terminology may vary between protein families, therefore the sequence boundaries and definitions used in a particular study are important. A propeptide can also play an important role before removal, although it is not present in the final mature protein. Treating proalbumin and propeptide as exact synonyms may therefore make it difficult to understand the described processing mechanism.

4. Is the signaling peptide the same as the propeptide?

No. A signaling peptide usually helps direct the newly synthesized protein to the appropriate intracellular pathway, while a propeptide is more often involved in subsequent maturation, folding, inhibition, assembly, or activation. Both can occur in the same pre-propeptide, but they perform different functions. Structural studies of signal recognition describe the intracellular mechanism of protein targeting, rather than the maturation function of each propeptide.[3] Therefore, one should specify the specific sequence and processing step, rather than assuming that each removed region performs the same function.

5. Why do cells produce precursors instead of only mature proteins?

The production of precursor molecules allows cells to coordinate many elements of protein biology, including their direction, folding, transport, assembly, and the moment of achieving biological activity. Some proteins may require remaining inactive or confined until they reach the appropriate site or obtain the correct three-dimensional structure. Studies of subtilisin show that the propeptide can assist folding while inhibiting the enzyme before the maturation process is complete.[1] This is a clear biological advantage of the precursor organization, although different proteins employ different maturation strategies.

6. Does cutting always activate the protein?

Cutting can activate a molecule, inactivate it, lead to the formation of an intermediate form, or create fragments with different or uncertain functions. Even after cutting, the removed segment may sometimes remain noncovalently bound to the mature part and continue to limit its activity. Studies of secreted TGF-β show why functional activation may require something more than just disrupting the peptide binding in the precursor.[7] Therefore, activation should be confirmed by appropriate functional evidence, and not inferred solely from cutting.

7. Can several peptides be formed from a single precursor?

Yes. One precursor can contain several regions, from which various peptide products eventually form. An important example is POMC. Experiments have shown that different convertases lead to different patterns of processing of the same precursor. [5] The enzymes that process the precursor in a specific cell therefore influence which mature products will form and in what proportions. The precursor sequence determines the possible products, but does not itself dictate which ones will predominate in each tissue or biological situation.

8. Do propeptides always appear at the beginning of a protein?

No. The precursor segments may be located in different places depending on the structure of the given protein. Procolagen contains terminal propeptides, including the C-terminal region removed during maturation, whereas proinsulin contains the internal connecting region removed during the formation of mature insulin.[6][4] The location is therefore a property of the specific precursor and should not be inferred from the mere prefix „pro”.

9. Are propeptides simply waste products?

No. Some propeptides perform important functions before being removed, and some released fragments have also been studied for their own biological activity. Studies of subtilizin provide experimental examples of both aiding folding and inhibiting the enzyme by the propeptide. [1] However, demonstrating the important functions of some propeptides does not mean that every released precursor segment has its own beneficial effect. The biological significance must be examined individually, rather than automatically assuming its presence or absence.

10. What is the intracellular protective protein?

The intracellular protective protein is a precursor region of the protein that helps another part of the same molecule take on the correct three-dimensional structure. A propeptide can perform this function, as shown in studies of subtilysin maturation. [2] This term describes the mechanism of the molecule folding, not the category of cosmetic ingredients. The evidence that one enzyme uses a propeptide as the intracellular protective protein does not mean that an unrelated peptide or cosmetic ingredient performs the same function.

11. Does every precursor use the same processing enzyme?

No. Different enzymes that process recognize different substrates and may have different preferences regarding cleavage sites. Experiments with proinsulin and POMC have shown different processing patterns associated with different convertases.[4][5] The outcome can be influenced by the availability of the enzyme, the local amino acid sequence, the protein conformation, the intracellular localization, and the availability of the cleavage site. Therefore, the general term „activating enzymes” provides less scientific information than the indication of a specific enzyme and precursor that was actually studied.

12. Can processing occur outside the cell?

Yes. Some stages of precursor processing occur extracellularly. An example related to the maturation of the extracellular matrix is the removal of C-propeptide procollagen by BMP-1. [6] This differs from the processing of many precursor hormones within the secretory pathway. The site of maturation therefore depends on the specific protein and should be determined experimentally, rather than generalized based on a single known precursor pathway.

13. Do propeptides have to be very short?

No. The term propeptide describes the relationship with precursor maturation, not the strictly defined number of amino acids. The subtilisin propeptide studied experimentally contains 77 amino acid residues, which shows that the propeptide does not necessarily mean a very small peptide that often appears in cosmetic marketing. [1] Other segments related to precursors and prodomains can vary significantly in length and structural complexity. The length of the sequence alone does not allow determining whether a given structure is a biological propeptide.

14. Does the detection of the precursor prove the presence of its mature product?

No. The detection of a precursor confirms the presence of the detected precursor form. It does not necessarily mean that the processing has taken place correctly or that a mature, biologically active product has accumulated. Processing may be incomplete or altered, and the mature product may subsequently undergo degradation. When this distinction is important, tests are needed to distinguish between the precursor, intermediate, and mature forms. The total amount of protein should not automatically be equated with the amount of active product.

15. Is KTTKS the same as the entire collagen propeptide?

No. KTTKS is a short sequence identified in the propeptide region of type I collagen, not the entire propeptide. Scientists have studied KTTKS in cultured cells and described its effect on the production of extracellular matrix components. [8] These results should remain linked to the exact sequence being studied and the experimental model. The full precursor segment, isolated KTTKS, chemically modified derivatives, and finished cosmetics containing similar components are different materials and should not automatically take over each other’s evidence.

16. Is the predicted cutting site sufficient to confirm the biological mechanism?

No. The predicted cutting site indicates the likely location of processing, but does not prove that in biological conditions the cutting actually occurs at that location. Experiments are needed to identify the resulting products and link their formation to the appropriate enzyme and conditions. Protein folding and the availability of the cutting site can also determine whether the theoretically recognizable site will actually be processed. Predictions are therefore useful for creating and testing hypotheses, but should not be presented as a completed validation of the mechanism.

17. What evidence can support the cosmetic claim regarding „pro-peptide”?

A clearly identified component, together with a properly designed study of the finished formulation, provides much more useful evidence than the terminology itself. The study should assess the claimed effect using an appropriate comparator, a suitable population, an appropriate duration, and an appropriate measurement method. If the claim specifically states that the cosmetic product changes the activation or processing of the precursor, then that mechanism also requires direct evidence. A general publication in the field of molecular biology regarding protein processing, their folding, or the activity of collagen precursors cannot by itself confirm the action of the cosmetic product in detail.

Summary

Propeptides are an important part of biological mechanisms through which many proteins are prepared, regulated, transported, assembled, and undergo the maturation process.

It simply does not define them as a short sequence of amino acids.

This term, however, describes their relationship with the precursor and the process of its maturation.

Individual propeptides can perform very different functions.

Some help in the proper folding of proteins.

Others limit biological activity to the appropriate stage of maturation.

Some participate in the formation of molecular structures, while certain released fragments can exhibit their own biological activity and become independent subjects of study.

The precursors of hormones, the precursor forms of enzymes, the systems of growth factors, and procollagen show how diverse these mechanisms can be.

It is particularly important to distinguish between cleavage and activation. Interrupting peptide binding can change the molecular identity without immediately triggering biological activity. The released segment may still remain bound to the mature molecule; cleavage can lead to the formation of an intermediate form instead of the final product, and proper processing does not automatically indicate functional activity.

Similarly, the detection of a precursor does not confirm the presence of a mature product.

These distinctions explain why the biology of precursors is usually determined based on complementary evidence regarding molecular identity, processing, enzymatic specificity, and biological function.

The terminology used in cosmetics requires a separate interpretation.

A product labeled „pro-peptide” does not have to contain a biological propeptide, replicate the natural process of precursor processing, or provide visible cosmetic benefits.

First, the actual identity of the component must be determined.

Then the finished formulation must be evaluated based on the evidence regarding the effect it actually claims to have.

The experiment involving folding the protein does not prove that it moisturizes the skin.

The study of procollagen processing is not in itself evidence of wrinkle reduction.

An experimental cell culture study regarding the sequence derived from the precursor does not demonstrate the efficacy of the finished cosmetic product in humans.

The most useful principle is therefore separating the molecular identity, the biological mechanism, the experimental evidence, and the claims regarding the finished product.

Such a distinction allows the scientific value of knowledge about propeptides to be preserved without extending the laboratory results beyond what was actually demonstrated in the experiments.

Disclaimer

This article is purely educational and scientifically informative. It explains issues related to precursor proteins, propeptides, protein processing, cutting, maturation, and the interpretation of related cosmetic terminology.

It does not constitute a recommendation regarding a cosmetic product, a peptide ingredient, a supplement, a treatment, a formulation, a dosage, or a route of administration.

Laboratory test results involving purified proteins, precursor systems, cultured cells, enzymes or individual peptide fragments should not be automatically interpreted as evidence of the clinical or cosmetic effects of similarly named commercial ingredients.

A product containing the term „pro-peptide” should be evaluated based on the actual composition and evidence regarding the finished formulation, and not on the assumption that it replicates the biological mechanism associated with the precursor.

Questions regarding treatment, illnesses, or individual health conditions should, if necessary, be discussed with a suitably qualified health care professional.

References

1. Ohta, Y., Hojo, H., Aimoto, S., Kobayashi, T., Zhu, X., Jordan, F., & Inouye, M. (1991). Pro-peptide as an intramolecular chaperone: Renaturation of denatured subtilisin E with a synthetic pro-peptide [corrected]. Molecular Microbiology, 5(6), 1507–1510. https://doi.org/10.1111/j.1365-2958.1991.tb00797.x
2. Yabuta, Y., Takagi, H., Inouye, M., & Shinde, U. (2001). Folding pathway mediated by an intramolecular chaperone: Propeptide release modulates activation precision of pro-subtilisin. Journal of Biological Chemistry, 276(48), 44427–44434. https://doi.org/10.1074/jbc.M107573200
3. Jomaa, A., Eitzinger, S., Zhu, Z., Chandrasekar, S., Kobayashi, K., Shan, S. O., & Ban, N. (2021). Molecular mechanism of cargo recognition and handover by the mammalian signal recognition particle. Cell Reports, 36(2), Article 109350. https://doi.org/10.1016/j.celrep.2021.109350
4. Smeekens, S. P., Montag, A. G., Thomas, G., Albiges-Rizo, C., Carroll, R., Benig, M., Phillips, L. A., Martin, S., Ohagi, S., & Gardner, P. (1992). Proinsulin processing by the subtilisin-related proprotein convertases furin, PC2, and PC3. Proceedings of the National Academy of Sciences of the United States of America, 89(18), 8822–8826. https://doi.org/10.1073/pnas.89.18.8822
5. Benjannet, S., Rondeau, N., Day, R., Chrétien, M., & Seidah, N. G. (1991). PC1 and PC2 are proprotein convertases capable of cleaving proopiomelanocortin at distinct pairs of basic residues. Proceedings of the National Academy of Sciences of the United States of America, 88(9), 3564–3568. https://doi.org/10.1073/pnas.88.9.3564
6. Kessler, E., Takahara, K., Biniaminov, L., Brusel, M., & Greenspan, D. S. (1996). Bone morphogenetic protein-1: The type I procollagen C-proteinase. Science, 271(5247), 360–362. https://doi.org/10.1126/science.271.5247.360
7. Shi, M., Zhu, J., Wang, R., Chen, X., Mi, L., Walz, T., & Springer, T. A. (2011). Latent TGF-β structure and activation. Nature, 474(7351), 343–349. https://doi.org/10.1038/nature10152
8. Katayama, K., Armendariz-Borunda, J., Raghow, R., Kang, A. H., & Seyer, J. M. (1993). A pentapeptide from type I procollagen promotes extracellular matrix production. Journal of Biological Chemistry, 268(14), 9941–9944. https://pubmed.ncbi.nlm.nih.gov/8486721/