Peptide Molecular Structure and Biological Roles

An evidence-based guide to how peptide structure—amino acid sequence, backbone features, and three-dimensional shape—drives biological function throughout the body.

Peptide Molecular Structure and Biological Roles

Peptides represent elegant chains of amino acids linked via specific covalent bonds. Numerous structural variables, ranging from basic sequence and electrostatic charge to three-dimensional folding and post-translational chemical additions, significantly dictate how these molecules engage with complex biological pathways and target receptors throughout the human body.

Amino Acids to Peptides

Each amino acid has an amine group, carboxyl group, and a hydrogen attached to a central carbon. Each amino acid also has a side chain attached to the central carbon, which determines the unique properties of each amino acid. The side chain is often referred to as an R group. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

When a carboxyl group of one amino acid reacts with the amine group of another amino acid, a peptide bond is formed. Water is lost during this reaction, and the resulting product is called a peptide. If more amino acids are added to this chain, it is considered a polypeptide. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Peptide Backbone

All peptides will have a peptide backbone consisting of multiple nitrogen and carbon atoms. At one end, there will be an unbound amino group known as the N-terminus, and the other end will have an unbound carboxyl group known as the C-terminus. Peptides are always written in order from the N-terminus to the C-terminus. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Structural Element

Description

Sequence of amino acids

Primary structure of the peptide. Amino acid sequence can have a large impact on biological function.

Peptide bond

Binds amino acids together.

R groups

Will vary based on the individual amino acid. These can vary by charge, polarity, hydrophobicity, etc.

Termini (N- and C-)

Can affect a peptide's interaction with its environment.

Conformation (Shape)

Can allow for specific interactions with certain molecules.

Structure of Peptides

Once you know the primary structure of a peptide (amino acid sequence), there can be secondary structures that occur. These will give the peptide a tertiary structure. There are also some peptides that can have disulfide bridges, which can give the peptide a unique shape. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

It is important to note that structure will dictate the function of a peptide. Just a minor change in the sequence can cause it to interact with a receptor differently, alter biological activity or stability, or change the half-life of the peptide. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Structure Dictates Function

There are many types of molecules that interact with one another to send signals in the body. Peptides have to have certain properties to be able to interact with these targets. Once a signaling molecule binds to a receptor, it can trigger many intracellular pathways. There are also peptides that can block enzymes, work on channels, and help with our immune system. Some even provide structure. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Structure and Function Specificity: A molecule's structure will determine what molecules it can interact with. What that molecule is located on can dictate how it affects the body.

Functions of Peptides

Application and Examples

Function

Examples

Endocrine Control

Peptide hormones serve as primary regulators for growth, reproductive cycles, and metabolic balance. (Muttenthaler et al., 2021)

Neurological Modulation

Peptides acting as neurotransmitters help manage vital processes like pain perception and stress responses. (Muttenthaler et al., 2021)

Immunity

Antimicrobial peptides provide defense by neutralizing pathogens or regulating inflammatory markers. (Muttenthaler et al., 2021)

Circulatory Health

Peptide-driven pathways are essential for maintaining vascular tone and internal homeostasis. (Muttenthaler et al., 2021)

Digestion

Specific peptides coordinate appetite signaling and metabolic secretions across the digestive tract. (Muttenthaler et al., 2021)

Regeneration

Growth factors utilize peptide pathways to facilitate cellular development and tissue restoration. (Muttenthaler et al., 2021)

Using Peptides as Drugs

Due to the structure and function of peptides, there are some that have been used as drugs. Because many of these pathways are very specific to a certain molecule, it can be beneficial to use these as drugs. However, just because a molecule has a pathway in the body doesn't mean it is safe to use as a drug. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

You want to make sure you understand what evidence you are looking at when evaluating if a peptide is a good drug candidate. There are FDA-approved peptides that are used as drugs. If the peptide is not FDA-approved, there may not be enough human data to back its safety and efficacy. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Evidence-Based Lens: Having knowledge of a mechanism of action is not the same as having clinical data to support its use as a drug.

Evaluating Information on Peptides

Naturally Occurring Peptides

There are many peptides that we can find in nature that play a role in our bodies. Insulin, glucagon, oxytocin, and vasopressin are just a few examples. As you can see, they can all play very different roles in the body. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Some may consider these peptides, polypeptides, or even proteins. The line can get blurry when discussing what a peptide is. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Degradation of Peptides

Peptides are constantly being created and broken down in the body. There are many proteases and peptidases that can break down peptides. The sequence and structure of the peptide can dictate how quickly it will be broken down. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

When creating peptide drugs, there can be modifications made to increase the half-life of the peptide. Changing certain amino acids, blocking the termini, or adding other moieties can alter the pharmacokinetic profile of the peptide. It is important to keep this in mind when discussing peptides as drugs. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Conclusion

Peptides are interesting molecules that can do many things in the body. They are made up of amino acids and can have different structures that allow them to interact with various targets. Knowing how a peptide is structured will allow you to better understand how it works in the body. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

As mentioned earlier, structure dictates the function of a molecule. Keep that in mind when learning about peptides and what they can do. (Fosgerau & Hoffmann, 2015; Muttenthaler et al., 2021)

Where to Learn More

PubMed is a great database to look up studies on peptides. The NIH, ClinicalTrials.gov, and the FDA are also good resources. Any biochemistry textbook will teach you more about peptides.

This information is for educational purposes only and is not meant to promote or provide medical advice on how to obtain or prepare peptides.

References

Fosgerau, K., & Hoffmann, T. (2015). Peptide therapeutics: Current status and future directions. Drug Discovery Today, 20(1), 122–128. https://doi.org/10.1016/j.drudis.2014.10.003

Muttenthaler, M., King, G. F., Adams, D. J., & Alewood, P. F. (2021). Trends in peptide drug discovery. Nature Reviews Drug Discovery, 20, 309–325. https://doi.org/10.1038/s41573-020-00135-8

U.S. Food and Drug Administration. (n.d.). Drugs@FDA: FDA-approved drugs. https://www.accessdata.fda.gov/scripts/cder/daf/