What Are Peptides?

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🧬 What Is a Peptide?

A peptide is a naturally occurring compound consisting of two or more amino acids linked by peptide bonds. A peptide bond is a type of covalent bond formed when the carboxyl group (C-terminus) of one amino acid reacts with the amino group (N-terminus) of another in a condensation reaction, releasing a molecule of water. The resulting CO–NH bond characterizes peptides, which are also referred to as amide molecules [1].

The term “peptide” originates from the Greek word πέσσειν (pessein), meaning “to digest.” Peptides are fundamental components in both biology and biochemistry, playing critical roles in signaling, enzymatic activity, structural function, and regulation. Thousands of peptides occur naturally in the human body and in animals. Additionally, new peptides are continually being discovered and synthesized in laboratories, driving innovation in pharmaceutical development, regenerative medicine, and biotechnology [2,3].

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🔬 How Are Peptides Formed?

Peptides can form naturally within the body or be synthesized in the lab. In biological systems, peptides are categorized as either:

• Ribosomal peptides, which are encoded by genes and synthesized by ribosomes (e.g., hormones and neurotransmitters)
• Or non-ribosomal peptides, which are assembled by specialized enzymes independent of mRNA translation.

In the lab, peptide synthesis is primarily achieved using solid-phase peptide synthesis (SPPS) or, less commonly, liquid-phase peptide synthesis (LPPS). SPPS, introduced by R. Bruce Merrifield in the 1960s, remains the industry standard due to its efficiency and scalability [4].

The first synthetic peptide was produced in 1901 by Emil Fischer and Ernest Fourneau. Later, in 1953, Vincent du Vigneaud achieved the first synthesis of a full polypeptide hormone — oxytocin — earning him the Nobel Prize in Chemistry.

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🧾 Peptide Terminology

Peptides are categorized based on their amino acid length:

• Dipeptide: 2 amino acids
• Tripeptide: 3 amino acids
• Oligopeptide: Generally fewer than 10 amino acids
• Polypeptide: Typically 10–50 amino acids
• Proteins: Usually over 50 amino acids

This classification is somewhat flexible. For instance, insulin, though relatively small, is often called a protein, while longer chains like amyloid beta are sometimes referred to as peptides due to their function or context [5].

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🧬 Classification of Peptides

Peptides can be classified by biosynthetic origin:

1. Ribosomal Peptides

Produced via mRNA translation, often serving as:

• Hormones (e.g., insulin, glucagon)
• Neurotransmitters
• Signaling molecules
• Antibiotics (e.g., microcins)

These peptides may undergo post-translational modifications (e.g., cleavage, glycosylation) to become biologically active.

2. Non-Ribosomal Peptides (NRPs)

Synthesized by non-ribosomal peptide synthetases (NRPS), primarily in bacteria, fungi, and some plants. These peptides are often:

• Cyclic in structure
• Pharmacologically active (e.g., antibiotics like vancomycin or immunosuppressants like cyclosporine)
• Glutathione, an antioxidant, is a well-known non-ribosomal peptide.

3. Milk Peptides

Generated from enzymatic breakdown of milk proteins, either through digestion or fermentation (e.g., by lactobacilli). Some exhibit antihypertensive, immune-modulating, or opioid-like effects.

4. Peptones and Peptide Fragments

• Peptones are enzymatically digested proteins used in microbial culture media.
• Peptide fragments can result from natural degradation or controlled enzymatic hydrolysis during laboratory studies.

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📚 Key Peptide Terms

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🔬 What Are Research Peptides?

Research peptides are peptides intended solely for in vitro scientific use — not for human or veterinary consumption. These compounds are frequently studied for their potential therapeutic effects, due to their selectivity, potency, and typically favorable safety profiles [6].

Research peptides are critical to modern drug development, helping scientists explore novel treatments in areas like metabolism, neurology, oncology, and regenerative medicine.

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🧪 Research Peptides vs. FDA-Approved Medications

It is important to distinguish research peptides from FDA-approved peptide drugs:

• Research peptides are unapproved compounds used in lab experiments.
• FDA-approved peptides (e.g., Lupro™, for prostate cancer, and Victoza™, for type 2 diabetes) have undergone extensive clinical testing and regulatory approval.

While over 60 peptide drugs have been approved by the FDA, research peptides remain investigational and are not intended to treat, diagnose, or cure any condition [7].

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🧭 Peptides as Future Therapeutics

To date, more than 7,000 naturally occurring peptides have been identified. These molecules act as:

• Hormones
• Neurotransmitters
• Growth factors
• Ion channel modulators
• Anti-infectives

Due to their target specificity, predictable metabolism, and tolerability, peptides are ideal candidates for therapeutic development.

🔬 Key Areas of Peptide-Based Drug Research:

• Metabolic diseases (e.g., obesity, type 2 diabetes)
• Cancer therapy
• Infectious diseases
• Inflammatory conditions
• Rare diseases
• Vaccine development
• Diagnostic tools

Peptides represent a promising frontier in pharmaceutical science, with research peptides at the core of ongoing innovation.

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📚 Citations:

1. Alberts B. et al., Molecular Biology of the Cell, 6th ed., Garland Science, 2015.
2. Fosgerau K., Hoffmann T. “Peptide therapeutics: current status and future directions.” Drug Discovery Today, 2015.
3. Craik DJ, Fairlie DP, Liras S, Price D. “The future of peptide-based drugs.” Chem Biol Drug Des. 2013.
4. Merrifield RB. “Solid phase peptide synthesis.” J Am Chem Soc. 1963.
5. Lodish H. et al., Molecular Cell Biology, 8th ed., W.H. Freeman, 2016.
6. Otvos L. “Peptide-based drug design: here and now.” Methods in Molecular Biology, 2017.
7. Lau JL, Dunn MK. “Therapeutic peptides: Historical perspectives, current development trends, and future directions.” Bioorganic & Medicinal Chemistry, 2018.