Introduction to Peptides
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What Are Peptides?
A peptide is a naturally occurring chemical molecule composed of two or more amino acids connected by peptide bonds. A peptide bond is a kind of covalent connection formed by the carboxyl group (C-terminus) of one amino acid reacting with the amino group (N-terminus) of another. This process releases a water molecule and generates a CO-NH bond, which serves as the backbone for a peptide or amide molecule. As a result, peptide bonds are categorized as amide bonds.
The name “peptide” is derived from a Greek word meaning “to digest.” Peptides are essential to biology and biochemistry. Thousands of distinct peptides exist naturally in people and animals, with many more being found or created as a result of continuing study. This busy field of research has great potential for future advancements in healthcare, biotechnology, and pharmaceutical sciences.
How Peptides Are Formed
Peptides can be produced both inside the body and in laboratory settings. In living organisms, the body naturally makes a variety of peptides, including ribosomal and nonribosomal types. In the lab, modern peptide synthesis techniques allow scientists to create a wide range of custom peptides. Two commonly used methods are liquid phase peptide synthesis and solid phase peptide synthesis. While liquid phase synthesis offers some specialized advantages, solid phase peptide synthesis has become the standard because it is efficient, adaptable, and well suited for automation.
The history of synthetic peptides dates back to 1901, when Emil Fischer, working with Ernest Fourneau, reported the first synthetic peptide. Later, in 1953, Vincent du Vigneaud successfully synthesized oxytocin, marking the first synthetic polypeptide and laying the foundation for today’s peptide research and manufacturing.
Peptide Terminology
Peptides are often described by the number of amino acids they contain. Common terms include:
- Dipeptide – a peptide made of 2 amino acids
- Tripeptide – a peptide made of 3 amino acids
- Oligopeptide – a shorter chain, usually fewer than 10 amino acids
- Polypeptide – a longer chain, typically more than about 10 amino acids
- Protein – a very long chain, usually more than 40–50 amino acids
While chain length is the main way to distinguish peptides from proteins, there are exceptions. Some relatively long chains are still commonly called peptides, such as amyloid beta. Conversely, certain smaller chains like insulin are often referred to as proteins.
Classification of Peptides
Peptides can be grouped into several main types, often based on how they are produced and how they function in living systems.
Ribosomal Peptides
Ribosomal peptides are produced through the translation of messenger RNA (mRNA). Many of these peptides act as hormones or signaling molecules that help coordinate processes within an organism. Examples include:
- Tachykinin peptides
- Vasoactive intestinal peptides
- Opioid peptides
- Pancreatic peptides
- Calcitonin peptides
Some antibiotics, such as microcins, are also ribosomal peptides made by specific microorganisms. Ribosomal peptides typically reach their active form through proteolysis, a process in which larger proteins or peptides are broken down into smaller pieces.
Nonribosomal Peptides
Nonribosomal peptides are assembled by specialized enzyme complexes rather than by the ribosome. These peptides are frequently cyclic (ring-shaped) instead of linear and can form highly intricate structures. They are commonly found in plants, fungi, and single-celled organisms.
A well-known example is glutathione, an important nonribosomal peptide that plays a key role in antioxidant defense in aerobic organisms.
Milk Peptides and Peptones
Milk peptides originate from milk proteins. They can be generated by digestive enzymes in the gastrointestinal tract or by enzymes produced by lactobacilli during milk fermentation.
Peptones are peptide mixtures derived from partially digested animal milk or meat. In laboratory settings, peptones are widely used as nutrient sources in culture media for growing bacteria and fungi.
Peptide Fragments
Peptide fragments are smaller pieces resulting from the breakdown of larger peptides or proteins. In research, they are often produced intentionally through controlled enzymatic digestion. Similar fragments can also form naturally as part of normal biological turnover and protein degradation.
Key Peptide Terms
Understanding peptide science is easier when you are familiar with a few essential concepts and definitions.
Amino Acids
Peptides are built from amino acids. An amino acid is a molecule that contains both an amine group and a carboxyl group. In peptide and protein chemistry, alpha-amino acids are the standard building blocks that link together to form chains.
Cyclic Peptides
Cyclic peptides have their amino acid chain arranged in a loop rather than in a straight line. This ring structure often increases their stability and can significantly influence how they behave in biological systems. Examples include melanotan-2 and PT-141 (Bremelanotide).
Peptide Sequence
A peptide sequence is the specific order in which amino acid residues are arranged along the chain. This sequence is a major factor in determining the peptide’s three-dimensional structure and its biological activity.
Peptide Bond
A peptide bond is the covalent link between two amino acids. It forms when the carboxyl group of one amino acid reacts with the amino group of another in a condensation reaction, releasing a molecule of water. The resulting CO-NH bond forms the core backbone of the peptide chain.
Peptide Mapping
Peptide mapping is an analytical method used to confirm or identify the amino acid sequence of a peptide or protein. In this technique, the peptide or protein is enzymatically cut into smaller fragments, and the resulting pattern is analyzed using chromatographic or similar tools. The final pattern, or “map,” helps verify structure, detect modifications, or compare different samples.
Peptide Mimetics
A peptide mimetic is a molecule designed to imitate the biological activity of a natural peptide ligand. It may interact with receptors or enzymes in a similar way to hormones, cytokines, enzyme substrates, viral proteins, or other active biomolecules. Peptide mimetics can be:
- Naturally occurring peptides
- Modified synthetic peptides
- Non-peptide molecules engineered to behave like peptides
Peptide Fingerprint
A peptide fingerprint is a distinctive pattern produced when a peptide is partially hydrolyzed into smaller fragments and then analyzed, often by chromatographic techniques. This pattern can be used to confirm identity, assess purity, or compare different lots or formulations.
Peptide Library
A peptide library is a large, organized collection of different peptides created to explore a wide range of amino acid sequence combinations. These libraries are powerful tools in biochemical and pharmaceutical research, allowing scientists to screen many variants for specific binding, activity, or other functional properties. Solid phase peptide synthesis is the most common method used to prepare peptide libraries.