Intro to Peptides

What is a Peptide?

A peptide is a biologically occurring chemical compound containing two or more amino acids connected to one another by peptide bonds. A peptide bond is a covalent bond that is formed between two amino acids when a carboxyl group or C-terminus of one amino acid reacts with the amino group or N-terminus of another amino acid in a condensation reaction (a molecule of water is released during the reaction).

The resulting bond is a CO-NH bond and forms a peptide, or amide molecule. Likewise, peptide bonds are amide bonds.

The word "peptide" itself comes from πέσσειν, the Greek word meaning "to digest." Peptides are an essential part of nature and biochemistry, and thousands of peptides occur naturally in the human body and in animals. In addition, new peptides are being discovered and synthesized regularly in the laboratory as well. Indeed, this discovery and innovation in the study of peptides holds great promise for the future in the fields of health and pharmaceutical development.

How Are Peptides Formed?

Peptides are formed both naturally within the body and synthetically in the laboratory. The body manufactures some peptides organically, such as ribosomal and non-ribosomal peptides. In the laboratory, modern peptide synthesis processes can create a virtually boundless number of peptides using peptide synthesis techniques like liquid phase peptide synthesis or solid phase peptide synthesis. While liquid phase peptide synthesis has some advantages, solid phase peptide synthesis is the standard peptide synthesis process used today.

Peptide Terminology

Peptides are generally classified according to the amount of amino acids contained within them.

• Dipeptide: A peptide composed of just two amino acids.
• Tripeptide: A peptide composed of three amino acids.
• Oligopeptides: Shorter peptides made up of relatively small numbers of amino acids, generally less than ten.
• Polypeptides: Typically composed of more than at least ten amino acids.
• Proteins: Much larger peptides (those composed of more than 40-50 amino acids).

While the number of amino acids contained is a main determinate when it comes to differentiating between peptides and proteins, exceptions are sometimes made. For example, certain longer peptides have been considered proteins (like amyloid beta), and certain smaller proteins are referred to as peptides in some cases (such as insulin).

Classification of Peptides

Peptides are generally divided into several classes based on how they are produced:

• Ribosomal Peptides: Produced from the translation of mRNA. They often function as hormones and signaling molecules (e.g., substance P, vasoactive intestinal peptide). They frequently undergo proteolysis to reach their mature form.
• Nonribosomal Peptides: Produced by peptide-specific enzymes, not the ribosome. These are frequently cyclic rather than linear and often appear in plants, fungi, and single-celled organisms. Glutathione is a common example.
• Milk Peptides: Formed from milk proteins via enzymatic breakdown by digestive enzymes or lactobacilli fermentation.
• Peptones: Derived from animal milk or meat digested by proteolytic enzymes, often used as nutrients for growing bacteria in labs.

Important Peptide Terms