What is a Peptide? (Part 2)
A peptide is nothing more than a string of amino acids that is similar to, but not identical to, a protein. To understand what a peptide is and how it differs from a protein, it is necessary to first understand what an amino acid is.
What Are Amino Acids?
Amino acids are biologically important molecules, but not all of them are used by living organisms. In fact, the human body requires only 20 different amino acids to function (the case for almost all living things), even though nearly 500 have been identified in the universe so far. Amino acids have two specific chemical structures, called amine and carboxylic acid groups, at opposite ends. These structures endow amino acids with a common set of functions and define how they interact with one another and with other molecules.
Though many people believe that amino acids are just “parts of proteins,” they actually have a number of important roles. In human biology, amino acids are the building blocks of proteins and act as intermediates in metabolism. They often serve individually, as precursors for hormones (e.g. human growth hormone) and signaling molecules (e.g. serotonin), particularly in the central nervous system. They even help to transfer energy throughout the body.
When the amino group of one amino acid interacts with the carboxyl group of another, a chemical reaction takes place. The result of that reaction is the release of water and the formation of a special type of bond (connection between two chemical components) called a peptide bond. Anytime two or more amino acids are linked together, the resulting molecule is called a peptide because of the presence of peptide bonds. Sometimes, the term polypeptide will be used to explain the fact that more than two amino acids have been linked together. The terms are virtually identical.
Proteins versus Peptides
Both proteins and peptides are made up of amino acids linked together by peptide bonds, but all proteins are peptides and not all peptides are proteins. The distinction is based on structure. Proteins fold back on themselves to form complicated three-dimensional shapes that have specific biological functions. Peptides do not fold on themselves, though they can still have biological function. In general, peptides contain 50 or fewer amino acids (chains that short are usually incapable of folding over on themselves).
Practical Peptide Definition
The easiest way to think of peptides is to remember that they are short (≤ 50) chains of amino acids. They often have biologic properties, and their short length tends to endow them with abilities that proteins don’t have (e.g. crossing the cell membrane). In fact, the research and development of synthetic peptides and proteins has become a common pursuit as scientists attempt to develop better options for treating many physiologic pathways and conditions in the future without the negative side effects of many pharmaceuticals.