Peptides are biological polymers formed by the linking of amino acids, the key elements of which are carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). The term "peptide sequence" refers to the order in which amino acids are linked. Peptides are very important in living systems and combine to form much larger chains called proteins. Although there are many varieties of biological peptides, most of these are made up from just 20 amino acids. Proteins perform a range of biological functions, including as enzymes and in the structure of cell walls in plants and membranes in animal cells.
Amino acids are so called because they have amine (-NH2) and carboxylic acid (-CO2H) groups. Most of the amino acids found in biological systems have the general formula R-CH(NH2)-CO2H, where R is a range of groups. The varying structure of R is what influences the structure and properties of peptides and proteins.
Peptide synthesis occurs when one amino acid's amine group reacts with the carboxylic acid group on another amino acid. The result is the formation of an amide group of structure -N(H)-CO- where the nitrogen-to-carbon bond is called a peptide bond. The reaction can take place any number of times to form a peptide sequence of different functional groups, each of which are interspersed by an amide group.
Peptides and proteins have different levels of structure. Primary protein structure is the order in which the amino acids are found; this is its peptide sequence. The functional groups on the protein interact with each other to pull it into a shape. The individual twists and turns of a peptide are called its secondary structure. Its overall shape, the net result of all these twists and turns, is the peptide's tertiary structure.
The biochemical properties of proteins or peptides depend on tertiary structure. As with all chemicals, proteins will assume the most energetically favorable shape. This shape is because of attractive and repulsive forces between the groups on the individual amino acids. Therefore, tertiary structure is because of primary structure, meaning that deducing the peptide sequence of a particular protein will ultimately explain its properties.
Working out the peptide sequence of a protein has applications in designing pharmaceuticals. Many biological processes depend on the actions of proteins, and drugs often work by either copying or blocking the action of these proteins. Understanding the behavior of proteins could lead to the design of more effective drugs. For this reason, the research and development department of a pharmaceutical company typically has a peptide library or peptide catalog that it will use in the search for new drugs.