A protein is any of a large group of macromolecules that plays a central role in the structure and functioning of living cells. Together with nucleic acids, carbohydrates, and lipids, proteins form the biochemical basis of life on Earth.
Types of protein
Classified according to molecular structure, there are three main types of protein:
Classified according to function, proteins fall into the following main groups:
Proteins are polymers made of hundreds or thousands of amino acid subunits linked by peptide bonds to give polypeptide chains. The specific amino acid sequence of a protein is referred to as its primary structure. Since, in principle, a protein can be formed from any long sequence of any or all of the 20 different amino acids found in terrestrial organisms, the potential diversity of proteins is enormous. For example, a protein containing 100 amino acids could have 20100 (about 1 followed by 130 zeros) different sequences. This almost unlimited diversity is perhaps the most important property of proteins since it enables them to perform such a wide range of functions in living organisms.
The amino acid sequence of a protein encourages the formation of hydrogen bonds between nearby amino acids. This results in two distinct types of structure. In the first, hydrogen bonds are established between different parts of the same polypeptide chain which pulls the chain into a spiral shape, known as an alpha helix. In the second, hydrogen bonds form between two adjacent chains resulting in a pleated configuration called a beta sheet. These types of folding are referred to as the protein's secondary structure. A more complex pattern of folding organizes the protein into its final, unique, three-dimensional configuration, or tertiary structure. The details of this final configuration are determined by the chemical nature of the side groups of the amino acids making up the primary structure. Many proteins associate with other polypeptide chains in clusters. Each contributing chain is referred to as a subunit and the overall structure of the cluster, the quaternary structure.
Since many of the bonds holding a protein in its normal shape are weak, they are easily broken by changes in the protein's environment, including the pH, the temperature, or the concentration of ions in the surrounding solution. When this happens, the protein undergoes a process called denaturation; that is, it changes shape or may even unfold and, as a result, usually becomes biologically inactive. This has particularly serious consequences if the protein is an enzyme, since such substances regulate the metabolism of living cells.