Proteins are cellular macromolecules made up of amino acid polymers (polypeptides). The sequence of amino acids, or primary structure of the protein, is dictated by the nucleotide sequence of the gene coding for that particular protein. Adjacent amino acids in a polypeptide chain will form stable, recurring arrangements based on steric constraints and weak interactions. The most common of these are the alpha helix and beta sheet conformations, making up the secondary structure of the protein. These segments of the protein then form a specific shape held by covalent (eg. disulfide) bonds or weaker interactions between amino acids, called the tertiary structure.
Proteins are very large, with molecular weights in the thousands to 100 thousands. Larger proteins might be made up of several polypeptides. The way the multiple “subunits” interact is referred to as quaternary structure.
The tertiary structure is not necessarily rigid and may change according to its surroundings. For example, the active site of an enzyme may have a specific shape under certain conditions that allows it to bind to the substrate (or ligand) prior to reaction catalysis. A conformational change during catalysis resulting in a change in the active site shape or charges (on nearby amino acid R-groups) allows the reaction product to be released. Conditions such as high temperatures, or certain chemicals, can cause a protein to lose its tertiary structure and, thus, its function. In the case of enzymes, partial loss of function, or a reduction in the rate of reaction due to less-than-ideal conditions, will result in a reduced specific activity.
Proteins play a number of different roles in cells. They are involved in reaction catalysis, transport and storage of nutrients and other molecules in and out of, or within cells, and have structural, defense and regulatory functions.
- Enzymes (reaction catalysis)
- Muscles (structural proteins)
- Antibodies (defense proteins)
- Hormones (regulatory proteins)
The hormone insulin is a protein that regulates sugar metabolism.
While muscles are structural proteins found within the body, fibronin and keratin are found outside the body, in spider webs and fingernails, respectively.
