Protein kinase C is a term used to describe a group of enzymes characterized by their ability to modify specific amino acids that compose other proteins, thereby altering their functions. A protein kinase, or phosphotransferase, is an enzyme that is involved in attaching a phosphate group to another molecule or protein, a process that can serve a variety of cellular purposes. This process, called phosphorylation, can activate or deactivate various aspects of proteins, including several that are related to human disease. The protein kinase C family is specifically involved in the phosphorylation of hydroxyl groups on two specific amino acids: serine and threonine. Different members of this protein family are involved in a massive variety of biological processes ranging from sweat secretion to muscle contraction.
Members of the protein kinase C family are made up of two different parts or domains that are referred to as the regulatory and catalytic domains. The regulatory domain is made up of several binding and sensing sites such as the C2 domain, which acts as a calcium ion sensor. Protein kinase C is often activated by high concentrations of the calcium ion in its surrounding cellular environment, so the ability to sense calcium ion levels is important.
Many different enzymes have a catalytic domain; it is not unique to protein kinase C. Enzymes function by greatly reducing the activation energy barriers that prevent biochemical reactions from proceeding quickly and efficiently. Without enzymes, most of the essential chemical reactions that occur within a cell would not occur quickly enough to support life. The catalytic domain is the specific region of an enzyme that catalyzes particular chemical reactions. One interesting feature of the catalytic domain of protein kinase C is that it can, itself, be phosphorylated; it is actually incapable of functioning properly if some of its phosphorylation sites are not phosphorylated.
The primary function of protein kinase C is signaling; it responds to various chemical stimuli by sending further chemical signals. When protein kinase C is activated, usually by a chemical signal such as high calcium ion concentration, transport proteins move the enzyme where it is needed. Generally, the enzyme must be moved to the cell's plasma membrane, as the signals it is responsible for sending must be sent beyond the cell. These signals can have a variety of different results based on the identity and location of the specific kinase involved. Possible functions include the secretion of stomach acid, vasoconstriction, and altering the chemical composition of saliva.