In cell biology, Protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins, or a member of this family. PKC enzymes in turn are activated by signals such as increases in the concentration of diacylglycerol (DAG) or calcium ions (Ca2+). Hence PKC enzymes play important roles in several signal transduction cascades. In biochemistry, the PKC family consists of fifteen isozymes in humans. They are divided into three subfamilies, based on their second messenger requirements: conventional (or classical), novel, and atypical. Conventional (c)PKCs contain the isoforms α, βI, βII, and γ. These require Ca2+, DAG, and a phospholipid such as phosphatidylserine for activation. Novel (n)PKCs include the δ, ε, η, and θ isoforms, and require DAG, but do not require Ca2+ for activation. Thus, conventional and novel PKCs are activated through the same signal transduction pathway as phospholipase C. On the other hand, atypical (a)PKCs (including protein kinase Mζ and ι / λ isoforms) require neither Ca2+ nor diacylglycerol for activation. The term "protein kinase C" usually refers to the entire family of isoforms. The different classes of PKCs found in jawed vertebrates originate from 5 ancestral PKC family members (PKN, aPKC, cPKC, nPKCE, nPKCD) that expanded due to genome duplication. The broader PKC family is ancient and can be found back in fungi, which means that the PKC family was present in the last common ancestor of opisthokonts. Human isozymes conventional - require DAG, Ca2+, and phospholipid for activation PKC-α () PKC-β1 () PKC-β2 () PKC-γ () novel - require DAG but not Ca2+ for activation PKC-δ () PKC-ε () PKC-η () PKC-θ () atypical - require neither Ca2+ nor DAG for activation (require phosphatidyl serine) PKC-ι () PKC-ζ () related PKD PKD1 () PKD2 () PKD3 () related PKN PK-N1 () PK-N2 () PK-N3 () Structure The structure of all PKCs consists of a regulatory domain and a catalytic domain (Active site) tethered together by a hinge region. The catalytic region is highly conserved among the different isoforms, as well as, to a lesser degree, among the catalytic region of other serine/threonine kinases. The second messenger requirement differences in the isoforms are a result of the regulatory region, which are similar within the classes, but differ among