PKC

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Protein Kinase-C (also known as PKC) is a family of protein kinase enzymes that are involved in controlling the function of other proteins via the phosphorylation of hydroxyl groups of serine and threonine amino acid residues. The PKC enzymes are activated by signals such as the increased concentration of diacylglycerol levels or calcium ions. This makes PKC enzymes important in several signal transduction cascades. That PKC family consists of fifteen isozymes in humans that are split into three subfamilies based on their second messenger requirements. This is conventional (or classical), novel and atypical. Conventional PKCs contain the isoforms α, βI, βII, and γ. They need DAG, Ca²⁺, and a phospholipid such as phosphatidylserine for activation. Novel PKCs include the δ, ε, η, and θ isoforms and require DAG. However, they do not require Ca²⁺ for activation. This means that conventional and novel PKCs are activated through the same signal transduction pathway as phospholipase C. Atypical PKCs (including protein kinase Mζ and ι / λ isoforms) don’t require Ca²⁺ or diacylglycerol for activation. Protein Kinase-C often refers to the entire family of isoforms.

Figure 1. Structure of Protein Kinase-C.

Protein Kinase-C Structure

All PKCs have a regulatory and catalytic domain that is tethered via a hinge region. The catalytic region is highly conserved among the different isoforms and the catalytic region of other serine/threonine kinases. The second messenger requirement in isoforms differs due to the regulatory region. Much of the crystal structure of the catalytic region of PKC has yet to be determined. The exception to this is PKC theta and iota. Due to its similarity to other kinases with documented crystal structures, there have been many predictions. The regulatory domain of PKCs contains multiple subregions that are shared. The C1 domain has a binding site for DaG and non-hydrolysable, non-physiological analogues known as phorbol esters. This domain is functional and can bind DAG in both conventional and novel isoforms. However, the C1 domain in atypical PKCs cannot bind to DAG or phorbol esters. The C2 domain acts as a Ca²⁺ sensor and can be found in conventional and novel isoforms. However, it only functions as a Ca²⁺ sensor in conventional isoforms. The pseudosubstrate region in all three classes of PKC is a small sequence of amino acids that mimic a substrate and bind the substrate-binding cavity in the catalytic domain. When DAG and Ca²⁺ are both present in the right concentrations, they will bind to the C1 and C2 domains respectively. They also recruit PKC to the membrane, resulting in the release of the pseudosubstrate from the catalytic site and activation of the enzyme. PKC must be properly folded and in the correct conformation permissive for these interactions to occur. The catalytic region of PKC enables different functions to be processed. It also allows for phosphorylation which is essential to its viability of the kinase. Conventional and novel PKCs have three phosphorylation sites known as the activation loop, the turn motif, and the hydrophobic motif. The consensus sequence of Protein Kinase-C enzymes is similar to Protein Kinase-A. This is because it contains basic amino acids close to the Ser/Thr to be phosphorylated.

Functions

Protein kinase C is a cytoplasmic enzyme. In unstimulated cells, PKC is mainly distributed in the cytoplasm in an inactive conformation. Once there is a second messenger, PKC will become a membrane-bound enzyme. It can activate enzymes in the cytoplasm and participate in the regulation of biochemical reactions. It can also act on transcription factors in the nucleus and participate in the regulation of gene expression.

Control of sugar metabolism

In liver cells, protein kinase C and protein kinase A cooperate to phosphorylate glycogen synthase, inhibit the activity of glucose-polymerizing enzyme, and promote glycogen metabolism

• Control of cell differentiation

Myogenin is a transcription factor that plays a key role in the differentiation of muscle cells. In myoblasts, protein kinase C can phosphorylate myogenin, which inhibits the ability of myogenin to bind to DNA, thereby preventing the differentiation of cells into muscle fibers.

• Participate in gene expression regulation

Protein kinase C can participate in the control of gene expression in at least two ways. One way is that protein kinase activates a phosphorylation cascade system to phosphorylate MAP protein kinase, and phosphorylated MAP protein kinase phosphorylates gene regulatory protein Elk-1 to activate it. Activated Elk-1 binds to a short DNA sequence (called serum response element, SRE), and then co-regulates gene expression with another factor (serum response factor, SRF). Another way is to phosphorylate protein kinases and activate the inhibitory protein Iκ-B, release the gene regulatory protein NF-κ-B, and make it enter the nucleus to activate the transcription of specific genes.

Activation

The activity of PKC depends on the presence of calcium ions and phospholipids, but only in the presence of the phospholipid metabolism intermediate diacylglycerol (DAG), the physiological concentration of calcium ions will work. This is because DAG can increase the affinity of PKC to the substrate. Phosphatidylinositol-4,5-bisphosphate (PIP2) is hydrolyzed under the action of phospholipase-C to produce DAG and IP3. IP3 promotes the release of intracellular calcium ions and plays a synergistic role with DAG in the process of activating PKC. A variety of chemical substances or antibiotics have inhibitory effects on PKC activity. According to the different target sites of inhibitors, inhibitors can be divided into two groups: one group is inhibitors acting on the catalytic region, which can be conserved with protein kinases. Residues bind, so there is no obvious selectivity to PKC; the other group is inhibitors acting on the regulatory region, they can be combined with Ca²⁺, phospholipids and diacylglycerol/phorbol esters, so they have higher selectivity.