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Protein Kinase A (PKA)
Dedicated protein kinases are those protein kinases that phosphorylate a single enzyme or a set of closely-related enzymes. On the other hand, multifunctional protein kinases are those that phosphorylate a wide range of enzyme substrates. The specificity of these protein kinases generally depends on the sequence of amino acids directly surrounding of the target amino acid to be phosphorylated. One particular example of a protein kinase found inside our body is protein kinase A (PKA). PKA is activated when the sympathetic nervous system kicks in and initiates the flight-or-fight response during stressful, dangerous or exciting situations. Once activated, PKA goes onto to activate other target enzymes via phosphorylation. PKA typically exists in its inactive form, which consists of the R2C2 complex. In this complex, there are two catalytic subunits and two regulatory subunits. There are a total of four allosteric sites on the two regulatory chains (two per chain) that allow the binding of cyclic adenosine monophosphate (cAMP). When the sympathetic nervous stimulates the release the epinephrine from the adrenal medulla, the epinephrine induces the conversion of ATP into cAMP. When the concentration of cAMP increases, these allosteric effectors begin to bind to the allosteric sites on the regulatory chains. When all four sites are filled, this causes a conformational change that dissociates the regulatory chains form the catalytic ones, which frees up the active sites of the catalytic chains and actives them.
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