Protein kinase A (EC 188.8.131.52) consists of two domains, a small domain with several β sheet structures and a larger domain containing several α helices. The binding sites for substrate and ATP are located in the catalytic cleft between the domains (or lobes). When ATP and substrate bind, the two lobes rotate so that the terminal phosphate group of the ATP and the target amino acid of the substrate move into the correct positions for the catalytic reaction to take place.
Protein kinase A has several functions in the cell, including regulation of glycogen, sugar, and lipid metabolism. It is controlled by cAMP: in the absence of cAMP, the kinase is a tetramer of two regulatory and two catalytic subunits (R2C2), with the regulatory subunits blocking the catalytic center of the catalytic subunits. Binding of cAMP to the regulatory subunit leads to dissociation of active RC dimers. Also, the catalytic subunit itself can be regulated by phosphorylation.
Downregulation of protein kinase A occurs by a feedback mechanism: one of the substrates that is activated by the kinase is a phosphodiesterase, which converts cAMP to AMP, thus reducing the amount of cAMP that can activate protein kinase A.
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