Molecular model of cAMP-dependent protein kinase in complex with an A-kinase anchoring protein (AKAP, red). This enzyme is also known as protein kinase A (PKA). This is the holoenzyme, which consists of two regulatory subunits (dark blue) and two catalytic subunits (light blue). Protein kinase enzymes modify other proteins by chemically adding phosphate groups to them (phosphorylation). PKA regulates a number of cellular processes, including lipid and glucagon metabolism. AKAP functions to tether PKA to a particular location within a cell.