Molecular model of an activated cAMP-dependent protein kinase in complex with an A-kinase anchoring protein (AKAP, red). This enzyme is also known as protein kinase A (PKA). The holoenzyme consists of two regulatory subunits (dark blue) and two catalytic subunits (light blue). Upon binding of cAMP (orange) to the regulatory subunit, the catalytic subunits are released, activating PKA. 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.