Phosphorylase kinase

Phosphorylase kinase (PhK) is a serine/threonine-specific protein kinase which activates glycogen phosphorylase to release glucose-1-phosphate from glycogen. PhK phosphorylates glycogen phosphorylase at two serine residues, triggering a conformational shift which favors the more active glycogen phosphorylase "a" form over the less active glycogen phosphorylase b.

The protein is a hexadecameric holoenzyme—that is, a homotetramer in which each subunit is itself a tetramer—arranged in an approximate "butterfly" shape. Each of the subunits is composed of an α, β, γ and δ subunit. The γ subunit is the site of the enzyme's catalytic activity while the other three subunits serve regulatory functions.

When unmodified, the α and β subunits inhibit the enzyme's catalysis, but phosphorylation of both these subunits by protein kinase A (PKA, or cAMP-dependent protein kinase) reduces their respective inhibitory activities. The δ subunit is the ubiquitous eukaryotic protein calmodulin which itself has 4 calcium ion binding sites. When cytosolic Ca2+ levels rise-to as low as 10−7 M—the δ subunit undergoes a large conformational change that activates the kinase's activity by binding to a complementary hydrophobic patch on the catalytic γ subunit.

Genes

Alpha: PHKA1, PHKA2

Beta: PHKB

Gamma: PHKG1, PHKG2

History

Phosphorylase kinase was the first protein kinase to be isolated and characterized in detail, accomplished first by Krebs, Graves and Fischer in the 1950s. At the time, the scientific community was largely unaware of the importance of protein phosphorylation in the regulation of cellular processes, and many in the field dismissed phosphoproteins as biologically unimportant.