<p>Protein phosphorylation, which plays a key role in most cellular activities, is a reversible process mediated by protein kinases and phosphoprotein phosphatases. Protein kinases catalyse the transfer of the gamma phosphate from nucleotide triphosphates (often ATP) to one or more amino acid residues in a protein substrate side chain, resulting in a conformational change affecting protein function. Phosphoprotein phosphatases catalyse the reverse process. Protein kinases fall into three broad classes, characterised with respect to substrate specificity [<cite idref="PUB00005115"/>]:</p><p> <ul> <li>Serine/threonine-protein kinases</li><li>Tyrosine-protein kinases</li><li>Dual specific protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)</li> </ul> </p><p>Protein kinase function has been evolutionarily conserved from <taxon tax_id="562">Escherichia coli</taxon> to human [<cite idref="PUB00020114"/>]. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation [<cite idref="PUB00015362"/>]. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins. The catalytic subunits of protein kinases are highly conserved, and several structures have been solved [<cite idref="PUB00034898"/>], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [<cite idref="PUB00034899"/>].</p>Phosphorylase B kinase (<db_xref db="EC" dbkey="2.7.1.38"/>) belongs to a family of proteins involved in glycogen biosynthesis [<cite idref="PUB00000241"/>]. The protein has a subunit compositionof (alpha, beta, gamma, delta)4, where the alpha and beta subunits are regulatory, delta is calmodulin, and the gamma subunit is catalytic. The enzyme is believed to have a dual role, the first is connected with glycogendegradation via phosphorylation of glycogen phosphorylase; the second controls glycogen biosynthesis on the sarcoplasmic reticular membrane moredirectly by phosphorylation, and thus inhibition, of glycogen synthase [<cite idref="PUB00001685"/>].<p>The gamma catalytic chain contains three domains; one protein kinase and twocalmodulin-binding domains. Calcium and magnesium ions, together with cyclicAMP, positively affect the efficiency of the enzyme, which is believed to be associated with its auto-kinase activity [<cite idref="PUB00000241"/>, <cite idref="PUB00000744"/>].</p><p>The full extent of the effects of deficiencies in this enzyme in humans is unknown; but case studies have been documented [<cite idref="PUB00005088"/>, <cite idref="PUB00004318"/>, <cite idref="PUB00003467"/>] that detail symptoms asmild as 'exercise intolerance' [<cite idref="PUB00005088"/>], to infant mortality arising from floppyinfant syndrome [<cite idref="PUB00004318"/>].</p>