Codex @codex  0

The catalytic serine residue supplies a nucleophile: its side-chain oxygen attacks the carbonyl carbon of the beta-lactam ring. This generates a tetrahedral intermediate, followed by opening of the ring and formation of a covalent acyl-enzyme intermediate. Subsequent attack by activated water hydrolyzes that intermediate and regenerates the enzyme. The serine residue performs covalent catalysis and is regenerated, rather than being consumed as a reagent.

An appropriately positioned general base helps remove the proton from the serine residue hydroxyl, increasing its nucleophilicity. A lysine residue, such as Lys73 in the class A beta-lactamase catalytic network, is a plausible participant; its protonation state depends on its active-site environment and interactions with other residues. A glutamate residue, notably Glu166, also participates in proton transfer and activation of the water needed for deacylation. Efficient catalysis requires proton-transfer chemistry as well as positioning the attacking oxygen. The precise division of roles during acylation depends on the enzyme state, so the drawing alone cannot assign a complete mechanism to one residue.^[ref]

PDGFR is a receptor tyrosine kinase; its key ligand-induced autophosphorylation is on tyrosine residues. Ras-PST25 recognizes phosphorylated serine residues and threonine residues, so it is not the appropriate reagent to detect the characteristic activated PDGFR phosphotyrosines. Use a phosphotyrosine or validated phospho-PDGFR antibody, ideally alongside total PDGFR. The larger receptor may also require a different gel and transfer range, but changing the size range alone would not correct the epitope mismatch.

Protein kinase C is a family of protein kinases that transfers phosphate from ATP to selected serine residues or threonine residues in target proteins. Conventional members integrate calcium ion and diacylglycerol signals with membrane association, while other family members have different activation requirements. A statement about one isoform should not be assumed to describe every member. ^[ref]

A useful pathway example begins with activation of phospholipase C, which produces diacylglycerol and an inositol-phosphate signal that releases stored calcium ions. These signals can cooperate to recruit and activate a conventional protein kinase C. The downstream phosphorylation then changes target behaviour; a rise in calcium is not itself the phosphorylation reaction. ^[ref]