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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.