= Solution
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. \b[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.<Probing the role of the conserved residue Glu166 in a class A beta-lactamase using neutron and X-ray protein crystallography by Patricia S. Langan and others>[Abstract and active-site protonation analysis]{ref}
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