Lysine-73 is involved in the acylation and deacylation of beta-lactamase

Lysine 73 is a conserved active-site residue in the class A beta-lactamases, as well as other members of the serine penicillin-sensitive enzyme family; its role in catalysis remains controversial and uncertain. Mutation of Lys73 to alanine in the beta-lactamase from Bacillus licheniformis resulted in a substantial reduction in both turnover rate (k(cat)) and catalytic efficiency (k(cat)/K(m)), and a very significant shift in pK(1) to higher pH in the bell-shaped pH-rate profiles (k(cat)/K(m)) for several penicillin and cephalosporin substrates. The increase in pK(1) is consistent with the removal of the positive ammonium group of the lysine from the proximity of Glu166, to which the acid limb has been ascribed. The alkaline limb of the k(cat)/K(m) vs profiles is not shifted appreciably, as might have been expected if this limb reflected the ionization of Lys73 in the wild-type enzyme. The k(cat)/K(m) at the pH optimum for the mutant was down about 200-fold for penicillins and around 10(4) for cephalosporins, compared to the wild-type, suggesting significant differences in the mechanisms for catalysis of penicillins compared to cephalosporins. Burst kinetics were observed with several substrates assayed with K73A beta-lactamase, indicating an underlying branched-pathway kinetic scheme, and rate-limiting deacylation. FTIR analysis was used to determine whether acylation or deacylation was rate-limiting. In general, acylation was the rate-limiting step for cephalosporin substrates, whereas deacylation was rate-limiting for penicillin substrates. The results indicate that Lys73 plays an important role in both the acylation and deacylation steps of the catalytic mechanism. The effects of this mutation (K73A) indicate that Lys73 does not function as a general base in the catalytic mechanism of beta-lactamase. The existence of bell-shaped pH-rate profiles for the K73A variant suggests that Lys73 is not directly responsible for either limb in such plots. It is likely that both Glu166 and Lys73 are important to each other in terms of maintaining the optimum electrostatic environment for fully efficient catalytic activity to occur.