Enzyme-induced strain/distortion in the ground-state ES complex in beta-lactamase catalysis revealed by FTIR

Class A beta-lactamases hydrolyze penicillins and other beta-lactams via an acyl-enzyme catalytic mechanism. Ser70 is the active site nucleophile. By constructing the S70A mutant, which is unable to form the acyl-enzyme intermediate, it was possible to make stable ES complexes with various substrates. The stability of such Michaelis complexes permitted acquisition of their infrared spectra. Comparison of the beta-lactam carbonyl stretch frequency (nu(CO)) in the free and enzyme-bound substrate revealed an average decrease of 13 cm(-)(1), indicating substantial strain/distortion of the lactam carbonyl when bound in the ES complex. Interestingly, regardless of the frequency of the C=O stretch in the free substrate, when complexed to Bacillus licheniformis beta-lactamase, the frequency was always 1755 +/- 2 cm(-)(1). This suggests the active site environment induces a similar conformation of the beta-lactam in all substrates when bound to the enzyme. Using deuterium substitution, it was shown that the "oxyanion hole", which involves hydrogen bonding to two backbone amides, is the major source of the enzyme-induced strain/distortion. The very weak catalytic activity of the S70A beta-lactamase suggests enzyme-facilitated hydrolysis due to substrate distortion on binding to the enzyme. Thus the binding of the substrate in the active site induces substantial strain and distortion that contribute significantly to the overall rate enhancement in beta-lactamase catalysis.