Enzyme-Induced Strain/Distortion in the Ground-State ES Complex in β-Lactamase Catalysis Revealed by FTIR

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Abstract
Class A β-lactamases hydrolyze penicillins and other β-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 β-lactam carbonyl stretch frequency (νCO) 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 CO stretch in the free substrate, when complexed to Bacilluslicheniformis β-lactamase, the frequency was always 1755 ± 2 cm-1. This suggests the active site environment induces a similar conformation of the β-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 β-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 β-lactamase catalysis.