PRESENCE OF LOW BARRIER HYDROGEN BONDS INVOLVED IN CATALYTIC MECH OF ENOLASE

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Low-barrier hydrogen bonds have been recently hypothesized to be involved in providing the energy required by enzymes for catalysis. These very strong, low-barrier hydrogen bonds exhibit very low field proton NMR signals (16-20 ppm) and low deuterium fractionation factors. The mechanism of enolase is believed to pass through a carbanion intermediate, which may be stabilized by such a low-barrier hydrogen bond present in the intermediate but not ground state. It has been demonstrated by Weiss, et al. (JACS, 1987) that the fractionation factor for the proton transferred from C-2 of D-TSP (a competitive inhibitor of enolase) to the catalytic base is far below unity. Because no cysteine residues are present in the catalytic site to explain this finding, as observed from x-ray crystal structure, it has been hypothesized that this result suggests the presence of a low-barrier hydrogen bond. Thus, proton NMR will be used to search for a low field signal indicative of such a low-barrier hydrogen bond. Methylene phosphoenolpyruvate, a slow substrate for enolase, and tartronate semialdehyde phosphate, a dead end inhibitor, will be employed for the studies. The use of such inhibitors in which the intermediate may be present in appreciable amounts is hoped to allow one to observe the low field signal representative of a low-barrier hydrogen bond. A low field proton NMR signal has been observed in chymotrypsin by Robillard, et al. (J. Mol. Biol., 1972), which has been postulated to represent a low-barrier hydrogen bond.