Polarization of substrate carbonyl groups by yeast aldolase: investigation by Fourier transform infrared spectroscopy

Abstract

The IR spectrum of the complex of D-fructose 1,6-bisphosphate bound to yeast aldolase displays 3 spectral features between 1700 and 1800 cm-1. One of these (at 1730 cm-1) corresponds to the carbonyl group of enzyme-bound D-fructose 1,6-bisphosphate and/or dihydroxyacetone phosphate. The frequency of this band, which is unaffected by the removal of the intrinsic Zn ion from the enzyme, demonstrates that this carbonyl group is not significantly polarized when the substrate binds to the enzyme. In contrast, the spectral band assigned to the carbonyl group of enzyme-bound D-glyceraldehyde 3-phosphate (at 1706 cm-1) appears at a frequency 24 cm-1 lower than when this substrate is in aqueous solution. This shift indicates considerable polarization of the carbonyl group when D-glyceraldehyde 3-phosphate is bound at the active site. The 3rd spectral feature (at 1748 cm-1), which is observed only in the presence of K ion, probably corresponds to an enzymic carboxyl group in a nonpolar environment.