title: Evidence from Fourier transform infrared spectroscopy for polarization of the carbonyl of oxaloacetate in the active site of citrate synthase.
abstract: The infrared spectrum of oxaloacetate bound in the active site of citrate synthase has been measured in the binary complex and in the ternary complex with the acetyl coenzyme A (CoA) enolate analogue carboxymethyl-CoA. The carbonyl stretching frequency of oxaloacetate in binary and ternary complexes is found at 1697 cm-1, a shift of 21 cm-1 to lower frequency relative to that of the free ligand. The line widths of the carbonyl absorption in enzyme complexes differ from that of the free ligand, decreasing from a value of 20 cm-1 for the free ligand to 10 cm-1 in the binary complex and 7 cm-1 in the ternary complex with carboxymethyl-CoA. The integrated absorbance of the carbonyl absorption in these enzyme complexes is significantly increased over that of the free ligand at the same concentration, increasing approximately 2-fold in the binary complex and approximately 3-fold in the ternary complex. These results indicate strong polarization of the carbonyl bond in the enzyme-substrate complexes and suggest that ground-state destabilization is a major catalytic strategy of citrate synthase.
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism; D001665:Binding Sites; D002950:Citrate (si)-Synthase / Q000378:metabolism*; D005583:Fourier Analysis; D009682:Magnetic Resonance Spectroscopy; D010071:Oxaloacetates / Q000378:metabolism*; D007652:Oxo-Acid-Lyases / Q000378:metabolism*; D011485:Protein Binding; D013055:Spectrophotometry, Infrared
chemical_list: D010071:Oxaloacetates; D000105:Acetyl Coenzyme A; D002950:Citrate (si)-Synthase; D007652:Oxo-Acid-Lyases