Phosphorus 31 nuclear magnetic resonance study of tryptophanase. Pyridoxal phosphate-binding site

The pyridoxal phosphate-dependent enzyme tryptophanase has been investigated using 31P nuclear magnetic resonance at 72.86 MHz. In the native enzyme, the pyridoxal-P 31P chemical shift was found to be 3.55 ppm and independent of pH, indicating that the dianionic phosphate group of the cofactor is not accessible to solvent. Binding of the competitive inhibitor, beta-phenyl-DL-serine, results in the formation of the transaldimination complex. This complex is fixed to the enzyme via the dianionic phosphate group of the cofactor: again the observed shift is independent of pH. In both cases, restricted rotational freedom of the phosphate group around the C-O bond linking the phosphate ester to the pyridine moiety of the cofactor could be asserted from line width data. Addition of the competitive inhibitor, L-alanine, to tryptophanase produces the quinonoid intermediate. The phosphate group of this complex has lost its specific interaction (probably a salt bridge) with the protein, as indicated by the pH dependence of the chemical shift.