Mutation of an Active Site Residue of Tryptophan Synthase (β-Serine 377) Alters Cofactor Chemistry

To better understand how an enzyme controls cofactor chemistry, we have changed a tryptophan synthase residue that interacts with the pyridine nitrogen of the pyridoxal phosphate cofactor from a neutral Ser (β-Ser 377 ) to a negatively charged Asp or Glu. The spectroscopic properties of the mutant enzymes are altered and become similar to those of tryptophanase and aspartate aminotransferase, enzymes in which an Asp residue interacts with the pyridine nitrogen of pyridoxal phosphate. The absorption spectrum of each mutant enzyme undergoes a pH-dependent change (p K a ∼ 7.7) from a form with a protonated internal aldimine nitrogen (λ max = 416 nm) to a deprotonated form (λ max = 336 nm), whereas the absorption spectra of the wild type tryptophan synthase β 2 subunit and α 2 β 2 complex are pH-independent. The reaction of the S377D α 2 β 2 complex with l -serine, l -tryptophan, and other substrates results in the accumulation of pronounced absorption bands (λ max = 498–510 nm) ascribed to quinonoid intermediates. We propose that the engineered Asp or Glu residue changes the cofactor chemistry by stabilizing the protonated pyridine nitrogen of pyridoxal phosphate, reducing the p K a of the internal aldimine nitrogen and promoting formation of quinonoid intermediates.