A tyrosine residue is involved in the allosteric binding of tryptophan to yeast chorismate mutase.

Comparative 1H-NMR studies have been carried out on wild-type chorismate mutase, which is activated by tryptophan and inhibited by tyrosine and phenylalanine, and mutant yeast chorismate mutase, which has a single point mutation (T226I) and is not allosterically regulated but 'locked' in the activated state. Double quantum-filtered COSY spectra show cross-peaks which have been assigned to a tyrosine that are absent in the mutant enzyme and in the wild-type enzyme plus tryptophan when compared with the wild-type enzyme alone. These observations indicate the involvement of a tyrosine at or near the allosteric binding site. The involvement of tyrosine in tryptophan binding was tested by modification of tyrosine in yeast chorismate mutase by nitration with tetranitromethane. All forms of the enzyme exhibited an approx. 50% reduction in specific activity, but it was found that preincubation of the wild-type with the allosteric activator, tryptophan, lead to partial protection against loss in specific activity. Only one tyrosine residue was nitrated in the wild-type enzyme and this tyrosine was identified by tryptic digestion and sequencing, and found to be very close to the site of the single point mutation in the mutant enzyme. It is proposed that Tyr-234 is located at or near the allosteric activation site.