CIRCULAR-DICHROISM STUDIES OF THE COENZYME ENVIRONMENT IN THE ACTIVE-SITES OF MUTANT FORMS OF THE BETA-SUBUNIT IN THE TRYPTOPHAN SYNTHASE ALPHA-2-BETA-2 COMPLEX

The circular dichroism has been used to evaluate the effect of mutation on the environment of the pyridoxal phosphate coenzyme in the active site of the beta-subunit in the tryptophan synthase alpha-2-beta-2-complex from Salmonella typhimurium. Seven mutant forms of the alpha-2-beta-2-complex with single amino acid replacements at residues 87, 109, 188, 306, and 350 of the beta-subunit have been prepared by site-directed mutagenesis, purified to homogeneity, and characterized by absorption and circular dichroism spectroscopy. Since the wild type and mutant alpha-2-beta-2-complexes all exhibit positive circular dichroism in the coenzyme absorption band, pyridoxal phosphate must bind asymmetrically in the active site of these enzymes. However, the coenzyme may have an altered orientation or active site environment in five of the mutant enzymes that display less intense ellipticity bands. The mutant enzyme in which lysine 87 is replaced by threonine has very weak ellipticity at 400 nm. Since lysine 87 forms a Schiff base with pyridoxal phosphate in wild type enzyme, our results demonstrate the importance of the Schiff base linkage for rigid or asymmetric binding. Although the mutant enzymes display spectra in the presence of L-serine that differ from that of the wild type enzyme, addition of alpha-glycerol 3-phosphate converts the spectra of two of the mutant enzymes to that of the wild type enzyme. We conclude that this alpha-subunit ligand may produce a conformational change in the alpha-subunit that is transmitted to the mutant beta-subunits and partially corrects conformational alterations in the mutant enzymes.