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Protein Science, Vol 7, Issue 5 1208-1213, Copyright © 1998 by Cold Spring Harbor Laboratory Press

ARTICLE

The role of tyrosine 121 in cofactor binding of 5-aminolevulinate synthase

D. TAN, M. J. BARBER and G. C. FERREIRA
Department of Biochemistry and Molecular Biology, College of Medicine, University of South Florida, Tampa, Florida 33612

5-Aminolevulinate synthase (EC 2.3.1.37) is the first enzyme in the heme biosynthesis in nonplant eukaryotes and some prokaryotes. It functions as a homodimer and requires pyridoxal 5'-phosphate as an essential cofactor. Tyr-121 is a conserved residue in all known sequences of 5-aminolevulinate synthases. Further, it corresponds to Tyr-70 of Escherichia coli aspartate aminotransferase, which has been shown to interact with the cofactor and prevent the dissociation of the cofactor from the enzyme. To test whether Tyr-121 is involved in cofactor binding in murine erythroid 5-aminolevulinate synthase, Tyr-121 of murine erythroid 5-aminolevulinate synthase was substituted by Phe and His using site-directed mutagenesis. The Y121F mutant retained 36% of the wild-type activity and the K(m) value for substrate glycine increased 34-fold, while the activity of the Y121H mutant decreased to 5% of the wild-type activity and the K(m) value for glycine increased fivefold. The pK(a1) values in the pH-activity profiles of the wild-type and mutant enzymes were 6.41, 6.54, and 6.65 for wild-type, Y121F, and Y121H, respectively. The UV-visible and CD spectra of Y121F and Y121H mutants were similar to those of the wild-type with the exception of an absorption maximum shift (420 -> 395 nm) for the Y121F mutant in the visible spectrum region, suggesting that the cofactor binds the Y121F mutant enzyme in a more unrestrained manner. Y121F and Y121H mutant enzymes also exhibited lower affinity than the wild-type for the cofactor, reflected in the K(d) values for pyridoxal 5'-phosphate (26.5, 6.75, and 1.78 {mu}M for Y121F, Y121H, and the wild-type, respectively). Further, Y121F and Y121H proved less thermostable than the wild type. Taken together, these findings indicate that Tyr-121 plays a critical role in cofactor binding of murine erythroid 5-aminolevulinate synthase.
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