THE REPLACEMENT OF TRP392 BY ALANINE INFLUENCES THE DECARBOXYLASE/CARBOLIGASE ACTIVITY AND STABILITY OF PYRUVATE DECARBOXYLASE FROM ZYMOMONAS-MOBILIS

The bulky tryptophan residue 392 located in the deep cleft leading to the active center of pyruvate decarboxylase (PDC) from Zymomonas mobilis was changed to alanine which is found in the equivalent position of PDC from yeast, The mutation reduced the decarboxylase activity towards pyruvate by a factor of two (60-70 U/mg), whereas the K-m (1.1 mM in Mes/KOH buffer) remains unchanged compared with the wild-type enzyme. The apparent K-m, for thiamine diphosphate (thiamin-P-2) in the presence of 5 mM MgSO4 was increased by a factor of 10 (84 mu M in Mes/KOH buffer) and the tetrameric mutant protein was less stable, as indicated by urea denaturation experiments. The mutation enhanced the carboligase activity of the enzyme towards benzaldehyde by a factor of four. The resulting alpha-hydroxyketone was identified as (R)-phenylacetylcarbinol.