3-Keto-L-gulonate 6-phosphate decarboxylase (KGPDC) and D-arabino-hex-3-ulose 6-phosphate synthase (HPS) are members of the orotidine 5′-monophosphate decarboxylase (OMPDC) suprafamily [Wise, E., Yew, W. S., Babbitt, P. C., Gerlt, J. A., and Rayment, I. (2002) Biochemistry 41, 3861-3869], a group of homologous enzymes that share the (β/α) 8-barrel fold. KGPDC catalyzes a Mg2+-dependent decarboxylation reaction in the catabolic pathway of L-ascorbate utilization by Escherichia coli K-12 [Yew, W. S., and Gerlt, J. A. (2002) J.Bacteriol. 184, 302-306]; HPS catalyzes a Mg2+-dependent aldol condensation between formaldehyde and D-ribulose 5-phosphate in formaldehyde-fixing methylotrophic bacteria [Kato, N., Ohashi, H., Hori, T., Tani, Y., and Ogata, K. (1977) Agric. Biol. Chem. 41, 1133-1140]. Our previous studies of the KGPDC from E. coli established the occurrence of a stabilized cis-enediolate intermediate [Yew, W. S., Wise, E., Rayment, I., and Gerlt, J. A. (2004) Biochemistry 43, 6427-6437; Wise, E., Yew, W. S., Gerlt, J. A., and Rayment, I. (2004) Biochemistry 43, 6438-6446]. Although the mechanism of the HPS-catalyzed reaction has not yet been investigated, it also is expected to involve a Mg2+-stabilized cis-enediolate intermediate. We now have discovered that the KGPDC from E. coli and the HPS from Methylomonas aminofaciens are both naturally promiscuous for the reaction catalyzed by the homologue. On the basis of the alignment of the sequences of orthologous KGPDC's and HPS's, four conserved active site residues in the KGPDC from E. coli were mutated to those conserved in HPS's (E112D/R139V/T169A/R192A): the value of the kcat for the promiscuous HPS activity was increased as much as 170-fold (for the E112D/R139V/T169A/R192A mutant), and the value of kcat/Km was increased as much as 260-fold (for the E112D/R139V/T169A mutant); in both cases, the values of the kinetic constants for the natural KGPDC activity were decreased. Together with the structures of mutants reported in the accompanying manuscript [Wise, E. L., Yew, W. S., Akana, J., Gerlt, J. A., and Rayment, I., accompanying manuscript], these studies illustrate that large changes in catalytic efficiency can be accomplished with only modest changes in active site structure. Thus, the (β/α)8-barrel fold shared by members of the OMPDC suprafamily appears well-suited for the evolution of new functions.
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