Role of Metal Ions in the Reaction Catalyzed by l-Ribulose-5-phosphate 4-Epimerase

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Abstract
H97N, H95N, and Y229F mutants of l-ribulose-5-phosphate 4-epimerase had 10, 1, and 0.1%, respectively, of the activity of the wild-type (WT) enzyme when activated by Zn2+, the physiological activator. Co2+ and Mn2+ replaced Zn2+ in Y229F and WT enzymes, although less effectively with the His mutants, while Mg2+ was a poorly bound, weak activator. None of the other eight tyrosines mutated to phenylalanine caused a major loss of activity. The near-UV CD spectra of all enzymes were nearly identical in the absence of metal ions and substrate, and addition of substrate without metal ion showed no effect. When both substrate and Zn2+ were present, however, the positive band at 266 nm increased while the negative one at 290 nm decreased in ellipticity. The changes for the WT and Y229F enzymes were greater than for the two His mutants. With Co2+ as the metal ion, the CD and absorption spectra in the visible region were different, showing little ellipticity in the absence of substrate and a weak absorption band at 508 nm. With substrate present, however, an intense absorption band at 555 nm (ε = 150−175) with a negative molar ellipticity approaching 2000 deg cm2 dmol-1 appears with WT and Y229F enzymes. With the His mutants, the changes induced by substrate were smaller, with negative ellipticity only half as great. The WT, Y229F, H95N, and H97N enzymes all catalyze a slow aldol condensation of dihydroxyacetone and glycolaldehyde phosphate with an initial kcat of 1.6 × 10-3 s-1. The initial rate slowed most rapidly with WT and H97N enzymes, which have the highest affinity for the ketopentose phosphates formed in the condensation. The EPR spectrum of enzyme with Mn2+ exhibited a drastic decrease upon substrate addition, and by using H217O, it was determined that there were three waters in the coordination sphere of Mn2+ in the absence of substrate. These data suggest that (1) the substrate coordinates to the enzyme-bound metal ion, (2) His95 and His97 are likely metal ion ligands, and (3) Tyr229 is not a metal ion ligand, but may play another role in catalysis, possibly as an acid−base catalyst.