Epimerization via carbon-carbon bond cleavage. L-ribulose-5-phosphate 4-epimerase as a masked class II aldolase

Studies indicating that the E. coli L-ribulose-5-phosphate 4-epimerase employs an "aldolase-like" mechanism are reported. This NAD+-independent enzyme epimerizes a stereocenter that does not bear an acidic proton and therefore it cannot utilize a simple deprotonation-reprotonation mechanism. Sequence similarities between the epimerase and the class II l-fuculose-1-phosphate aldolase suggest that the two may be evolutionarily related and that the epimerization may occur via carbon-carbon bond cleavage and re-formation. Conserved residues thought to provide the metal ion ligands of the epimerase have been modified using site-directed mutagenesis. The resulting mutants show low kcat values in addition to a reduced affinity for Zn2+. These observations serve to establish that there is a structural link between between the active site geometry of the epimerase and the aldolase. In addition, the H97N mutant was found to catalyze the condensation of dihydroxyacetone and glycolaldehyde phosphate to produce a mixture of L-ribulose-5-phosphate and D-xylulose-5-phosphate. This observation of aldolase activity establishes that the epimerase active site is capable of promoting carbon-carbon bond cleavage. Furthermore, glycolaldehyde phosphate was shown to be a competitive inhibitor of the mutant enzyme (KI = 0.37 mM) but not of the wild-type enzyme. The mutation apparently causes the epimerase to become "leaky" and enables it to bind/generate the normal reaction intermediates from the unbound aldol cleavage products.