d-Phosphoarabinoisomerase and d-Ribulokinase in Escherichia coli

An enzyme, D-phosphoarabinoisomerase, which stoichiometrically interconverts D-arabinose 5-phosphate and Dribulose S-phosphate, was isolated from Escherichia coli.At equilibrium, 75% of D-arabinose-5-P and 25% of D-ribulose-5-P are found.This unstable enzyme does not show any requirement for added cofactor, and is inhibited by Mn++, Co++, Zn+f, Cd++, p-chloromercuribenzoate, and phosphate.The optimum pH is 8.0.The Km is 1.36 X 10u3 M for D- arabinose-5-P and 5.40 X low4 M for D-ribulose-5-P.The enzyme lacks activity toward D-arabinose, D-ribulose, Dribose, and D-ribose-5-P.This enzyme not only accounts for the metabolic origin of the precursor for Z-keto-3-deoxyoctonate (KDO), a constituent in the cell wall lipopolysaccharide of E. coli and some other gram-negative bacteria, but also fits into the schema suggested for the utilization of D-arabinose-1-P generated in the metabolism of D-arabinosyl nucleosides.The enzyme is absent from yeast and mouse fibroblasts.D-Arabinose-5-P was synthesized by phosphorylating D-glucosamine followed by ninhydrin degradation.The product was contaminated with D-ribulose-5-P and was purified by precipitation of the latter with borate.D-Ribulose-5-P was obtained by phosphorylating D-ribulose with a D-ribulokinase isolated from E. coli.The product showed a distinct carbonyl absorption band in its infrared spectrum.Upon periodate oxidation, phosphoglycolaldehyde was detected as a major degradation product, as was expected for D-ribulose-5-P.However, this purified preparation of D-ribulokinase also phosphorylated L-fuculose at position 1.We are reporting studies on the isolation and characterization of n-phosphoarabinoisomerase and n-ribulokinase derived from