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d-Phosphoarabinoisomerase and d-Ribulokinase in Escherichia coli

From the ¹ From the Department of Biochemistry and the Department of Therapeutic Research, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104

An enzyme, d-phosphoarabinoisomerase, which stoichiometrically interconverts d-arabinose 5-phosphate and d-ribulose 5-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⁺⁺, Cd⁺⁺, p-chloromercuribenzoate, and phosphate. The optimum pH is 8.0. The K_m is 1.36 x 10^-3 m for darabinose-5-P and 5.40 x 10^-4 m for d-ribulose-5-P. The enzyme lacks activity toward d-arabinose, d-ribulose, d-ribose, and d-ribose-5-P. This enzyme not only accounts for the metabolic origin of the precursor for 2-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.

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