SummaryInterconversion of D-ribose 5-phosphate and D-ribulose 5-phosphate is an important step in the pentose phosphate pathway. Two unrelated enzymes with D-ribose-5-phosphate isomerase activity were first identified in Escherichia coli, RpiA and RpiB. In this organism, the essential 5-carbon sugars were thought to be processed by RpiA, while the primary role of RpiB was suggested instead to be interconversion of the rare 6-carbon sugars, D-allose 6-phosphate and D-allulose 6-phosphate. In Mycobacterium tuberculosis, where only an RpiB is found, the 5-carbon sugars are believed to be the enzyme's primary substrates. Here we present kinetic studies examining the D-allose-6-phosphate isomerase activity of the RpiBs from these two organisms, and show that only the E. coli Keywords: Ribose-5-phosphate isomerase; allose-6-phosphate isomerase; rare sugar; Xray crystallography; Rv2465c.
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