Crystal structure of a class 2 D-xylose isomerase from the human intestinal tract microbe Bacteroides thetaiotaomicron

The human intestinal microbe Bacteroides thetaiotaomicron has an extensive starch utilization system and multiple genes that are involved in starch binding and utilization. We determined the structure of a xylose isomerase (XI) of Bacteroides thetaiotaomicron, which belongs to the class 2 XIs. Although the overall structures of XIs from different organisms were highly similar to each other, some large deviations were found in the class 1 and class 2 XIs; the long N-terminal extension of class 2 XIs made additional contact with the adjacent subunit of XI, and the C-terminal small α helical domain of XIs exhibited distinguishable structural features between the class 1 and class 2 XIs. However, the structures around the metal and substrate binding site were also almost identical regardless of the XI classes, suggesting common enzymological properties between the class 1 and class 2 XIs.