Xylose isomerase (EC 5.3.1.5) [1] is an enzyme found in microorganisms which catalyzes the interconversion of an aldo sugar D-xylose to a keto sugar D-xylulose. It can also isomerize D-ribose to D-ribulose and D-glucose to D-fructose. Xylose isomerase seems to require magnesium for its activity, while cobalt is necessary to stabilize the tetrameric structure of the enzyme.

Xylose isomerase also exists in plants [2] where it is manganese-dependent. The enzyme has also been found in anaerobic fungi [3].

A number of residues are conserved in all known xylose isomerases. A histidine in the N-terminal section of the enzyme has been shown [4] to be involved in the catalytic mechanism of the enzyme. Two glutamate residues, a histidine and four aspartate residues are the metal-binding sites that bind two ions of magnesium, cobalt, or manganese [5,6,7].

Three-dimensional structures of xylose isomerases show a that each subunit contains a common α/β-barrel fold (see <PDB:2GLK; A>) [7] similar to that of other divalent metal-dependent TIM barrel enzymes, such as rhamnose isomerase [8] and endonuclease 4 (see <PDOC00599>) [1,5,6]. The C-terminal smaller part forms an extended helical fold that seems to be implicated in multimerization.

We have developed a profile that covers the entire xylose isomerase structure.