The evolution of sugar isomerases

L-Arabinosc isomerase (EC 5.3.1.4 EC] ) catalyzes the isomerizationof L-arabinose to L-ribulose. Here we report on the purification,kinetic mechanism and chemical mechanism of L-arabinose isomerasefrom Escherichia coli. The enzyme catalyzes the isomerizationof L-arabinose to L-ribulose by a proton transfer mechanism,in contrast to xylose isomerase which uses a hydride transfermechanism to perform a similar isomerization. Arabinose isomeraseactivity is metal dependent, although the enzyme can catalyzethe exchange of the proton attached to carbon 2 of arabinosewith the solvent in the absence of metal ion. Manganese(II)is the only metal ion which renders the enzyme active for theisomerization reaction. Arabinose isomerase has high substratespecificity for L-arabinose. The difference in chemical mechanismbetween xylose isomerase and arabinose isomerase suggests thatthese enzymes are not related by convergent evolution. Thiswork also suggests that unless convergent evolution has beendemonstrated, the mechanism of one enzyme may not give any insightinto the mechanism of a second enzyme catalyzing the same reaction