D-glucose isomerase(GI) can isomerize D-glucose and D-xylose into D-fructose and D-xylulose，respectively. It is a crucial enzyme in the production of high fructose corn syrup on industrial scale. In the hydride shift mechanism proposed for GI，the main features are ring opening of the substrate，isomerization of GI via a hydtride shift from C₂ to C₁，and ring closure of the product. GI gene has been cloned，sequenced and overexpressed in the homologous and heterogous hosts. GI whose properties have been improved by protein engineering may be one of the most important industrial enzymes of the future.