Insights into Fungal Xylose Reductases and Its Application in Xylitol Production

Abstract

Xylose reductase (EC 1.1.1.21), an aldo-keto reductase enzyme, catalyzes the conversion of xylose into xylitol. It is present in animals, plants, and many microorganisms. In microorganisms, in addition to its production by many fungal (yeasts and molds) cultures, a few members of bacteria such as Corynebacterium sp. and Enterobacter sp. have also been reported to produce NADPH-dependent xylose reductase (XR). In fungi, XR directly converts xylose into xylitol during the metabolism of xylose by using NADH and/or NADPH as coenzyme. The tetrad of amino acids (Tyr, His, Asp, and Lys) at catalytic site is responsible for XR activity. Several attempts have been made to improve XR production using recombinant DNA technology by introducing xylose reductase gene (xyl1) into different fungal strains from other microorganisms for efficient conversion of xylose to xylitol. Site-directed mutagenesis at the catalytic site is another approach to increase the turnover number and catalytic efficiency of XRs. Xylitol is a rare pentol sugar whose global market is increasing at a very fast pace due to its applications in food, cosmetic, odontological, pharmaceutical, and medical sector. The microbial production of xylitol is emerging as a good alternative due to abundance of agriculture waste material. The present chapter will describe the different aspects of fungal XRs including their structural characteristics, sources, production, purification and characterization, immobilization, patent status, and xylitol applications.