The structure and function of yeast xylose (aldose) reductases

Yeast xylose (aldose) reductases are members of the aldo-keto reductase family of enzymes which are widely distributed in a variety of other organisms. In yeasts, these enzymes catalyse the first step of xylose metabolism where xylose is converted to xylitol. In the past 16 years, xylose reductases from yeasts able to ferment or utilize xylose have been isolated and studied mainly because of their importance in xylose bioconversions. In recent years, genes encoding xylose reductases from several yeasts have been cloned and sequenced. A comparison of the primary sequences of yeast xylose reductases with the much better characterized human aldose reductase and human aldehyde reductase reveals that the yeast enzymes are hybrids between aldo-keto reductases and the short chain dehydrogenases/reductases families of enzymes. Why this is so and its evolutionary significance is presently not known. This short review will critically examine the structure and function information that can be gleaned from the sequence comparison. Several interesting questions arise from the sequence comparison and these can provide fruitful areas for further investigations.