A D-galacturonic acid reductase and the corresponding gene were identified from the mold Hypocrea jecorina (Trichoderma reesei). We hypothesize that the enzyme is part of a fungal D-galacturonic acid catabolic pathway which has not been described previously and which is distinctly different from the bacterial pathway. H. jecorina grown on D-galacturonic acid exhibits an NADPH-dependent D-galacturonic acid reductase activity. This activity is absent when the mold is grown on other carbon sources. The D-galacturonic acid reductase was purified, and tryptic digests of the purified protein were sequenced. The open reading frame of the corresponding gene was then cloned from a cDNA library. The open reading frame was functionally expressed in the yeast Saccharomyces cerevisiae. A histidine-tagged protein was purified, and the enzyme kinetics were characterized. The enzyme converts in a reversible reaction from D-galacturonic acid and NADPH to L-galactonic acid and NADP. The enzyme also exhibits activity with D-glucuronic acid and DL-glyceraldehyde.
- Hypocrea jecorina
- Saccharomyces cerevisiae