Identification and characterization of d ‐xylulokinase from the d ‐xylose‐fermenting fungus, Mucor circinelloides

d ‐Xylulokinase catalyzes the phosphorylation of d ‐xylulose in the final step of the pentose catabolic pathway to form d ‐xylulose‐5‐phosphate. The d ‐xylulokinase activity was found to be induced by both d ‐xylose and l ‐arabinose, as well as some of the other enzymes involved in the pentose catabolism, in the d ‐xylose‐fermenting zygomycetous fungus, Mucor circinelloides NBRC 4572. The putative gene, xyl3 , which may encode d ‐xylulokinase, was detected in the genome sequence of this strain. The amino acid sequence deduced from the gene was more similar to d ‐xylulokinases from an animal origin than from other fungi. The recombinant enzyme was purified from the E. coli transformant expressing xyl3 and then characterized. The ATP‐dependent phosphorylative activity of the enzyme was the highest toward d ‐xylulose. Its kinetic parameters were determined as K m ( d ‐xylulose) = 0.29 mM and K m (ATP) = 0.51 mM, indicating that the xyl3 gene encoded d ‐xylulokinase (McXK). Western blot analysis revealed that McXK was induced by l ‐arabinose as well as d ‐xylose and the induction was repressed in the presence of d ‐glucose, suggesting that the enzyme may be involved in the catabolism of d ‐xylose and l ‐arabinose and is subject to carbon catabolite repression in this fungus. This is the first study on d ‐xylulokinase from zygomycetous fungi.