Properties of a pentulose-5-phosphate phosphoketolase from yeasts grown on xylose

Summary

Phosphoketolase activity from nine yeasts grown on xylose occurred with both xylulose 5-phosphate (Xu5P) and ribulose 5-phosphate (Ru5P) as substrates. With extracts from five yeasts (Candida curvata, C. famata, Lipomyces starkeyi, Rhodotorula glutinis and Pachysolen tannophilus) activity was approximately the same with either substrate; with C. boidinii, Pichia media and Yarrowia lipolytica Ru5P was the preferred substrate; and with Rhodosporidium toruloides Xu5P was the better substrate. Partial purification of the phosphoketolase from C. famata was attempted: although activity of phosphoketolase towards Ru5P was decreased it was not eliminated and it is concluded that either (i) the phosphoketolase does have dual substrate specificity, in which case it should be referred to as a pentulose-5-phosphate phosphoketolase (Pu5PPK) or (ii) Ru5P-3-epimerase activity, which can interconvert Xu5P and Ru5P, may be closely associated with phosphoketolase activity. The Pu5PPK has a $K_{\hat m}$ of 2.4mm for Xu5P, a pH optimum of 7.2–7.4 and a M _r of 5x10⁵ daltons. It is not sensitive to inhibition by citrate or acetyl-CoA at physiological concentrations.