Classical transketolase functions as the formaldehyde‐assimilating enzyme during growth of a dihydroxyacetone synthase‐negative mutant of the methylotrophic yeast Hansenula polymorpha on mixtures of xylose and methanol in continuous cultures

Abstract

AbstractContrary to expectation, a mutant of Hansenula polymorpha blocked in dihydroxyacetone (DHA) synthase was able to assimilate methanol‐carbon when grown in chemostat culture on mixtures of xylose and methanol. Incubation of a DHA synthase‐ and DHA kinase‐negative double mutant resulted in DHA accumulation, indicating that a DHA synthase‐type of reaction was involved. Low residual DHA synthase activity subsequently was shown to be present when using an assay with improved sensitivity. This activity was not associated with the (mutated) DHA synthase protein, which was still present in the peroxisomes, but with the enzyme transketolase. Transketolase from methanol grown cells was purified (525‐fold) to homogeneity in 9% yield. The native enzyme was dimeric, as has been reported fro other transketolases, with a subunit molecular weight of 74000. The affinity of the purified enzyme for formaldehyde was low (Km = 5 mM), but high for xylulose‐5‐phosphate (ca. 10 μM). The in vivo functioning of transketolase in formaldehyde assimilation, and the influence of the hydration state of formaldehyde is discussed.