Glycerol metabolism in the methylotrophic yeast Hansenula polymorpha: phosphorylation as the initial step

Abstract

In hansenula polymorpha glycerol is metabolized via glycerol kinase and NAD(P)-independent glycerol-3-phosphate (G3P) dehydrogenase, enzymes which hitherto were reported to be absent in this methylotrophic yeast. Activity of glycerol kinase was readily detectable when cell-free extracts were incubated at pH 7–8 with glycerol/ATP/Mg²⁺ and a discontinuous assay for G3P formation was used. This glycerol kinase activity could be separated from dihydroxyacetone (DHA) kinase activity by ion exchange chromatography. Glycerol kinase showed relatively low affinities for glycerol (apparent K _m=1.0 mM) and ATP (apparent K _m=0.5 mM) and was not active with other substrates tested. No inhibition by fructose-1,6-bisphosphate (FBP) was observed. Both NAD-dependent and NAD(P)-independent G3P dehydrogenases were present. The latter enzyme could be assayed with PMS/MTT and cosedimented with the mitochondrial fraction. Glucose partly repressed synthesis of glycerol kinase and NAD(P)-independent G3P dehydrogenase, but compared to several other non-repressing carbon sources no clear induction of these enzymes by glycerol was apparent. Amongst glycerolnegative mutants of H. polymorpha strain 17B (a DHA kinase-negative mutant), strains blocked in either glycerol kinase or membrane-bound G3P dehydrogenase were identified. Crosses between representatives of the latter mutants and wild type resulted in the isolation of, amongst others, segregants which had regained DHA kinase but were still blocked in the membrane-bound G3P dehydrogenase. These strains, employing the oxidative pathway, were only able to grow very slowly in glycerol mineral medium.