Enzymological aspects of the pathways for trimethylamine oxidation and C1 assimilation of obligate methylotrophs and restricted facultative methylotrophs

Oxidation of C1-compounds in Pseudomonas C

Extracts of Pseudomonas C grown on methanol as a sole carbon and energy source contain a methanol dehydrogenase activity which can be coupled to phenazine methosulfate. This enzyme catalyzes two reactions namely the conversion of methanol to formaldehyde (phenazine methosulfate coupled) and the oxidation of formaldehyde to formate (2,6-dichloroindophenol-coupled). Activities of glutathione-dependent formaldehyde dehydrogenase (NAD+) and formate dehydrogenase (NAD+) were also detected in the extracts. The addition of D-ribulose 5-phosphate to the reaction mixtures caused a marked increase in the formaldehyde-dependent reduction of NAD+ or NADP+. In addition, the oxidation of [14C]formaldehyde to CO2, by extracts of Pseudomonas C, increased when D-ribulose 5-phosphate was present in the assay mixtures. The amount of radioactivity found in CO2, was 6;8-times higher when extracts of methanol-grown Pseudomonas C were incubated for a short period of time with [1-14C]glucose 6-phosphate than with [U-14C]glucose 6-phosphate. These data, and the presence of high specific activities of hexulose phosphate synthase, phosphoglucoisomerase, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase indicate that in methanol-grown Pseudomonas C, formaldehyde carbon is oxidized to CO2 both via a cyclic pathway which includes the enzymes mentioned and via formate as an oxidation intermediate, with the former predominant.

Solubilization, partial purification and properties of N-methylglutamate dehydrogenase from Pseudomonas aminovorans

1. Extracts of amine-grown Pseudomonas aminovorans contained a particle-bound N-methylglutamate dehydrogenase (EC 1.5.99.5). The enzyme was not present in succinate-grown cells, and activity appeared before growth began in succinate-grown cells which had been transferred to methylamine growth medium. 2. Membrane-containing preparations from methylamine-grown cells catalysed an N-methylglutamate-dependent uptake of O2 or reduction of cytochrome c, which was sensitive to inhibitors of the electron-transport chain. 3. N-Methylglutamate dehydrogenase activity with phenazine methosulphate or 2,6-dichlorophenol-indophenol as electron acceptor could be solubilized with 1% (w/v) Triton X-100. The solubilized enzyme was much less active with cytochrome c as electron acceptor and did not sediment in 1 h at 150000g. Solubilization was accompanied by a change in the pH optimum for activity. 4. The solubilized enzyme was partially purified by Sepharose 4B and hydroxyapatite chromatograpy to yield a preparation 22-fold increased in specific activity over the crude extract. 5. The partially-purified enzyme was active with sarcosine, N-methylalanine and N-methylaspartate as well as with N-methylglutamate. Evidence suggesting activity with N-methyl D-amino acids as well as with the L-forms was obtained. 6. The enzyme was inhibited by p-chloromercuribenzoate, iodoacetamide and by both ionic and non-ionic detergents. 2-Oxoglutarate and formaldehyde were also inhibitors. 7. Kinetic analysis confirmed previous workers' observations of a group transfer (Ping Pong) mechanism. 8. Spectral observations suggested that the partially purified preparation contained flavoprotein and a b-type cytochrome. 9. The role of the enzyme in the oxidation of methylamine is discussed.

S-formylgluthathione: the substrate for formate dehydrogenase in methanol-utilizing yeasts

Formaldehyde dehydrogenase and formate dehydrogenase were purified 45- and 16-fold, respectively, from Hansenula polymorpha grown on methanol. Formaldehyde dehydrogenase was strictly dependent on NAD and glutathione for activity. The Km values of the enzyme were found to be 0.18 mM for glutathione, 0.21 mM for formaldehyde and 0.15 mM for NAD. The enzyme catalyzed the glutathione-dependent oxidation of formaldehyde to S-formylglutathione. The reaction was shown to be reversible: at pH 8.0 a Km of 1 mM for S-formylglutathione was estimated for the reduction of the thiol ester with NADH. The enzyme did not catalyze the reduction of formate with NADH. The NAD-dependent formate dehydrogenase of H. polymorpha showed a low affinity for formate (Km of 40 mM) but a relatively high affinity for S-formylglutathione (Km of 1.1 mM). The Km values of formate dehydrogenase in cell-free extracts of methanol-grown Candida biodinii and Pichia pinus for S-formylglutathione were also an order of magnitude lower than those for formate. It is concluded that S-formylglutathione rather than free formate is an intermediate in the oxidation of methanol by yeasts.

Tricarboxylic acid-cycle and related enzymes in restricted facultative methylotrophs

The isolation is described of pure cultures of three non-methane-utilizing methylotrophic bacteria which, together with the previously described Bacillus PM6, have a very limited range of growth substrates; these organisms are designated "restricted facultative' methylotrophs. Two of these isolates, W6A and W3A1, grow only on glucose out of 50 non-C1 compounds tested, whereas the third isolate S2A1 and Bacillus PM6 grow on betaine, glucose, gluconate, alanine, glutamate, citrate and nutrient agar, but not on any of a further 56 non-C1 compounds. Crude sonic extracts of trimethylamine-grown and glucose-grown W6A and W3A1 isolates, and of trimethylamine-grown C2A1 (an obligate methylotroph) contain (i) no detectable 2-oxogltarate dehydrogenase activity, (ii) very low or zero specific activities of succinate dehydrogenase and succinyl-CoA synthetase and (iii) NAD+-dependent isocitrate dehydrogenase activity. Extracts of trimethylamine-grown PM6 and S2A1 methylotrophs have (i) very low 2-oxoglutarate dehydrogenase specific activities, (ii) comparatively high specific activities of succinate dehydrogenase, malate dehydrogenase and succinyl-CoA synthetase and (iii) NADP+-dependent isocitrate dehydrogenase activity but no NAD+-dependent isocitrate dehydrogenase activity. The activities of most of these enzymes are increased during growth on glucose, alanine, glutamate or citrate, but only very low 2-oxoglutarate dehydrogenase activities are present under all growth conditions. The restricted facultative methylotrophs grow on certain non-C1 compounds in the absence of 2-oxoglutarate dehydrogenase and, in some cases, of other enzymes of the tricarboxylic acid cycle; these lesions cannot therefore be the sole cause of obligate methylotrophy.