ABSTRACT
The extensive range of biotransformation catalyzed by aerobic C1 utilizing (methane, methanol) organisms, methanotrophs, has opened up the possibility of their exploitation for biocatalysis. The broad substrate specificity of enzymes involved in the oxidation of methane is catalytically useful for the production of chemicals. The epoxidation of alkenes (C2 - C5) to produce ethylene oxide, propylene oxide, the hyroxylation of alkanes (C1- C8) to produce alcohols, and the production of methylketones such as acetone and 2-butanone from their corresponding n-alkanes (propane, butane) or secondary alcohols (2-propanol, 2-butanol) are industrially important biotransformations catalyzed by methanotrophs. Soluble methane monooxygenase from a methane-utilizing organism, Methylobacterium sp. CRL-26, catalyzed the NAD(P)H-dependent epoxidation/hydroxylation of a variety of hydrocarbons, including terminal alkenes, internal alkenes, substituted alkenes, branch-chain alkenes, alkanes, carbon monoxide, ethers, cyclic, and aromatic compounds. The NAD+ linked dehydrogenases such as formate dehydrogenase or secondary alcohol dehydrogenase in the presence of formate or secondary alcohol, respectively, regenerated NAD/NADH required for the methane monooxygenase in a coupled enzyme reactions. 328Methane monooxygenase was resolved into two components, a hydroxylase and a flavoprotein. In contrast to methane-utilizing bacteria, other hydrocarbon-utilizing bacteria (ethane, propane, butane) catalyzed the epoxidation of alkenes and the hydroxylation of alkanes, except the oxidation of methane, indicating that the oxygenase enzyme system from these microorganisms is different from that of methane monooxygenase. Oxidation of secondary alcohols to the corresponding methylketones in methanotrophs is catalyzed by an NAD+-dependent, zinc-containing, secondary alcohol dehydrogenase. Primary alcohols were oxidized to the corresponding aldehydes by a phenazine methosulfate-dependent, pyrollo quinoline quinone (methoxatin or PQQ) containing methanol dehydrogenase in methanotrophs.
