Role of monooxygenase reaction during assimilation of non-growth substrates by methanotrophs

Methanotrophic bacteria utilize only methane (some species methanol) as a source of carbon and energy. They can co-metabolize a number of non-growth substrates due to the low specificity of methane monooxygenase (MMO). The reducer for monooxygenation is obtained during dehydrogenation of intermediates of methane oxidation. During growth on methane, co-metabolism of non-growth substrates leads to exhaustion of the methanotroph energy sources, accumulation of toxic products and growth cessation. To evaluate a role of the monooxygenation process in the metabolism of methanotrophs, co-metabolism of methanol and a substrate of MMO (ethane or carbon monoxide) was studied during the growth of Methylomonas rubra 15sh and Methylococcus thermophilus 111p. These organisms, being grown at low copper content, did not oxidize naphthalene. Thus, they possess particulate MMO, which can use reducing equivalents at the level of methanol dehydrogenase. Methanol did not support growth of Mc. thermophilus 111p. Growth of Mm. rubra 15sh on methanol was inefficient and was accompanied by accumulation of formaldehyde. When a second substrate (ethane or carbon monoxide) was added into the gas phase (5–20 vol.%), the growth of Mc. thermophilus 111p occurred, and the yield from methanol of Mm. rubra 15sh was enhanced. At low methanol concentration (to 12.5 mM), the yield of the Mm. rubra 15sh biomass from methanol was proportional to the amount of the second substrate co-oxidized, and the whole methanol utilized was converted into biomass. Theoretical calculations showed that such high efficiency of growth could be achieved when MMO received reducing equivalents from methanol dehydrogenase and, moreover, electron transport chain from NADH to MMO contained a proton translocating segment. Thus, monooxygenation of the MMO substrate was profitable for the methanotroph metabolism. During the growth of methanotrophs studied on methanol plus ethane or carbon monoxide, the major part of electrons of methanol utilized was used for the monooxygenation of the MMO substrate. For methanotrophs, this allows to consider the monooxygenation process as the energy yielding mechanism of the electron transport to oxygen.