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Original: "Methylotroph"

General microbiology

Methylotrophs are a diverse group, including both Gram-negative and Gram-positive genera. None of them make resting structures like exospores or cysts and none of them have the complex intracellular membrane systems that characterize methanotrophs growing on methane

There are two sub groups:

1. obligate methylotrophs.
2. facultative methylotrophs.

Obligate methylotrophs

A single obligate methylotroph (methylophilus) is known. It is Gram-negative, polarly flagellated rod capable of rapid growth with methanol. Some strains can also utilize formaldehyde or methylamines. Carbon is assimilated via the ribulose mono phosphate pathway.

Facultative methylotrophs

It is relatively widely distributed trait among heterotrophic bacteria. It may also be common among chemoautotrophs: several thiobacilli and nitrifying bacteria can drive CO₂ assimilation via the Calvin-Benson cycle by formate oxidation.

Edit: "Methylotroph"

Metabolism

The key intermediate in methylotrophic metabolism is formaldehyde, which can be diverted to either assimilatory or dissimilatory pathways. Methylotrophs produce formaldehyde through oxidation of methanol and/or methane. Methane oxidation requires the enzyme methane monooxygenase (MMO). Methylotrophs with this enzyme are given the name methanotrophs. The oxidation of methane (or methanol) can be assimilatory or dissimilatory in nature (See Figure 1). If dissimilatory, the formaldehyde intermediate is oxidized completely into ${\ce {CO2}}$ to produce reductant and energy. If assimilatory, the formaldehyde intermediate is used to synthesize a 3-Carbon (${\ce {C3}}$) compound for the production of biomass. Many methylotrophs use multi-carbon compounds for anabolism, thus limiting their use of formaldehyde to dissimilatory processes, however methanotrophs are generally limited to only ${\textstyle {\ce {C1}}}$metabolism.

Compounds known to support methylotrophic metabolism

Single Carbon Compounds	Chemical Formula	Multi-Carbon Compounds	Chemical Formula
Carbon monoxide	${\ce {CO}}$	Dimethyl ether	${\ce {(CH3)2O}}$
Formaldehyde	${\ce {CH2O}}$	Dimethylamine	${\ce {(CH3)2NH}}$
Formamide	${\ce {HCONH2}}$	Dimethyl sulfide	${\ce {(CH3)2S}}$
Formic acid	${\ce {HCOOH}}$	Tetramethylammonium	${\ce {(CH3)4N+}}$
Methane	${\ce {CH4}}$	Trimethylamine	${\ce {(CH3)3N}}$
Methanol	${\ce {CH3OH}}$	Trimethylamine N-oxide	${\ce {(CH3)3NO}}$
Methylamine	${\ce {CH3NH2}}$	Trimethylsuphonium	${\ce {(CH3)3S+}}$
Methyl halide	${\ce {CH3X}}$

Catabolism

Methylotrophs use the electron transport chain to conserve energy produced from the oxidation of ${\ce {C1}}$ compounds. An additional activation step is required in methanotrophic metabolism to allow degradation of chemically-stable methane. This oxidation to methanol is catalyzed by MMO, which incorporates one oxygen atom from ${\ce {O2}}$ into methane and reduces the other oxygen atom to water, requiring two equivalents of reducing power. Methanol is then oxidized to formaldehyde through the action of methanol dehydrogenase (MDH) in bacteria, or a non-specific alcohol oxidase in yeast. Electrons from methanol oxidation are passed to a membrane-associated quinone of the electron transport chain to produce ${\ce {ATP}}$.

In dissimilatory processes, formaldehyde is completely oxidized to ${\ce {CO2}}$ and excreted. Formaldehyde is oxidized to formate via the action of Formaldehyde dehydrogenase (FALDH), which provides electrons directly to a membrane associated quinone of the electron transport chain, usually cytochrome b or c. In the case of ${\ce {NAD+}}$ associated dehydrogenases, ${\ce {NADH}}$ is produced. 

Finally, formate is oxidized to ${\ce {CO2}}$ by cytoplasmic or membrane-bound Formate dehydrogenase (FDH), producing ${\ce {NADH}}$ and ${\ce {CO2}}$. 

Anabolism

The main metabolic challenge for methylotrophs is the assimilation of single carbon units into biomass. Through de novo synthesis, Methylotrophs must form carbon-carbon bonds between 1-Carbon (${\ce {C1}}$) molecules. This is an energy intensive process, which facultative methylotrophs avoid by using a range of larger organic compounds. However, obligate methylotrophs must assimilate ${\ce {C1}}$ molecules. There are four distinct assimilation pathways with the common theme of generating one ${\ce {C3}}$ molecule . Bacteria use three of these pathways while Fungi use one. All four pathways incorporate 3 ${\ce {C1}}$ molecules into multi-carbon intermediates, then cleave one intermediate into a new ${\ce {C3}}$ molecule. The remaining intermediates are rearranged to regenerate the original multi-carbon intermediates.

CodeSwitch (talk) 14:55, 2 November 2017 (UTC)

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