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In enzymology, a methanol dehydrogenase (MDH) is an enzyme that catalyzes the chemical reaction:

CH₃OH ${\displaystyle \rightleftharpoons }$ CH₂O + 2 electrons + 2H

How the electrons are captured and transported depends upon the kind of methanol dehydrogenase. There are three main types of MDHs: NAD-dependent MDH, pyrrolo-quinoline quinone dependent MDH, and oxygen-dependent alcohol oxidase.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD or NADP as acceptor. The systematic name of this enzyme class is methanol:NAD oxidoreductase. This enzyme participates in methane metabolism.

Classes of Methanol Dehydrogenase

NAD Dependent MDH

A common electron acceptor in biological systems is nicotinamide adenine dinucleotide (NAD); some enzymes use a related molecule called nicotinamide adenine dinucleotide phosphate (NADP). An NAD-dependent methanol dehydrogenase(EC 1.1.1.244) was first reported in a Gram-positive methylotroph and is an enzyme that catalyzes the chemical reaction:

CH₃OH + NAD ${\displaystyle \rightleftharpoons }$ CH₂O + NADH + H

Thus, the two substrates of this enzyme are methanol and NAD, whereas its 3 products are formaldehyde (CH₂O), NADH, and H. This can be performed under both aerobic and anaerobic conditions.

NAD -dependent MDHs are found in thermophilic, Gram positive methylotrophs, but can also been obtained from some non-methylotrophic bacteria. NAD-dependent MDHs have so far been found in Bacillus sp., Lysinibacillus sp.,and Cupriavidus sp.

PQQ-Dependent MDH

For Gram-negative bacteria, methanol oxidation occurs in the periplasmic space, facilitated by PQQ-dependent MDH. PQQ-dependent MDHs contain a PQQ prosthetic group, which has the role of capturing electrons from methanol oxidation and passing them to the cytochrome.

MxaFI and XoxF are the genes that encode for PQQ-dependent MDHs. In MxaFI-type MDH, calcium (Ca) is encoded as the cofactor for PQQ-dependent methylotrophy. XoxF-type MDHs use lanthanides (Ln) as cofactors and are highly selective towards early lanthanides (typically La-Nd). Sm, Eu, and Gd can support some XoxF-type organisms, but less effectively. Pm and Tb-Lu have shown no evidence of utilization so far.

Many methylotrophs encode both MxaFI and XoxF, but those that encode only one will encode exclusively for XoxF.

O₂-Dependent Alcohol Oxidase

Oxygen-dependent alcohol oxidase (AOX) can be obtained from eukaryotic methylotrophs in the peroxisome of yeasts. Formaldehyde and hydrogen peroxide (H₂O₂) are formed through the oxidation of methanol. Dihydroxyacetone synthase (DAS) and catalase (CTA) must then transform these toxic chemicals into non-toxic forms to protect the cell. In this process, electrons from methanol are not captured as usable energy by the cell, and are thus lost.

References

1. Deng YW, Ro SY, Rosenzweig AC (October 2018). "Structure and function of the lanthanide-dependent methanol dehydrogenase XoxF from the methanotroph Methylomicrobium buryatense 5GB1C". Journal of Biological Inorganic Chemistry. 23 (7): 1037–1047. doi:10.1007/s00775-018-1604-2. PMC 6370294. PMID 30132076.
2. ^ Le TK, Lee YJ, Han GH, Yeom SJ (2021). "Methanol Dehydrogenases as a Key Biocatalysts for Synthetic Methylotrophy". Frontiers in Bioengineering and Biotechnology. 9: 787791. doi:10.3389/fbioe.2021.787791. PMC 8741260. PMID 35004648.
3. Arfman N, Watling EM, Clement W, van Oosterwijk RJ, de Vries GE, Harder W, et al. (1989). "Methanol metabolism in thermotolerant methylotrophic Bacillus strains involving a novel catabolic NAD-dependent methanol dehydrogenase as a key enzyme". Archives of Microbiology. 152 (3): 280–288. doi:10.1007/BF00409664. PMID 2673121.
4. Chistoserdova L, Kalyuzhnaya MG (August 2018). "Current Trends in Methylotrophy". Trends in Microbiology. 26 (8): 703–714. doi:10.1016/j.tim.2018.01.011. PMID 29471983.

Further reading

• Arfman N, Watling EM, Clement W, van Oosterwijk RJ, de Vries GE, Harder W, et al. (1989). "Methanol metabolism in thermotolerant methylotrophic Bacillus strains involving a novel catabolic NAD-dependent methanol dehydrogenase as a key enzyme". Archives of Microbiology. 152 (3): 280–288. doi:10.1007/BF00409664. PMID 2673121.

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