Public defence, Biochemistry, MSc Maxime Laird

Title of the thesis: Probing methanogenesis enzymes for C2-substrate reactivity - Towards understanding the metabolism of ethane-oxidising archaea

Thesis defender: Maxime Laird
Opponent:  Prof. Antonio J. Pierik, RPTU Kaiserslautern-Landau, Germany
Custos: Prof. Silvan Scheller, Aalto University School of Chemical Engineering

The study focused on enzymes from methanogens and the ethane oxidiser Candidatus Ethanoperedens thermophilum, and on their ability to process two-carbon (C2) compounds. The purpose was to address how E. thermophilum metabolises ethane by testing the hypothesis that this process relies on a single-carbon (C1)-processing machinery, well characterised in methanogens, in which enzymes have an extended substrate range for C2 compounds. This work is highly relevant from a fundamental perspective, as it challenges the prevailing view that methanogenesis enzymes are restricted to processing C1 units and addresses the currently unknown metabolic conversions linking the ethyl group derived from ethane to a conventional C1-processing pathway.

In this study, tetrahydromethanopterin (H4MPT), the carbon carrier that enables C1 oxidoreductions by methanogenesis enzymes, was shown to be chemically competent in carrying C2 groups, with higher efficiency than its more widespread analogue tetrahydrofolate. Furthermore, our results suggest that E. thermophilum expresses a methylene-H4MPT reductase (Mer) and a methyl-H4MPT:coenzyme M methyltransferase (Mtr), canonical C1 enzymes, that can catalyse C2 conversions. For a better thermodynamic understanding of alkane oxidation pathways, we also re-estimated the standard redox potential of the heterodisulfide of coenzyme B and coenzyme M, which plays a central role in methanogens and anaerobic alkane oxidisers.

Taken together, these results support a plausible new hypothesis for the missing steps in the ethane metabolism of E. thermophilum, involving a C1-analogous, H4MPT-mediated C2 pathway with acetaldehyde as an intermediate. Beyond its fundamental contribution, this work suggests that the enzymatic reactions investigated may be extended from C1 to multi-carbon substrates and provides new perspectives for the development of biosynthetic applications, including the design of alkanogenesis pathways towards bio-based hydrocarbon production. 

Keywords: tetrahydromethanopterin, ethane, methyltransferase, substrate promiscuity, 
oxidoreductase, Ethanoperedens thermophilum, Methanothermobacter marburgensis

Contact information: 
maximelaird@gmail.com 
https://www.linkedin.com/in/maxime-laird/ 

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.