Autotrophic CO2 Fixation Pathway in Methanobacterium thermoautotrophicum

Abstract

The autotrophic methanogenic bacterium Methanobacterium thermoautotrophicum assimilates CO₂ via a novel CO₂ fixation pathway rather than via the Reductive Pentosephosphate Cycle (Calvin Cycle). The first known fixation product of this pathway is acetyl CoA which is the starting compound for the synthesis of the carbon skeleton of all cell compounds except for one-carbon units. This central intermediate appears to be synthesized from 2 CO₂ via bound one-carbon units rather than via cleavage of a compound which itself is synthesized from acetyl CoA and CO₂. Further CO₂ fixation proceeds via the reductive carboxylation of acetyl CoA to pyruvate, the carboxylation of phosphoenolpyruvate to oxaloacetate, and the reductive carboxylation of succinyl CoA to α-ketoglutarate. Triosephosphate and hexosephosphate synthesis proceeds from acetyl CoA and CO₂ via pyruvate and phosphoenolpyruvate.

This contribution describes the outlines of the proposed new pathway and summarizes the experimental evidence on which it is based.