[NiFe]-Containing acetyl-CoA synthase and corrinoid-[FeS]-protein
The reductive acetyl-CoA pathway (Wood-Ljungdahl pathway) accounts for the production of about 1010 tonnes of acetic acid from CO2 per year and contains unique metalloenzymes like the [NiFe]-containing CODHs (see E2-1), [NiFe]-containing acetyl-CoA synthases (ACS) and the corrinoid-[FeS] protein (CoFeSP) (see Figure).
Within the last few years we have developed heterologous expression systems for CODH, ACS and CoFeSP and defined their active site structures using protein X-ray crystallography.[U24] ACS and CoFeSP together catalyse the reversible condensation of CO, CH3+ and coenzyme A to form acetyl-CoA. The reaction is analogous to the Monsanto and Cativa-processes, but in contrast to these largescale industrial processes does not depend on rare 4d-metals.
- A [NiNi]-[Fe4S4] cluster has been described as the active site of ACS, which provides the binding-site for two one-carbon units (Fig. E2-3a).[D28a] The order of substrate binding events and the arrangement of the respective substrates upon binding to the active site are as yet unknown. The reaction will be analysed at different levels of complexity from the ternary protein complex between CODH-ACS and CoFeSP down to a “minimal” model for ACS consisting of a single protein domain encapsulating the [NiNi]-[Fe4S4] cluster.
- This biosynthetic model of ACS offers the possibility of analysing the metal cluster structure at sufficient resolution to detect redox-dependent structural changes in the metal part and to define the binding sites for the substrates CO, CH3+ and CoASH crystallographically.
- The structural approach will be complemented by spectroscopic investigations and single-turnover kinetic studies including stopped-flow and rapid-freeze quenched flow experiments with XAS, EPR, RR/IR spectroscopic detection, as well as by theoretical studies.
- Further mechanistic details to be investigated concern the underlying redox state changes of the cofactors.
- Finally, we will address the mechanism of methyl-group transfer between CH3-Co(III)FeSP and the [NiNi]-[Fe4S4] cluster as it is a unique reaction step in biology.