In addition to acetyl-CoA production, the reverse can occur with ACS producing CO and returning the methyl piece back to the corrinoid protein.

[6] It has been discovered that the CODH/ACS enzyme in the bacteria M. theroaceticum can make dinitrogen (N2) from nitrous oxide in the presence of an electron-donating species.

Furthermore, they resolved the structure of the A-cluster active site and found an [Fe4S4]-X-Cu-X-Ni centre which is highly unusual in biology.

[7] The debate towards the absolute structure and identity of the metals in the A-cluster active site of ACS continued, with a competing model presented.

[4] A later review article attempted to reconcile the different observations of Mp and stated that this proximal position in the active site of ACS was prone to substitution and could contain any one of Cu, Zn and Ni.

[8] It is now generally accepted that the ACS active site (A-cluster) is a Ni-Ni metal centre with both nickels having a +2 oxidation state.

This active site is also responsible for the C-C and C-S bond formations in the product acetyl-CoA (and its reverse reaction).

It also appears to interact with a ferredoxin compound which may activate the subunit during the CO transferring process from CODH to ACS.

This channel is most likely to protect the carbon monoxide molecules from the outside environment of the enzyme and to increase efficiency of acetyl-CoA production.

With the "open" position, the active site rotates itself to interact with the CFeSP protein in the methyl transfer step of the Wood–Ljungdahl pathway.

These two configurations are opposite one another in that access to CO blocks off interaction with CFeSP, and when methylation occurs, the active site is buried and does not allow CO transfer.

[3] Both are similar in terms of the binding of substrates and the general steps, but differ in the oxidation state of the metal centre.

The nickel then binds to either carbon monoxide from CODH or the methyl group donated by the CFeSP protein in no particular order.

Furthermore, there is evidence of the ACS catalytic cycle without any external reducing complex, which refutes the ferrodoxin activation step.

[9] The order in which the carbon monoxide molecule and the methyl group bind to the nickel centre has been highly debated, but no solid evidence has demonstrated preference for one over the other.