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. Author manuscript; available in PMC: 2018 Oct 16.
Published in final edited form as: Nat Chem. 2018 Apr 16;10(5):568–572. doi: 10.1038/s41557-018-0029-4

Figure 1. Assembly of the M-cluster.

Figure 1

(a) The assembly of M-cluster involves SAM-dependent conversion of K-cluster to L-cluster on NifB, transfer of L-cluster to NifEN, maturation of L-cluster on NifEN upon insertion of Mo and homocitrate (HC) by NifH, and transfer of the resultant M-cluster to NifDK. (b) Coupling of the 4Fe units of K-cluster (2x[Fe4S4]) into an 8Fe L-cluster ([Fe8S9C]) concomitant with insertion of an interstitial carbide and a ‘9th sulfur’, followed by conversion of the L-cluster to a mature M-cluster ([MoFe7S9C]) via insertion of Mo and HC. Carbide insertion begins with methyl transfer from SAM to the K-cluster and hydrogen abstraction from the SAM-derived methyl group by 5′-dA• and continues with further deprotonation/dehydrogenation of the carbon-radical until a carbide appears in the center of the L-cluster. c. The [Fe4S4]Syn cluster ([Fe4S4(SCH2CH2OH)4]2–) with ligands that are exchangeable for protein-bound cysteines. The hydrogen atoms are omitted for clarity. Color code: Fe, orange; S, yellow; Mo, cyan; C, gray; N, blue; O, red.