Figure 1. Proposed carbide insertion pathway.

It begins with the transfer of a methyl group from one equivalent of SAM to the K-cluster (I) and abstraction of a hydrogen atom from the methyl group by a 5-dA· radical that is generated upon homolytic cleavage of a second equivalent of SAM (II). The process continues with further dehydrogenation/deprotonation of the methylene radical concomitant with its incorporation into the L-cluster as an interstitial carbide atom (III). The L-cluster is further matured into an M-cluster on insertion of Mo and homocitrate (IV). The Nif proteins hosting events I–IV during the assembly of M-cluster are indicated in the figure. Atoms of the clusters are coloured as follows: Fe, orange; S, yellow; C, gray; Mo, cyan. HC, homocitrate.