Stearoyl-acyl carrier protein desaturase (Δ9D) catalyzes the O₂ and 2e^– dependent desaturation of stearoyl-acyl carrier protein (18:0–ACP) to yield oleoyl-ACP (18:1–ACP). The 2e^– are provided by essential interactions with reduced plant-type [2Fe–2S] ferredoxin (Fd). This presentation is on the protein-protein interfaces used for complex formation as revealed by chemical cross-linking, site-directed mutagenesis, steady-state kinetic approaches and molecular docking studies. Crosslinking revealed that carboxylate residues from Fd and lysine residues from Δ9D contribute to the protein interface while mutagenesis strongly implicated the triad of K56, K60 and K230 of Δ9D in formation of the complex with Fd. Molecular docking studies indicated that interactions between K56 and K60 and carboxylate groups on Fd may situate the [2Fe-2S] cluster of Fd near to W62, a surface residue that is structurally conserved in both ribonucleotide reductase and mycobacterial putative acyl-ACP desaturase DesA2. Owing to the considerably larger effects on catalysis, K230 appears to have other contributions arising from its positioning in helix–7 and its close sequential location to the diiron center ligands E229 and H232. These results are considered in the light of the presently available models for Fd-mediated electron transfer in Δ9D and other protein-protein complexes.

This work funded by NIH GM50853.