The flexible N-terminus of BchL protects its cluster in oxygenic environments and autoinhibits activity

The dark-operative protochlorophyllide oxidoreductase (DPOR) enzyme contains two [4Fe-4S]- containing component proteins (BchL and BchNB) that assemble in an ATP-dependent fashion to coordinate electron transfer and reduction of protochlorophyllide to chlorophyllide. Photosynthesis generates an oxygenic environment that is non-optimal for [Fe-S] clusters and we here present an elegant evolutionarily conserved mechanism in BchL to protect its [4Fe-4S] cluster. We present a crystal structure of BchL in the nucleotide-free form with an ordered N-terminus that shields the [4Fe-4S] cluster at the docking interface between BchL and BchNB. Amino acid substitutions that perturb the shielding of the [4Fe-4S] cluster produce an unstable, but hyper-active enzyme complex, suggesting a role for the N-terminus in both auto-inhibition and enzyme stability. Upon ATP binding, a patch of amino acids, Asp-Phe-Asp ( DFD patch), situated at the mouth of the BchL ATP-binding pocket promotes inter-subunit cross stabilization of the two subunits and relieves the auto-inhibition by the N-terminus. A linked BchL dimer with one defective ATP-binding site does not support substrate reduction, illustrating that nucleotide binding to both subunits is a prerequisite for the inter-subunit cross stabilization. We propose that ATP-binding produces a conformational compaction of the BchL homodimer leading to a release of the flexible N-terminus from blocking the [4Fe-4S] cluster and promotes complex formation with BchNB to drive electron transfer. The auto-inhibitive feature and release mechanism appear unique to DPOR and is not found in the structurally similar nitrogenase.