Fig. 4.
Different catalytic forms of SDHA orthologs. (A) The catalytic form has a closed interdomain orientation that optimizes the active site for the interconversion of succinate and fumarate. It has been observed in structures of assembled bacterial and mitochondrial complex II (34, 44–46) and is exemplified here by assembled porcine complex II (PDB ID 3SFD) (34). (B) The flavinylation product form, is hallmarked by the rotation of the capping domain, which in turn alters the alignment of active site residues and disfavors succinate-fumarate conversion (22, 23). This form also has ready solvent access to an active site arginine (equivalent to SDHAR451) and contains an essential dicarboxylate cofactor bound in a position distinct from that observed in the catalytic form of the assembled enzyme. The flavinylation product form has been observed in the human SDHA-SDHAF2 assembly intermediate and the E. coli FrdA-SdhE assembly intermediate (PDB ID 6B58) (22) and is represented by the human SDHA-SDHAF2 assembly intermediate here. (C) The open form contains many of the hallmarks of the flavinylation product form, including a misaligned active site and solvent access to an active site arginine. The capping domain rotation is along a different axis and is of much larger magnitude, and this form lacks the dicarboxylate cofactor necessary for the covalent flavinylation reaction. To date, this form has only been observed in E. coli SDHA-SdhE assembly intermediate (PDB ID 6C12) (23), which is shown here. (D) The apo form, contains the largest interdomain rotation and lacks bound FAD, thus it is catalytically incompetent. The apo form has only been observed in the structure of the E. coli L-aspartate oxidase (PDB ID 1CHU) (58).