Inner Secrets of the Respirasome

Abstract

Structure determination by cryo-electron microscopy has approached atomic resolution and helped solve structures of large membrane-protein complexes that resisted crystallography. The 4.0 Å cryo-EM structure of one of the most intricate enzyme systems, the respirasome, in the mitochondrial inner membrane is reported in this issue of Cell.

Figure 1. Catalytic and Putative Regulatory Mechanisms of the Respirasome

(A) The (I + III₂ + IV) respirasome couples the oxidation of NADH to reduction of molecular oxygen. CI (outlined in blue) oxidizes NADH and transfers two electrons through its FMN cofactor and seven of its eight Fe-S clusters to reduce ubiquinone (Q) to ubiquinol (QH₂). The dimeric CIII (outlined in green) oxidizes QH₂ and transfers one electron to cytochrome c via a heme and a Fe-S cluster and the second electron is used to reduce Q in the Q cycle of proton pumping. CIV (outlined in red) receives four electrons from successive cytochrome c transfers and electrons pass through a binuclear copper A center, a heme a moiety to a heterobimetallic copper B-heme a₃ center for the 4 electron reduction of molecular oxygen. IMM is the inner mitochondrial membrane, IMS is the intermembrane space and matrix is the space enclosed by the inner membrane. (B) This side view of the respirasome highlights additional CI cofactors. The ACP protein SDAP-β and the corresponding LYRM protein, NDUFB22 (A, purple/dark purple) and ACP protein SDAP-α and its corresponding LYRM protein, NDUFB14 (B, purple/magenta) are associated by a C:10 acyl chain on the phosphopantetheine (PPT) cofactor of each ACP. These ACP-LYRM dimers provide a potential regulatory link between fatty acid biosynthesis and respiration. A NADPH molecule bound to NDUFA9 (C, yellow) and zinc ion bound to NDUFS6 (D, orange) may act as redox sensors for CI function. (C) The IMS view of the respirasome shows subunits in all three complexes that contain stabilizing disulfide bonds. Redox changes in these subunits may modulate the respirasome function. The disulfide subunits include COX6B (E, red) in complex CIV, UQCRH (F, green) in CIII, and CI subunits in blue NDUFB7 (G), NDUFB10 (H), NDUFA11 (I), NDUFS5 (J) and NDUFA8 (K).