Crystal Structure of Mitochondrial Respiratory Membrane Protein Complex II

Abstract

The mitochondrial respiratory Complex II or succinate:ubiquinone oxidoreductase (SQR) is an integral membrane protein complex in both the tricarboxylic acid cycle and aerobic respiration. Here we report the first crystal structure of Complex II from porcine heart at 2.4 A resolution and its complex structure with inhibitors 3-nitropropionate and 2-thenoyltrifluoroacetone (TTFA) at 3.5 A resolution. Complex II is comprised of two hydrophilic proteins, flavoprotein (Fp) and iron-sulfur protein (Ip), and two transmembrane proteins (CybL and CybS), as well as prosthetic groups required for electron transfer from succinate to ubiquinone. The structure correlates the protein environments around prosthetic groups with their unique midpoint redox potentials. Two ubiquinone binding sites are discussed and elucidated by TTFA binding. The Complex II structure provides a bona fide model for study of the mitochondrial respiratory system and human mitochondrial diseases related to mutations in this complex.

α-Tocopheryl succinate induces apoptosis by targeting ubiquinone-binding sites in mitochondrial respiratory complex II

“…UbQ5 was chosen as it is of similar size to a-TOS ( Figure 5e) and contains a similar number of rotatable bonds (16 and 17, respectively). Figure 5a indicates that UbQ5 docked in Q P to a deeper position than that observed for the portion of UbQ resolved in the published crystal structure (Sun et al, 2005). UbQ5 was also found to dock into the proposed Q D site, with the UbQ ring positioned in front of the binding site and the hydrophobic tail located inside the site (Figure 5c).…”

“…To establish the relationship between inhibition of SDH and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, initial experiments with 3BP and 3NPA were performed in the absence of succinate that might otherwise compete for binding to the enzyme, and the results confirmed that 3BP and 3NPA inhibited CII-mediated MTT reduction and that SDH activity and MTT reduction were closely related ( Figure 2a). Thenoyltrifluoroacetone (TTFA) is an inhibitor of CII that blocks the succinate-dependent MTT reduction by targeting the UbQ-binding sites rather than the SDH enzyme catalytic site (Sun et al, 2005). The CII UbQ sites are situated beneath the active site of SDH, in the transmembrane region of the CII.…”

“…Also, data analysis from the published crystal structure only shows electron density for the very start of the isoprenoid chain suggesting that the rest of the tail is not particularly tightly bound at this site (Sun et al, 2005). Therefore, the expected interaction energies of UbQ10 for the two sites will be similar to those calculated for UbQ5.…”

“…MitoQ overcame both the TTFAand a-TOS-mediated inhibition of SDH activity. While TTFA binds to both Q P and Q D sites in CII (Sun et al, 2005) like a-TOS, it displays less selective toxicity ) than the VE analogue for killing cancer cells (Neuzil et al, 2001b). Further studies are required to understand this differential specificity.…”

Shaping the mitochondrion: mitochondrial biogenesis, dynamics and dysfunction

“…More recently, investigators have begun to identify some of the molecules required for mitophagy. (Sun et al, 2005). bc 1 , cytochrome c reductase; CcO, cytochrome c oxidase; Cyc, cytochrome c; Fe/S, iron/sulphur; SDH, succinate dehydrogenase.…”