Structural Basis for Malfunction in Complex II

Abstract

Complex II couples oxidoreduction of succinate and fumarate at one active site with that of quinol/quinone at a second distinct active site over 40 Å away. This process links the Krebs cycle to oxidative phosphorylation and ATP synthesis. The pathogenic mutation or inhibition of human complex II or its assembly factors is often associated with neurodegeneration or tumor formation in tissues derived from the neural crest. This brief overview of complex II correlates the clinical presentations of a large number of symptom-associated alterations in human complex II activity and assembly with the biochemical manifestations of similar alterations in the complex II homologs from Escherichia coli. These analyses provide clues to the molecular basis for diseases associated with aberrant complex II function.Over the past 6 decades, complex II (succinate dehydrogenase, succinate:quinone oxidoreductase (SQR) 3 ) has been at the forefront in the discovery of redox cofactors involved in bioenergetics. Helmut Beinert noted that in the 1950s-1960s, complex II was instrumental in the discovery of covalently bound flavin, tightly bound iron, and acid-labile sulfur associated with Fe-S clusters and bound ubiquinone of the mitochondrial respiratory chain (1). In 1995, a mutation of human complex II was the first report of a nuclear gene mutation shown to cause a mitochondrial respiratory chain deficiency (2). More recently, germ-line mutations in complex II have been found to be associated with hereditary tumors, suggesting that complex II genes may act as tumor suppressors (3).Complex II enzymes ( Fig. 1A) are heterotetramers containing two soluble subunits (SdhA (flavoprotein) and SdhB (Fe-S protein)) and two integral membrane subunits (SdhC and SdhD). This architecture houses two distinct active sites, and the coordinated catalysis at these sites links two key biological pathways, i.e. succinate oxidation to the Krebs cycle and quinone reduction to the electron transport chain. The SdhA subunit harbors a covalently attached FAD redox moiety and the dicarboxylate-binding site (Fig. 1B), where succinate is oxidized to fumarate. The product protons of this oxidation are transferred to bulk solvent, and fumarate acts as the next substrate in the Krebs cycle. The product electrons are transferred over 40 Å via three Fe-S clusters in the SdhB protein. These electrons act as co-substrates at the second active site, located at the interface of the integral membrane subunits. At this quinonereducing site (Fig. 1C), 2H ϩ and 2eϪ reduce ubiquinone to ubiquinol. The resultant quinol pool supports ATP synthesis by oxidative phosphorylation.The complex II superfamily contains both SQRs and quinol: fumarate reductases (QFRs). QFRs are enzymes that are kinetically poised to catalyze quinol oxidation and fumarate reduction as part of anaerobic electron transport chains. Both enzymes have evolved to function optimally in the physiological niche in which they are expressed. SQR and QFR are capable of functionally replacing each other in vivo in E...

Structural and functional consequences of succinate dehydrogenase subunit B mutations

“…SDHB mutants p.Cys101Tyr and p.Arg242His were both associated with significantly reduced SDH activity when compared with the WT. Using our in silico results we predicted loss of bonding to the 2Fe-2S centre at p.Cys101, which is more tolerant to amino acid substitution than the other two (4Fe-4S), (3Fe-4S) domains of SDHB (Werth et al 1990, Iverson et al 2012. Mutation at p.Arg242 was predicted to disrupt hydrogen bonds between SDH subunits, thereby reducing their biochemical efficiency and perhaps could also explain the loss of function in an otherwise fully compartmentalised SDH complex.…”

Catalytic mechanisms of complex II enzymes: A structural perspective

“…Despite the sequence and cofactor differences of the membrane-spanning domain, at a global level the members of these subfamilies appear structurally similar (Fig. 1), and the membrane-spanning regions between subfamily B and subfamily C have surprisingly high structural similarity (for example, an RMS deviation of 1.7 Å between W. succinogenes FrdC and avian SdhC/D over 130 C α atoms) despite a lack of detectable sequence identity when using automated sequence alignments [11]. …”

“…Indeed, the architecture immediately surrounding the b -type heme and Q-site (Fig. 4B,C, Table 3) are very strongly conserved across the structurally characterized enzymes in subfamily C [11]. …”

“…The mutation of the amino acid equivalent in position to Gly-A464, which is located between the FAD-binding and capping-domains and is predicted to influence interdomain dynamics [11], is associated with a debilitating neurodegenerative disease known as Leigh’s syndrome in humans [48]. The patient with this substitution had reduced succinate oxidase activity in fibroblasts suggesting that altering the interdomain association influences enzyme turnover.…”

Molecular mechanisms of superoxide production by complex III: A bacterial versus human mitochondrial comparative case study

“…1). More recently, complex II (CII) is implicated in this process as well [27, 28]. It has been suggested that about 2–5% of O 2 consumed might lead to O 2 .− generation, of which roughly 70–80% is linked to the mechanism of function of CIII [29] based on quantitative data obtained using isolated mitochondria.…”