The respiratory complex I transduces redox energy into an electrochemical proton gradient in aerobic respiratory chains, powering energy-requiring processes in the cell. However, despite recently resolved molecular structures, the mechanism of this gigantic enzyme remains poorly understood. By combining large-scale quantum and classical simulations with site-directed mutagenesis and biophysical experiments, we show here how the conformational state of buried ion-pairs and water molecules control the protonation dynamics in the membrane domain of complex I and establish evolutionary conserved long-range coupling elements. We suggest that an electrostatic wave propagates in forward and reverse directions across the 200 Å long membrane domain during enzyme turnover, without significant dissipation of energy. Our findings demonstrate molecular principles that enable efficient long-range proton–electron coupling (PCET) and how perturbation of this PCET machinery may lead to development of mitochondrial disease.

NADH:ubiquinone oxidoreductase (respiratory complex I) plays a major role in energy metabolism by coupling electron transfer from NADH to quinone with proton translocation across the membrane. Complex I deficiencies were found to be the most common source of human mitochondrial dysfunction that manifest in a wide variety of neurodegenerative diseases. Seven subunits of human complex I are encoded by mitochondrial DNA (mtDNA) that carry an unexpectedly large number of mutations discovered in mitochondria from patients’ tissues. However, whether or how these genetic aberrations affect complex I at a molecular level is unknown. Here, we used Escherichia coli as a model system to biochemically characterize two mutations that were found in mtDNA of patients. The V253AMT-ND5 mutation completely disturbed the assembly of complex I, while the mutation D199GMT-ND1 led to the assembly of a stable complex capable to catalyze redox-driven proton translocation. However, the latter mutation perturbs quinone reduction leading to a diminished activity. D199MT-ND1 is part of a cluster of charged amino acid residues that are suggested to be important for efficient coupling of quinone reduction and proton translocation. A mechanism considering the role of D199MT-ND1 for energy conservation in complex I is discussed.

IntroductionDefects in the DNA mismatch repair (MMR) protein MLH1 are frequently observed in sporadic and hereditary colorectal cancers (CRC). Affected tumors generate much less metastatic potential than the MLH1 proficient forms. Although MLH1 has been shown to be not only involved in postreplicative MMR but also in several MMR independent processes like cytoskeletal organization, the connection between MLH1 and metastasis remains unclear. We recently identified non-erythroid spectrin αII (SPTAN1), a scaffolding protein involved in cell adhesion and motility, to interact with MLH1. In the current study, the interaction of MLH1 and SPTAN1 and its potential consequences for CRC metastasis was evaluated.MethodsNine cancer cell lines as well as fresh and paraffin embedded colon cancer tissue from 12 patients were used in gene expression studies of SPTAN1 and MLH1. Co-expression of SPTAN1 and MLH1 was analyzed by siRNA knock down of MLH1 in HeLa, HEK293, MLH1 positive HCT116, SW480 and LoVo cells. Effects on cellular motility were determined in MLH1 deficient HCT116 and MLH1 deficient HEK293T compared to their MLH1 proficient sister cells, respectively.ResultsMLH1 deficiency is clearly associated with SPTAN1 reduction. Moreover, siRNA knock down of MLH1 decreased the mRNA level of SPTAN1 in HeLa, HEK293 as well as in MLH1 positive HCT116 cells, which indicates a co-expression of SPTAN1 by MLH1. In addition, cellular motility of MLH1 deficient HCT116 and MLH1 deficient HEK293T cells was impaired compared to the MLH1 proficient sister clones. Consequently, overexpression of SPTAN1 increased migration of MLH1 deficient cells while knock down of SPTAN1 decreased cellular mobility of MLH1 proficient cells, indicating SPTAN1-dependent migration ability.ConclusionsThese data suggest that SPTAN1 levels decreased in concordance with MLH1 reduction and impaired cellular mobility in MLH1 deficient colon cancer cells. Therefore, aggressiveness of MLH1-positive CRC might be related to SPTAN1.

Summary The NADH:ubiquinone oxidoreductase (respiratory complex I) is the main entry point for electrons into the Escherichia coli aerobic respiratory chain. With its sophisticated setup of 13 different subunits and 10 cofactors, it is anticipated that various chaperones are needed for its proper maturation. However, very little is known about the assembly of E. coli complex I, especially concerning the incorporation of the iron‐sulfur clusters. To identify iron‐sulfur cluster carrier proteins possibly involved in the process, we generated knockout strains of NfuA, BolA, YajL, Mrp, GrxD and IbaG that have been reported either to be involved in the maturation of mitochondrial complex I or to exert influence on the clusters of bacterial complex. We determined the NADH and succinate oxidase activities of membranes from the mutant strains to monitor the specificity of the individual mutations for complex I. The deletion of NfuA, BolA and Mrp led to a decreased stability and partially disturbed assembly of the complex as determined by sucrose gradient centrifugation and native PAGE. EPR spectroscopy of cytoplasmic membranes revealed that the BolA deletion results in the loss of the binuclear Fe/S cluster N1b.

Energy-converting NADH:ubiquinone oxidoreductase, respiratory complex I, plays a major role in cellular energy metabolism. It couples NADH oxidation and quinone reduction with the translocation of protons across the membrane, thus contributing to the protonmotive force. Complex I has an overall L-shaped structure with a peripheral arm catalyzing electron transfer and a membrane arm engaged in proton translocation. Although both reactions are arranged spatially separated, they are tightly coupled by a mechanism that is not fully understood. Using redox-difference UV-vis spectroscopy, an unknown redox component was identified in Escherichia coli complex I as reported earlier. A comparison of its spectrum with those obtained for different quinone species indicates features of a quinol anion. The re-oxidation kinetics of the quinol anion intermediate is significantly slower in the D213GH variant that was previously shown to operate with disturbed quinone chemistry. Addition of the quinone-site inhibitor piericidin A led to strongly decreased absorption peaks in the difference spectrum. A hypothesis for a mechanism of proton-coupled electron transfer with the quinol anion as catalytically important intermediate in complex I is discussed.

SummaryThe NADH:ubiquinone oxidoreductase, respiratory complex I, couples electron transfer from NADH to ubiquinone with the translocation of protons across the membrane. The complex consists of a peripheral arm catalyzing the redox reaction and a membrane arm catalyzing proton translocation. The membrane arm is almost completely aligned by a 110 Å unique horizontal helix that is discussed to transmit conformational changes induced by the redox reaction in a piston-like movement to the membrane arm driving proton translocation. Here, we analyzed such a proposed movement by cysteine-scanning of the helix of the Escherichia coli complex I. The accessibility of engineered cysteine residues and the flexibility of individual positions were determined by labeling the preparations with a fluorescent marker and a spinprobe, respectively, in the oxidized and reduced states. The differences in fluorescence labeling and the rotational flexibility of the spin probe between both redox states indicate only slight conformational changes at distinct positions of the helix but not a large movement.

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