Characterisation of the last Fe-S cluster-binding subunit of Neurospora crassa complex I

The role of Coenzyme Q in mitochondrial electron transport

In mitochondria, most Coenzyme Q is free in the lipid bilayer; the question as to whether tightly bound, non-exchangeable Coenzyme Q molecules exist in mitochondrial complexes is still an open question. We review the mechanism of inter-complex electron transfer mediated by ubiquinone and discuss the kinetic consequences of the supramolecular organization of the respiratory complexes (randomly dispersed vs. super-complexes) in terms of Coenzyme Q pool behavior vs. metabolic channeling, respectively, both in physiological and in some pathological conditions. As an example of intra-complex electron transfer, we discuss in particular Complex I, a topic that is still under active investigation.

Neurospora strains harboring mitochondrial disease-associated mutations in iron-sulfur subunits of complex I

We subjected the genes encoding the 19.3-, 21.3c-, and 51-kDa iron-sulfur subunits of respiratory chain complex I from Neurospora crassa to site-directed mutagenesis to mimic mutations in human complex I subunits associated with mitochondrial diseases. The V135M substitution was introduced into the 19.3-kDa cDNA, the P88L and R111H substitutions were separately introduced into the 21.3c-kDa cDNA, and the A353V and T435M alterations were separately introduced into the 51-kDa cDNA. The altered cDNAs were expressed in the corresponding null-mutants under the control of a heterologous promoter. With the exception of the A353V polypeptide, all mutated subunits were able to promote assembly of a functional complex I, rescuing the phenotypes of the respective null-mutants. Complex I from these strains displays spectroscopic and enzymatic properties similar to those observed in the wild-type strain. A decrease in total complex I amounts may be the major impact of the mutations, although expression levels of mutant genes from the heterologous promoter were sometimes lower and may also account for complex I levels. We discuss these findings in relation to the involvement of complex I deficiencies in mitochondrial disease.

The location of NuoL and NuoM subunits in the membrane domain of the Escherichia coli complex I: implications for the mechanism of proton pumping

The molecular organization of bacterial NADH: ubiquinone oxidoreductase (complex I or NDH-1) is not established, apart from a rough separation into dehydrogenase, connecting and membrane domains. In this work, complex I was purified from Escherichia coli and fragmented by replacing dodecylmaltoside with other detergents. Exchange into decyl maltoside led to the removal of the hydrophobic subunit NuoL from the otherwise intact complex. Diheptanoyl phosphocholine led to the loss of NuoL and NuoM subunits, whereas other subunits remained in the complex. The presence of N,N-dimethyldodecylamine N-oxide or Triton X-100 led to further disruption of the membrane domain into fragments containing NuoL/M/N, NuoA/K/N, and NuoH/J subunits. Among the hydrophilic subunits, NuoCD was most readily dissociated from the complex, whereas NuoB was partially dissociated from the peripheral arm assembly in N,N-dimethyldodecylamine N-oxide. A model of subunit arrangement in bacterial complex I based on these data is proposed. Subunits NuoL and NuoM, which are homologous to antiporters and are implicated in proton pumping, are located at the distal end of the membrane arm, spatially separated from the redox centers of the peripheral arm. This is consistent with proposals that the mechanism of proton pumping by complex I is likely to involve long range conformational changes.

From NADH to ubiquinone in Neurospora mitochondria

The respiratory chain of the mitochondrial inner membrane includes a proton-pumping enzyme, complex I, which catalyses electron transfer from NADH to ubiquinone. This electron pathway occurs through a series of protein-bound prosthetic groups, FMN and around eight iron-sulfur clusters. The high number of polypeptide subunits of mitochondrial complex I, around 40, have a dual genetic origin. Neurospora crassa has been a useful genetic model to characterise complex I. The characterisation of mutants in specific proteins helped to understand the elaborate processes of the biogenesis, structure and function of the oligomeric enzyme. In the fungus, complex I seems to be dispensable for vegetative growth but required for sexual development. N. crassa mitochondria also contain three to four nonproton-pumping alternative NAD(P)H dehydrogenases. One of them is located in the outer face of the inner mitochondrial membrane, working as a calcium-dependent oxidase of cytosolic NADPH.

Disruption of iron-sulphur cluster N2 from NADH: ubiquinone oxidoreductase by site-directed mutagenesis

We have cloned and inactivated, by repeat-induced point mutations, the nuclear gene encoding the 19.3 kDa subunit of complex I (EC 1.6.5.3) from Neurospora crassa, the homologue of the bovine PSST polypeptide. Mitochondria from mutant nuo19.3 lack the peripheral arm of complex I while its membrane arm accumulates. Transformation with wild-type cDNA rescues this phenotype and assembly of complex I is restored. To interfere with assembly of a proposed bound iron-sulphur cluster, site-directed mutants were constructed by introducing cDNA with altered codons for two adjacent cysteines, Cys-101 and Cys-102. The mutant complexes were purified and their enzymic activities and EPR and UV/visible spectra were analysed. Either of the mutations abolishes assembly of iron-sulphur cluster N2, showing that this redox group is bound to the 19.3 kDa protein. We also observed an interference with the reduction of redox group X, suggesting that cluster N2 is the electron donor to this high-potential redox group.

On complex I and other NADH:ubiquinone reductases of Neurospora crassa mitochondria

The mitochondrial complex I is the first component of the respiratory chain coupling electron transfer from NADH to ubiquinone to proton translocation across the inner membrane of the organelle. The enzyme from the fungus Neurospora crassa is similar to that of other organisms in terms of protein and prosthetic group composition, structure, and function. It contains a high number of polypeptide subunits of dual genetic origin. Most of its subunits were cloned, including those binding redox groups. Extensive gene disruption experiments were conducted, revealing many aspects of the structure, function, and biogenesis of complex I. Complex I is essential for the sexual phase of the life cycle of N. crassa, but not for the asexual stage. In addition to complex I, the fungal mitochondria contain at least three nonproton-pumping alternative NAD(P)H dehydrogenases feeding electrons to the respiratory chain from either matrix or cytosolic substrates.