Molecular genetic studies of complex I inNeurospora crassa, Aspergillus niger andEscherichia coli

Abstract

The proton pumping NADH:ubiquinone oxidoreductase of mitochondria (complex I) is an assembly of some 25 different nuclear-encoded and 7 mitochondrially-encoded subunits. One FMN and 4-6 iron-sulfur (FeS) clusters have been identified as internal redoxgroups [1]. Recent electron microscopic and biochemical studies have shown that the complex is made up of two arms forming an L-shaped structure. The peripheral arm which protrudes into the mitochondrial matrix contains most of the nuclear-encoded subunits, while the membrane arm contains all mitochondrially encoded subunits [2]. The two arms are assembled on separate pathways and possibly emerged independently in evolution [1,3,4]. Sequence analysis revealed homology between subunits of the peripheral arm of complex I and subunits of a soluble NAD-reducing hydrogenase of Alcaligenes eutrophus. Based on this relationship, binding sites for NADH, FMN and the FeS clusters N-l, N-3 and N-4 were assigned to several subunits [1,5-7]. The ubiquinone binding domain in the membrane arm of complex I was identified by sequence comparison with a bacterial glucose dehydrogenase [8]. Homology was also found between complex I subunits (essentially of the membrane arm) and subunits of a membrane bound formate hydrogen lyase of Escherichia coli [1,9]. This relatiofiship has, however, not yet contributed to a better understanding of complex I, because the membrane part of the formate hydrogen lyase is itself poorly understood.