Structural studies of the yeast type II NADH:quinone oxidoreductase NDI-1

Membrane proteins perform a staggering range of biological functions including respiration, signal transduction and molecular transport. The results of various genome projects have shown that up to 30 % of proteins encoded by eukaryotic cells are membrane proteins. A more fundamental understanding of the structure-function relationships of membrane proteins would make invaluable contributions to structural biology, pharmacology and medicine. Recently, we have obtained well-diffracting crystals of a yeast membrane protein NDI-1. This enzyme is a homologue of the mammalian protein and is an essential enzyme for yeast respiration. Structural information for this membrane protein will provide important insights into this respiratory membrane protein, particularly on its substrate (NADH and coenzyme Q10) and cofactor (FAD) binding sites. The structure is also medically important as it can be used for gene therapy of human genetic diseases.

The oxygen respiratory chain is the final stage of energy catabolism for aerobic organisms. The system exists in the inner membrane of mitochondria and is composed of five membrane protein complexes including Complex I, NADH-ubiquinone oxidoreductase. Many organisms have an alternative enzyme called type II NADH:quinone oxidoreductases, which catalyses the same redox reaction as Complex I without proton translocation. NDI-1 from yeast is a unique type II NADH:quinone oxidoreductase since it is solely responsible for the NADH:quinone redox reaction in yeast, which lacks Complex I. Structural information for this membrane protein will provide important insights into the type II NADH dehydrogenases, particularly on their substrate and cofactor binding sites. The structure is also medically important as it can be used for gene therapy of human genetic diseases caused by defective Complex I. We have well-diffracting crystals of NDI-1 to address these important questions. Very little is known about the structures of eukaryotic membrane proteins, thus the NDI-1 structure will make invaluable contributions to general structural biology and biochemistry.