Characterization of the Nqo5 subunit of bacterial complex I in the isolated state.

The subunits that comprise bacterial complex I (NADH:ubiquinone oxidoreductase) are also found in more complicated mitochondrial enzymes in eukaryotic organisms. Although the Nqo5 subunit is one of these conserved components and important for the formation of complex, it has been little studied. Here, we report structure analyses of isolated Nqo5 from Thermus thermophilus. Biochemical studies indicated that the C-terminal region following the 30-Kd subunit motif is disordered in the isolated state, while the remaining portion is already folded. Crystallographic studies of a trypsin-resistant fragment revealed detailed structural differences in the folded domain between the isolated and complexed states.