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 Neu
rospora crassa is similar to that of other organisms in terms of protein an
d prosthetic group composition, structure, and function. It contains a high
number of polypeptide subunits of dual genetic origin. Most of its subunit
s were cloned, including those binding redox groups. Extensive gene disrupt
ion experiments were conducted, revealing many aspects of the structure, fu
nction, 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 add
ition to complex I, the fungal mitochondria contain at least three nonproto
n-pumping alternative NAD(P)H dehydrogenases feeding electrons to the respi
ratory chain from either matrix or cytosolic substrates.