The iron‐sulfur clusters in the two related forms of mitochondrial NADH: ubiquinone oxidoreductase made by Neurospora crassa

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Abstract
Two related forms of the respiratory‐chain complex, NADH: ubiquinone oxidoreductase (Complex I) are synthesized in the mitochondria of Neurospora crassa. Normally growing cells make a large, piericidin‐A‐sensitive form, which consists of some 23 different nuclear‐ and 6–7 mitochondrially encoded subunits. Cells grown in the presence of chloramphenicol make a small, piericidin‐A‐insensitive form which consists of only ∼ 13 nuclear‐encoded subunits. The subunits of the small form are either identical or similar to nuclear‐encoded subunits of the large form. The iron‐sulfur clusters in these two forms of Complex I are characterized by redox potentiometry and EPR spectroscopy. The large form of Complex I contains four EPR‐detectable iron‐sulfur clusters, N1, N2, N3 and N4, with the spin concentration of the individual clusters equivalent to the flavin concentration, similar to the mammalian counterparts. The small Complex I contains clusters N1, N3 and N4, but it is devoid of cluster N2. A model of the electron‐transfer route through the large form of Complex I has been derived from these findings and an evolutionary pathway which leads to the emergence of large Complex I is discussed.