Effects of bc 1 ‐site electron transfer inhibitors on the absorption spectra of mitochondrial cytochromes b

Changes are described that are brought about by antimycin, NoHOQnO, funiculosin, myxothiazol and mucidin in the α‐, β‐ and γ‐absorption bands of reduced and oxidized cytochromes b in the isolated complex bc 1 , form beef heart mitochondria. The inhibitors can be divided into 2 groups. Antimycin, funiculosin and NoHOQnO are likely to shift the spectrum of b ‐562 and compete for specific binding with complex bc 1 with each other but not with myxothiazol and mucidin. The spectral effects of the latter two inhibitors are more difficult to interpret and may involve contributions not only from b ‐562 but from b ‐566 as well. The existence of 2 independent inhibitor binding‐sites in the complex bc 1 corroborates the Q‐cycle hypothesis.