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Complex III was purified from submitochondrial particles prepared from Euglena gracilis. The purified complex consisted of 10 subunits and lost antimycin sensitivity. The Euglena complex III showed an atypical difference absorption spectrum for cytochrome c1 with its α‐band maximum at 561 nm. The pyridine ferrohemochrome prepared from covalently bound heme in the Euglena complex III had an α‐peak at 553 nm. This wavelength is the same as that of pyridine ferrohemochrome prepared from Euglena mitochondrial cytochrome c (c‐558), the heme of which is linked to only a single cysteine residue through a thioether bond. Cytochrome c1 which was a heme‐stained subunit with a molecular mass of 32.5 kDa was isolated from the purified complex III and its N‐terminal sequence of 46 amino acids was determined. On the basis of apparent homologies to cytochromes c1 from other sources, this sequence included the heme‐binding region. However, the amino acid at position 36, corresponding to the first cysteine involved in heme linkage in other cytochromes c1, was phenylalanine. Position 39, corresponding to the second cysteine, was not identified despite the treatment for removal of the heme and carboxymethylation of the expected cysteine. The unidentified amino acid is assumed to be a derivative of cysteine to which the heme is linked through a single thioether bond. The histidine‐40 corresponding to the probable fifth ligand for heme iron was conserved in Euglena cytochrome c1.

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