The cytochrome b subunit of the bc1 complex contains two cytochrome components, cytochrome bH and cytochrome bL. Sequence comparisons of this polypeptide from a number of organisms have revealed four invariant histidines which have been postulated to be the heme ligands for the two protoheme IX prosthetic groups. In Rhodobacter sphaeroides, these correspond to His97, His111, His198, and His212. In this paper, the results of amino acid substitutions at each of these positions are reported. Replacement of His97 by either Asp or Asn and of His198 by Asn or Tyr resulted in loss of both cytochrome components. However, His111 Asn, His111Asp, and His212Asp all resulted in the selective loss of cytochrome bH and the retention of cytochrome bL. Furthermore, flash kinetics studies show that the myxothiazol-sensitive quinol oxidase (Qz) site associated with cytochrome bL is still functional. These data support the assignment of the axial ligands to cytochrome bH (His111 and His212) and cytochrome bL (His97 and Hisl98). This pairing is consistent with current models of the cytochrome b subunit with eight transmembrane α-helices.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Jul 1 1991|
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