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Structural basis for the quinone reduction in the bc1 complex: a comparative analysis of crystal structures of mitochondrial cytochrome bc1 with bound substrate and inhibitors at the Qi site. | LitMetric

AI Article Synopsis

  • - Cytochrome bc(1) is a crucial protein complex for cellular respiration and photosynthesis, working through a mechanism called the Q cycle, which features separate sites for quinone reduction and quinol oxidation.
  • - The study reveals how specific inhibitors like antimycin A and NQNO interact with the bc(1) complex by binding at distinct sites, illustrating competition between these inhibitors and the substrate ubiquinone.
  • - Structural analysis suggests that certain amino acids in the Q(i) site are important for acomprehending the binding affinity of these inhibitors and introduces a proposed mechanism for how ubiquinone gets protonated in the process.

Article Abstract

Cytochrome bc(1) is an integral membrane protein complex essential to cellular respiration and photosynthesis. The Q cycle reaction mechanism of bc(1) postulates a separated quinone reduction (Q(i)) and quinol oxidation (Q(o)) site. In a complete catalytic cycle, a quinone molecule at the Q(i) site receives two electrons from the b(H) heme and two protons from the negative side of the membrane; this process is specifically inhibited by antimycin A and NQNO. The structures of bovine mitochondrial bc(1) in the presence or absence of bound substrate ubiquinone and with either the bound antimycin A(1) or NQNO were determined and refined. A ubiquinone with its first two isoprenoid repeats and an antimycin A(1) were identified in the Q(i) pocket of the substrate and inhibitor bound structures, respectively; the NQNO, on the other hand, was identified in both Q(i) and Q(o) pockets in the inhibitor complex. The two inhibitors occupied different portions of the Q(i) pocket and competed with substrate for binding. In the Q(o) pocket, the NQNO behaves similarly to stigmatellin, inducing an iron-sulfur protein conformational arrest. Extensive binding interactions and conformational adjustments of residues lining the Q(i) pocket provide a structural basis for the high affinity binding of antimycin A and for phenotypes of inhibitor resistance. A two-water-mediated ubiquinone protonation mechanism is proposed involving three Q(i) site residues His(201), Lys(227), and Asp(228).

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Source
http://dx.doi.org/10.1021/bi0341814DOI Listing

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