title: Effects of inhibitors on the activity of the cytochrome b(6)f complex: evidence for the existence of two binding pockets in the lumenal site.
abstract: The effects of two inhibitors of electron transfer in the cytochrome b(6)f complex have been studied in whole cells of Chlamydomonas reinhardtii. DNP-INT affected equally the two steps of the concerted oxidation of plastoquinol at the Q(o) site; it decreased the rates of both cytochrome f reduction and cytochrome b(6) turnover, without affecting the amplitude of their redox signals. On the contrary, DBMIB inhibited only the rate of cytochrome f reduction while reducing, at the same time, the amplitude of cytochrome b(6) signals. The accessibility of DNP-INT to the Q(o) site was unaffected by preillumination, while that of DBMIB was greatly enhanced, even after a single turnover of the cytochrome b(6)f complex. Similar results were obtained with a mutant strain (FUD2) where the Q(o) site has an affinity for plastoquinol that is diminished by a factor of approximately 50 [Finazzi, G., et al. (1997) Biochemistry 36, 2867-2874]. However, the binding of the two inhibitors was differentially influenced by the mutation: a factor of approximately 250 was calculated for DNP-INT and a factor of only approximately 5 for DBMIB. This suggests that they bind within the Q(o) site in two distinct pockets, which are differentially involved in the process of quinol oxidation, in agreement with a recent model where two distinct positions for the reduced and semireduced quinones are considered [Crofts, A. R., and Berry, E. A. (1998) Curr. Opin. Struct. Biol. 8, 501-509].
authors.affiliation: Centro di Studio del CNR sulla Biologia Cellulare e Molecolare delle Piante, via Celoria 26, 20133 Milano, Italy.
authors.forename: R P
authors.initials: RP
authors.lastname: Barbagallo
authors.forename: G
authors.initials: G
authors.lastname: Finazzi
authors.forename: G
authors.initials: G
authors.lastname: Forti
mesh_terms: D000818:Animals; D001665:Binding Sites; D016825:Chlamydomonas reinhardtii / Q000201:enzymology / Q000502:physiology; D003573:Cytochrome b Group / Q000037:antagonists & inhibitors* / Q000378:metabolism; D045346:Cytochrome b6f Complex; D004579:Electron Transport; D004791:Enzyme Inhibitors / Q000378:metabolism / Q000494:pharmacology*; D008564:Membrane Potentials
chemical_list: D003573:Cytochrome b Group; D004791:Enzyme Inhibitors; D045346:Cytochrome b6f Complex