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Titolo:
Unbinding of oxidized cytochrome c from photosynthetic reaction center of Rhodobacter sphaeroides is the bottleneck of fast turnover
Autore:
Gerencser, L; Laczko, G; Maroti, P;
Indirizzi:
Univ Szeged, Dept Biophys, H-6722 Szeged, Hungary Univ Szeged Szeged Hungary H-6722 , Dept Biophys, H-6722 Szeged, Hungary
Titolo Testata:
BIOCHEMISTRY
fascicolo: 51, volume: 38, anno: 1999,
pagine: 16866 - 16875
SICI:
0006-2960(199912)38:51<16866:UOOCCF>2.0.ZU;2-S
Fonte:
ISI
Lingua:
ENG
Soggetto:
BACTERIAL REACTION CENTERS; PHOTOOXIDIZED BACTERIOCHLOROPHYLL DIMER; ELECTRON-TRANSFER COMPLEXES; SITE-DIRECTED MUTAGENESIS; OPTICAL LINEAR DICHROISM; L-SUBUNIT PLAYS; RHODOPSEUDOMONAS-SPHAEROIDES; PHOSPHOLIPID-VESICLES; MEDIATED REREDUCTION; TETRAHEME CYTOCHROME;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Life Sciences
Citazioni:
49
Recensione:
Indirizzi per estratti:
Indirizzo: Maroti, P Univ Szeged, Dept Biophys, Egyetem Utca 2, H-6722 Szeged, Hungary Univ Szeged Egyetem Utca 2 Szeged Hungary H-6722 zeged, Hungary
Citazione:
L. Gerencser et al., "Unbinding of oxidized cytochrome c from photosynthetic reaction center of Rhodobacter sphaeroides is the bottleneck of fast turnover", BIOCHEM, 38(51), 1999, pp. 16866-16875

Abstract

To understand the details of rate limitation of turnover of the photosynthetic reaction center, photooxidation of horse heart cytochrome c by reaction center from Rhodobacter spheroides in detergent dispersion has been examined by intense continuous illumination under a wide variety of conditions of cytochrome concentration, ionic strength, viscosity, temperature, light intensity, and pH. The observed steady-state turnover rate of the cytochromewas not Light intensity limited. In accordance with recent findings [Larson, J. W., Wells, T. A., and Wraight, C. A. (1998) Biophys. J. 74 (2), A76],the turnover rate increased with increasing bulk ionic strength in the range of 0-40 mM NaCl from 1000 up to 2300 s(-1) and then decreased at high ionic strength under conditions of excess cytochrome and ubiquinone and a photochemical rate constant of 4500 s(-1). Furthermore, we found the following: (i) The contribution of donor (cytochrome c) and acceptor (ubiquinone) sides as well as the binding of reduced and the release of oxidized cytochrome c could be separated in the observed kinetics. At neutral and acidic pH (when the proton transfer is not rate limiting) and at low or moderate ionicstrength, the turnover rate of the reaction center was limited primarily by the low release rate of the photooxidized cytochrome c (product inhibition). At high ionic strength, however, the binding rate of the reduced cytochrome c decreased dramatically and became the bottleneck. The observed activation energy of the steady-state turnover rate reflected the changes in limiting mechanisms: 1.5 kcal/mol at 4 mM and 5.7 kcal/mol at 100 mM ionic strength. A similar distinction was observed in the viscosity dependence of the turnover rate: the decrease was steep (eta(-1)) at 40 and 100 mM ionic strengths and moderate (eta(-0.2)) under low-salt (4 mM) conditions. (ii) Therate of quinone exchange at the acceptor side with excess ubiquinone-30 orubiquinone-50 was higher than the cytochrome exchange at the donor side and did not limit the observed rate of cytochrome turnover. (iii) Multivalentcations exerted effects not only through ionic strength (screening) but also by direct interaction with surface charge groups (ion-pair production). Heavy metal ion Cd2+ bound to the RC with apparent dissociation constant of14 mu M. (iv) A two-stale model of collisional interaction between reaction center and cytochrome c together with simple electrostatic considerationsin the calculation of rate constants was generally sufficient to describe the kinetics of photooxidation of dimer and cytochrome c. (v) The pH dependence of cytochrome turnover rate indicated that the steady-state turnover rate of the cytochrome under high light conditions was not determined by theisoelectric point of the reaction center (pI = 6.1) but by the carboxyl residues near the docking site.

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Documento generato il 11/07/20 alle ore 07:50:42