The Modified Q-cycle Explains the Apparent Mismatch between the Kinetics of Reduction of Cytochromes c1 and bH in the bc1 Complex

Antony R Crofts,Vladimir P Shinkarev,Derrick R.J Kolling,Sangjin Hong

doi:10.1074/jbc.m305461200

Antony R Crofts, Vladimir P Shinkarev + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m305461200

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Abstract

Crystallographic structures of the bc1 complex from different sources have provided evidence that a movement of the Rieske iron-sulfur protein (ISP) extrinsic domain is essential for catalysis. This dynamic feature has opened up the question of what limits electron transfer, and several authors have suggested that movement of the ISP head, or gating of such movement, is rate-limiting. Measurements of the kinetics of cytochromes and of the electrochromic shift of carotenoids, following flash activation through the reaction center in chromatophore membranes from Rhodobacter sphaeroides, have allowed us to demonstrate that: (i) ubiquinol oxidation at the Qo-site of the bc1 complex has the same rate in the absence or presence of antimycin bound at the Qi-site, and is the reaction limiting turnover. (ii) Activation energies for transient processes to which movement of the ISP must contribute are much lower than that of the rate-limiting step. (iii) Comparison of experimental data with a simple mathematical model demonstrates that the kinetics of reduction of cytochromes c1 and bH are fully explained by the modified Q-cycle. (iv) All rates for processes associated with movement of the ISP are more rapid by at least an order of magnitude than the rate of ubiquinol oxidation. (v) Movement of the ISP head does not introduce a significant delay in reduction of the high potential chain by quinol, and it is not necessary to invoke such a delay to explain the kinetic disparity between the kinetics of reduction of cytochromes c1 and bH.

Highlights

In the simplest of these schemes, the kinetic disparity between rates of reduction of cyt c1 and bH seen in mitochondrial complexes has been taken as evidence for a kinetic impediment at the level of iron-sulfur protein (ISP) movement
Several groups have espoused more complex schemes involving “conformational gating” of electron transfer between the Qo-site and cyt c1 through a conformational coupling; the reduced ISP is held in position close to cyt b until released by conformational changes linked to electron transfer events elsewhere in the complex
At the ambient redox potential of the experiment (Eh ϳ 100 mV) the reduction of cyt bH seen after a saturating flash in the presence of antimycin represents ϳ90% of the total accessible bc1 complex

Summary

Introduction

Cluster of the Rieske iron-sulfur protein (ISP), which feeds electrons via a bound c-type heme (cytochrome (cyt) c1 in the bc complex) to a mobile electron carrier protein (cyt c) that reduces a terminal acceptor (cytochrome oxidase in the mitochondrial chain, an oxidized photochemical reaction center in photosynthetic systems). It is supposed from general principles of quinone chemistry [3] that a semiquinone intermediate is formed at the Qo-site, but this has not been detected. In the explication of this mechanism, a critical feature was the recognition that the complex operated as an independ-

Results

Discussion

Conclusion