Light, time and micro-organisms

9 > pts > 3
Nanoseconds to milliseconds. The second time domain

A crucial set of secondary electron and proton transfers occur in bioenergetic membranes, including photosynthetic membranes if the donor happens to be a reaction centre. One intermediate in these reactions is particularly short-lived, but may provide a key to understanding quite distant biological responses to light and time.

The Q-cycle (Mitchell, 1976) is an important component of energy coupling in most photosynthetic and respiratory systems, and appears to be the primary function of cytochrome bc1 complexes. The Q-cycle takes two electrons from the bulk quinone pool in the membrane, passes one on to an iron-sulphur protein and hence to cytochrome c, and recycles the other back into the pool by means of its transfer between two cytochrome b haems, arranged across the membrane.

One inexplicable feature of the Q-cycle has, until recently, been the requirement for bifurcation of the electron transport chain at the Qo site: by what means may the quinol be forbidden from donating both of the electrons it carries in the thermodynamically-favoured direction, that is, to the Fe-S centre? Even after a single electron transfer, the intermediate semiquinone should be a good donor to the iron-sulphur protein. In other words, why does recycling through cytochrome b occur at all?

Recent structures from X-ray crystallography for the iron-sulphur and for the intrinsic membrane domain of the bc1 complex have provided Crofts et al. (1997) with the structural basis for an ingenious solution to this problem. An animation presents the fundamentals of Crofts's idea. The quinol in its Qo site is initially much closer in space to the to the iron-sulphur centre than it is to the low-potential b-haem, and electron transfer to the iron-sulphur centre is then kinetically favoured. After the first electron transfer, the semiquinone centre moves closer to the low-potential b-haem, permitting the second electron transfer to occur in a different direction. After its reduction, the iron-sulphur centre moves away from the Qo site and towards cytochrome c as a result of rotation of the mobile head-group of the iron-sulphur protein.

The kinetics of the component reactions are known in some detail, and are consistent with this proposal (1997). I suggest here that one way of considering the Qo site mechanism in functional terms is to view the semiquinone anion radical as indispensable but dangerous. This dilemma that may provide an insight into the persistence, in evolution, of extra-nuclear genetic systems in eukaryotes, as discussed in time domain five. Thus the steady-state concentration of the semiquinone is maintained at the lowest possible value by the first electron transfer, from the quinol to the iron-sulphur centre, being around ten times slower than the second electron transfer, from the semiquinone to the low-potential b-haem. Quinol oxidation to semiquinone occurs in 600 ms: pts = 3.2. the semiquinone is oxidised to quinol in 60 ms: pts = 4.2. The electron transfer between the low-potential and high-potential b-haems takes around 200 ms: pts = 3.7.

A graphic of the Crofts model for the Qo site

An animation of the Crofts model for the Qo site

Cytochrome b-c complex site