Length, time, and energy scales of photosystems

The design of photosynthetic systems reflects the length scales of the fundamental physical processes. Energy transfer is rapid at the few angstrom scale and continues to be rapid even at the 50-A scale of the membrane thickness. Electron tunneling is nearly as rapid at the shortest distances, but becomes physiologically too slow well before 20 A. Diffusion, which starts out at a relatively slow nanosecond time scale, has the most modest slowing with distance and is physiologically competent at all biologically relevant distances. Proton transfer always operates on the shortest angstrom scale. The structural consequences of these distance dependencies are that energy transfer networks can extend over large, multisubunit and multicomplex distances and take leaps of 20 A before entering the domain of charge separating centers. Electron transfer systems are effectively limited to individual distances of 15 A or less and span the 50 A dimensions of the bioenergetic membrane by use of redox chains. Diffusion processes are generally used to cover the intercomplex electron transfer distances of 50 A and greater and tend to compensate for the lack of directionality by restricting the diffusional space to the membrane or the membrane surface, and by multiplying the diffusing species through the use of pools. Proton transfer reactions act over distances larger than a few angstroms through the use of clusters or relays, which sometimes rely on water molecules and which may only be dynamically assembled. Proteins appear to place a premium on robustness of design, which is relatively easily achieved in the long-distance physical processes of energy transfer and electron tunneling. By placing cofactors close enough, the physical process is relatively rapid compared to decay processes. Thus suboptimal conditions such as cofactor orientation, energy level, or redox potential level can be tolerated and generally do not have to be finely tuned. The most fragile regions of design tend to come in areas of complex formation and catalysis involving proton management, where relatively small changes in distance or mutations can lead to a dramatic decrease in turnover, which may already be limiting the overall speed of energy conversion in these proteins. Light-activated systems also face a challenge to robust function from the ever-present dangers of high redox potential chemistry. This can turn the protein matrix and wandering oxygen molecules into unintentional redox partners, which in the case of PSII requires the frequent, costly replacement of protein subunits.