niiicholas
Posts: 319
Joined: May 2002
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I came across a new article on the evolution of photosynthesis; there are a number of articles on this topic, I will post them as I rediscover them, others may have come across interesting stuff also.
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Reaction centres: the structure and evolution of biological solar power
Peter Heathcote b, Paul K. Fyfe a and Michael R. Jones a
Trends in Biochemical Sciences 2002, 27:79-87
Abstract
Reaction centres are complexes of pigment and protein that convert the electromagnetic energy of sunlight into chemical potential energy. They are found in plants, algae and a variety of bacterial species, and vary greatly in their composition and complexity. New structural information has highlighted features that are common to the different types of reaction centre and has provided insights into some of the key differences between reaction centres from different sources. New ideas have also emerged on how contemporary reaction centres might have evolved and on the possible origin of the first chlorophyll–protein complexes to harness the power of sunlight.
[...I'll quote the last part of the review to give a sense of where things are at...]
Common structural blueprint
The crystallographic information summarized in Fig. 4 highlights structural features that are common to all types of reaction centre [3,10,25] . At the heart of each complex is a core domain consisting of an arrangement of two sets of five transmembrane helices. This protein scaffold encases six (bacterio)chlorin and two quinone cofactors that are arranged in two pseudosymmetric membrane-spanning branches. These cofactors catalyse the photochemical transmembrane electron transfer reaction that is the key to the photosynthetic process. Added to this basic structural blueprint are a variety of protein–cofactor structures, such as antenna complexes, the oxygen-evolving complex or Fe–S centres, which represent further adaptations. In particular, in the PSII reaction centre and all known Type I reaction centres, the core electron transfer domain is flanked by two homologous antenna domains, each consisting of a bundle of six membrane-spanning helices binding antenna pigments [24], and antenna chlorophylls are also bound to the ten-helix core ( Fig. 4). These antenna domains are not present in purple bacteria such as Rhodobacter sphaeroides or green filamentous bacteria such as Chloroflexus.
Which is the oldest reaction centre?
The realization that all reaction centres are based on a common design has provoked much discussion over the evolutionary links between the different complexes and the nature of the ancestral reaction centre. This is a challenging topic because it is clear that chlorophyll-based photosynthesis is a very old process that appeared during the first few hundred million years of evolution [38]. One approach to this problem has been to examine which of the five distinct groups of photosynthetic bacteria represents the oldest photosynthetic lineage, through phylogenetic studies of both photosynthetic and non-photosynthetic proteins. However, such studies have produced conflicting results, with green filamentous bacteria, heliobacteria and purple bacteria all being identified as the oldest lineage in different studies [39–42] . The problem of tracing the evolutionary development of modern day photosystems is not helped by some of the variety and complexity exhibited by photosynthetic organisms, which indicates some interchange of photosynthetic components by lateral gene transfer between groups during the course of evolution [41,43] . At present, it is probably prudent to conclude that the use of this approach requires additional data and a more extensive analysis.
Primordial reaction centre: Type I, Type II or both?
Setting aside the question of which is the oldest photosynthetic organism, several models have been proposed to account for the development of modern day reaction centres from simpler ancestors [41]. Most recently, a new evolutionary scheme for contemporary reaction centres has been proposed that envisages the ancestral reaction centre as homodimeric, with the three-domain antenna–core–antenna organization seen in extant Type I complexes [37]. It is proposed that this ancestral reaction centre had two membrane-spanning electron transfer chains, each terminating in a loosely bound quinone that could dissociate when reduced and move into the membrane pool, and that it occupied a membrane that had already developed a fully functional anaerobic respiratory chain, in accordance with the 'respiration early' hypothesis [44]. Therefore, the ancestral reaction centre proposed had a mixed character, with the three-domain organization and (possibly) symmetric electron transfer characteristic of contemporary Type I reaction centres but a capacity to reduce the intramembrane quinone pool, as seen in contemporary Type II reaction centres [37].
The future ... and the dim, distant past
The increasingly detailed crystallographic information now available for the cyanobacterial Type I and Type II reaction centres is provoking renewed interest in the detailed mechanism of these elegant transducers of energy. In particular, the first crystallographic glimpses of the machinery for oxygen evolution are both intriguing and exciting, and will trigger much re-evaluation of our current understanding of a reaction that is of obvious importance to aerobes such as ourselves. It is also becoming apparent that a detailed understanding of quinone chemistry of the homodimeric reaction centres from heliobacteria and green sulfur bacteria might help to focus ideas about the nature of the ancestral reaction centre and the evolutionary route that has led to contemporary complexes.
Finally, peering even further back in evolutionary time, an intriguing question that remains relatively unexplored concerns the origins of the ancestral reaction centre. What was the function of this (bacterio)chlorophyll-containing membrane protein before it evolved into a system capable of harnessing light energy? One suggestion is that early organisms used pigment–protein complexes to protect themselves against the ultraviolet (UV) radiation that bathed the surface of the planet before the development of the atmospheric ozone layer [45]. Such proteins might originally have operated by absorbing high-energy UV photons and dissipating the energy through internal conversion between the (bacterio)chlorophyll Soret absorbance transition and the visible-region absorbance bands, before emitting the energy as a much more benign visible or near-infrared photon [45]. Light-activated electron transfer might originally have developed as an extension to this photoprotective function, excited state energy being converted first into the energy of a charge separated state (similar to the P870+HA- state formed in the purple bacterial reaction centre) and subsequently lost as heat as the charge-separated state recombines (as occurs in purple bacterial reaction centres when forward electron transfer from HA- is blocked). Another suggestion is that photosynthetic function evolved from bacteriochlorophyll-containing proteins involved in infrared thermotaxis [46]. Whatever the truth, addressing these questions requires a journey back to an early stage in the evolution of life, and presents a fascinating challenge.
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Biochim. Biophys. Acta, 1507:291-310. MEDLINE Cited by
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Nature, 409:1083-1091. Cited by
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J. Bacteriol., 176:1-6. MEDLINE Cited by
[40] Gupta R.S. et al. (1999) Evolutionary relationships among photosynthetic prokaryotes (Heliobacterium chlorum, Chloroflexus aurantiacus, cyanobacteria, Chlorobium tepidum and proteobacteria): implications regarding the origin of photosynthesis.
Mol. Microbiol., 32:893-906. MEDLINE Cited by
[41] Xiong J. et al. (1998) Tracking molecular evolution of photosynthesis by characterization of a major photosynthesis gene cluster from Heliobacillus mobilis.
Proc. Natl. Acad. Sci. U. S. A., 95:14851-14856. Full text MEDLINE Cited by
[42] Xiong J. et al. (2000) Molecular evidence for the early evolution of photosynthesis.
Science, 289:1724-1730. Full text MEDLINE Cited by
[43] Blankenship R.E. (2001) Molecular evidence for the evolution of photosynthesis.
Trends Plant Sci., 6:4-6. Full text Cited by
[44] Castresana J. et al. (1994) Evolution of cytochrome oxidase, an enzyme older than atmospheric oxygen.
EMBO J., 13:2516-2525. MEDLINE Cited by
[45] Mulkidjanian A.Y. and Junge W. (1997) On the origin of photosynthesis as inferred from sequence analysis.
Photosynth. Res., 51:27-42.
[46] Nisbet E.G. et al. (1995) Origins of photosynthesis.
Nature, 373:479-480.
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