The evolution of the oxygenic photosynthetic reaction center is of paramount importance in evolutionary biology. 1
Photosystem II, a Bioenergetic Nanomachine
"Of all the biochemical inventions in the history of life, the machinery to oxidize water — photosystem II — using sunlight is surely one of the grandest." (Sessions, A. et al, 2009)
PSII "is one of nature's most complicated enzymes — complicated partly because it involves a four-electron oxidation process resulting in four intermediate states and the concurrent chemistry of O-O bond formation."
3D picture :
Architecture of the photosynthetic oxygen evolving center
The advent of oxygenic photosynthesis brought about an increase in protein complexity from the three to four subunits found in anoxygenic reaction centres to around thirty subunits found in PSII. A few of these proteins show homology to one another, and as such may have arisen by gene duplication - for instance, the D1 (PsbA) and D2 (PsbD) reaction centre core proteins, the CP43 (PsbC) and CP47 (PsbB) core antenna proteins, and the PsbE and PsbF subunits of cytochrome b559. However, most of the proteins making up PSII are unrelated to one another or to other protein families, suggesting a period of rapid protein diversification, perhaps in response to the toxic effects of oxygen from which cells would need protection. The development of oxygenic photosynthesis brought about many changes, requiring alterations to existing pigments, the generation of an oxygen evolution complex, and protection against the toxic effects of oxygen by-products.
PSII is a multi-subunit, pigment-protein complex localised in the chloroplast thylakoid membranes. It consists of around 30 subunits and several cofactors. The major redox components are present in the heterodimer reactive centre core, which is composed of polypeptides D1 (PsbA) and D2 (PsbD) that bind to chlorophyll a, beta-carotene and iron. These chlorophylls participate in energy transfer from the proximal antennae complexes of CP43 (PsbC) and CP47 (PsbB) to the reactive centre core chromophores. The antenna pigment-protein complex CP43-CP47 also binds chlorophyll a and beta-carotene, and acts to transfer excitation energy from the peripheral antenna of PSII toward the photochemical reaction centre. Cytochrome b559 (proteins PsbE and PsbF) is closely associated with the core, and may be involved in a secondary electron transfer pathway that helps to protect PSII from photodamage. Associated with the core is an oxygen-evolving complex (OEC) that acts as the active site of the water oxidation centre. The OEC is composed of the extrinsic polypeptides OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ), as well as a tetranuclear manganese (Mn) cluster, one calcium ion and one chloride ion. OEE1 acts to stabilise the ligation of the Mn cluster in the dark and to promote rapid redox cycling in the light. Finally, there are at least ten small (<10 kDa) hydrophobic peptides, many of which contain transmembrane helices, which are required for the assembly, stability or dimerisation of the PSII complex, as well as for facilitating the fast conformational changes required for photosynthetic activity. Some of the small polypeptides, such as PsbH and PsbT, are involved in photoprotection, which help protect against the damaging effects of the reactive oxygen species generated during photosynthesis.
Years of research have shown that the structure and function of photosystem II is similar in plants, algae and certain bacteria, so that knowledge gained in one species can be applied to others. This homology is a common feature of proteins that perform the same reaction in different species. This homology at the molecular level is important because there are estimated to be 300,000-500,000 species of plants. If different species had evolved diverse mechanisms for oxidizing water, research aimed at a general understanding of photosynthetic water oxidation would be hopeless.[/b][/b]
Photosynthesis reactor: Speaking of photosynthesis, Japanese scientists have achieved the imaging of the “Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9?Å,” zooming in almost twice as far as previous studies. Their paper, published in Nature,1 spoke of the reactor as “indispensable for sustaining life on Earth.” It includes detailed drawings of the 20 subunits involved with numerous molecular contacts.
The particular part of the reactor that splits water molecules and combines oxygen atoms into the O2 gas we breathe they said is “one of nature’s most fascinating and important reactions.” Understanding Photosystem II may help humans to mimic plants’ ability to split water efficiently at ambient temperatures, leading to renewable energy for a multitude of applications. The ability lives all around us if we can tap into its secrets.
A number of problems have not been taken into consideration until now. The terms photosystem I and photosystem II, for example, have been introduced and all participating pigments have been mentioned but the following subjects remain to be discussed:
How are the photosystems organized?
How are the pigments arranged?
Why does one of the chlorophyll molecules react different than all the others?
Why are action and absorption spectra not quite congruent?
Why reacts P 680 (chlorophyll a) different than P 700 (chlorophyll a, too)?
How are electron transport chain and ATP production coupled?
How are photosystem I and II linked?
Which structural prerequisites have to exist in order for the two systems to co-operate?
Blankenship, molecular mechanisms of photosynthesis, pg.214
The two different classes of reaction centers have only minimal sequence similarity to each other, not significantly above what would be expected randomly. However, it is well known that very distantly related proteins can exhibit minimal sequence identity, yet still be homologous (descended from a common ancestor) (Doolittle, 1994).
Thats indeed telling. Cannot infer common ancestry through phylogeny comparison ? Its descended from a common ancestor anyway.... Thats religion at its best. That way you can turn the ToE however you want, it will be always right.
Attempting to understand the most important chemical reaction on the planet: Photosynthesis
The synthesis of organic compounds made possible by coupling light to the splitting of water is probably the most important set of chemical reactions on the planet.
There’s even more info here
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