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Intelligent Design, the best explanation of Origins » Photosynthesis, Protozoans,Plants and Bacterias » Photosystem I and II

Photosystem I and II

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1 Photosystem I and II on Sat 1 Mar 2014 - 22:35

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Photosystem I and II

The evolution of the oxygenic photosynthetic reaction center is of paramount importance in evolutionary biology. 1

Photosystem
Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplasts), or in the cytoplasmic membrane of photosynthetic bacteria. There are kinds of photosystems: II and I, respectively.
Photosystem  II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme captures photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By obtaining these electrons from water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP+ to NADPH or are used in cyclic photophosphorylation.
The Photosystems: a presentation
http://www.powershow.com/view/11a36d-NGQzY/Folie_1_powerpoint_ppt_presentation





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


http://www.ebi.ac.uk/interpro/potm/2004_11/Page2.htm



    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]

http://creationsafaris.com/crev201105.htm#gene499

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.


http://www.biologie.uni-hamburg.de/b-online/e24/24d.htm

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.


1) http://iiti.ac.in/people/~sksingh/OEC.pdf

Attempting to understand the most important chemical reaction on the planet: Photosynthesis
http://www.extremetech.com/extreme/172329-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.
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/antpho.html#c3

http://www.artinaid.com/2013/04/chloroplasts-what-are-and-what-they-are-made%E2%80%8B%E2%80%8B-of/
http://www.chm.bris.ac.uk/motm/oec/motm.htm
http://www.nature.com/nature/journal/v473/n7345/full/nature09913.html
http://www.spring8.or.jp/pdf/en/res_fro/11/016-017.pdf
http://www.pnas.org/content/109/7/2257.long

There’s even more info here



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2 Re: Photosystem I and II on Thu 6 Mar 2014 - 7:49

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Photosystem II at 3.0 Å Resolution



The Photosystem II (PSII) is enzyme at the beginning of the photosynthetic light reactions that splits water into protons and oxygen.

Light energy is absorbed by antenna complexes and funneled into the PSII complex reaction center P680. The reaction center is a special pair of chlorophyll molecules buried in the heart of this multisubunit membrane protein complex. These chlorophylls are poised in just the right environment relative to other cofactors such that when energy is absorbed by them, charge separation occurs. This means that light causes the movement of electrons (as opposed to energy transfer along antenna systems flowing energetically downhill). When an electron has been separated from the reaction center, it travels through several other cofactors within PSII and finally onto a plastoquinone molecule (PQ). PQ is a small organic molecule that can move within the thylakoid membrane and carry its electrons over to the next complex in the light reactions (cytochrome b6f). With the light energy causing electrons to be displaced from the reaction center, PSII must replace those electrons from somewhere. It does this by pulling electrons from water molecules. In order to balance the reaction for products and reactants, PSII sequentially pulls four electrons from two water molecules to make oxygen, release 4 protons and make 2 PQH2 molecules (that’s just PQ with 2 electrons). PSII performs this water-splitting reaction using an inorganic manganese-calcium-chloride cluster buried on the lumenal face of the enzyme.

Here are the highlights of what we still don’t know:

PSII Structure: There are structural models of PSII, but they have only been solved for the cyanobacterial enzyme. See figure below for an example. This structure represents a highly uniform complex, which may not tell the entire story of what the PSII complexes really looks like. Also, there are some differences in the PSII components between cyanobacteria and plants making it difficult to infer what may be going on in the more complex plant system.

Water Oxidation Mechanism: Splitting water to form oxygen is a very challenging reaction and we’re still not exactly sure how PSII does it. Structural data as well as sophisticated spectroscopy data are getting us closer to understanding how water splitting works, but it is technically challenging to probe the details of this enzymatic reaction.

PSII Assembly: PSII is made up of more than twenty different protein subunits plus cofactors. All of these must come together precisely to create the highways for electron transfer within the enzyme. We don’t know all of the details as to how this works either. Also, there are a handful of other proteins that only act as chaperones to facilitate assembly of the complex, but do not actually become part of PSII. Understanding the details of PSII assembly is more than just an intellectual pursuit because of the topic listed below.

Photosystem II lifecycle

PSII Damage-Repair Cycle: PSII is at the heart of the ‘light problem’ for photosynthetic organisms. During the normal course of PSII function, its proteins become damaged by the electrons zipping through the complex. A sophisticated mechanism exists for recognizing the damaged protein, removing it and replacing it with a freshly made protein. There are lots of details still to be determined as far as how his process works and how it may be regulated. Researchers are also working to tease apart the differences and/or overlap between the de novo assembly pathway and the damage-repair cycle in the Life Cycle of PSII (see figure above).

PSII Regulation: PSII function is also finely tuned according to environmental conditions, and its regulation in response to light and the redox balance within the thylakoid membranes on a wide-ranging timescale represents a frontier in photosynthesis research. Some general components and strategies are known, but many new discoveries are waiting on the horizon in this area.


Photosynthesis is one of the most fundamental and important natural processes. Responsible for the maintenance of the oxygen level in the atmosphere and the reduction of CO2 to carbohydrates, a detailed understanding of photosynthesis has been developed following decades of careful investigations. A complete picture of the process requires the synthesis of the information from many physical and bio-physical techniques, in addition to the three dimensional structures of the proteins involved.




Two views of PSII isolated from the cyanobacterium Synechococcus elongatus. (a) view along the membrane plane, the pseudo-twofold axis relating the monomers in the homodimer is indicated. The principal protein subunits (reaction center D1, D2; antennae subunits CP43, CP47; Cyt b-559 and membrane-extrinsic subunits) in different colours and small subunits with unknown function in grey. Cofactors are indicated, chlorophylls (green), ß-carotenes (orange), lipids (black), haems (blue), Mn4Ca cluster (red spheres). (b) view from the "top" side on the membrane plane, colours and labelling (in one monomer) as in (a).

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3 Re: Photosystem I and II on Sat 8 Mar 2014 - 3:11

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Regulation of photosystem synthesis in Rhodobacter capsulatus
Abstract

Control of the synthesis of the purple bacterial photosystem has been an active area of research for many decades. The period of the 1960s involved physiological characterization of photosystem synthesis under different growth conditions. In the 1970s Barry Marrs and coworkers developed genetic tools that were used to define and map genes needed for synthesis of photopigments. The 1980s was a period of cloning and physical mapping of photosynthesis genes onto the chromosome, the demonstration that regulation of photosystem synthesis involved Regulation of gene expression , and sequence analysis of photosynthesis genes. The 1990s was a period of the discovery and characterization of regulatory genes that control synthesis of the photosystem in response to alterations in oxygen tension and light intensity. Although several photosynthetic organisms are mentioned for comparison and contrast, the focus of this minireview is on Rhodobacter capsulatus.



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4 Re: Photosystem I and II on Sat 8 Mar 2014 - 12:27

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photosystem II subunits CP47 (white) and PsbH (blue)



Top view on photosystem II subunits CP47 (white) and PsbH (blue) with bound chlorophyll (green) and β-carotene (orange) molecules.  Function of the CP47 ‘antenna‘ protein is to direct the energy of photons into the reaction center of the photosystem II. Structure of the monomeric photosystem II is shown in the right figure

Our work is aimed to elucidate how is the chlorophyll biosynthetic pathway controlled by the photosynthetic cell, what way is this pigment built into photosynthetic complexes and what is the fate of chlorophylls once these complexes are degraded.



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5 Re: Photosystem I and II on Sun 9 Mar 2014 - 13:48

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Some questions that proponents of evolution might to answer in regard of Photosystems :

http://reasonandscience.heavenforum.org/t1544-photosystem-i-and-ii

the history of photosynthesis development is hard to trace. It's difficult to conceptualize, let alone trace, when explained by a paradigm of purposelessness and mindless direction. One reason for this is the interdependency of different cellular functions. For example, ATP is utilized during photosynthesis. So are by-products of metabolic pathways. If one assumes all related processes (including the Calvin Cycle) evolved prior to photosynthesis, chicken-egg ( irreducible complexity ) dilemnas arise. They also present themselves if the opposite is assumed. The interdependency of basic universal pathways is strong evidence for design.

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."

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.

http://www.sciencemag.org/site/feature/data/prizes/ge/2006/loll.xhtml#1

Take for example Photosystem II. Without it, life on earth would cease to exist. Despite rarely mentioned in the creationism - naturalism debate,i regard it as the PRIMA FACIE example to be studied and debated. How did it arise , and why ? PSII is at the heart of photosynthesis, and catalyzes the thermodynamically most demanding reaction in biological systems, the splitting of water into oxygen and reducing equivalents. The monomer of this bewilderingly complex machinery consists of 20 different protein subunits that bind 77 organic cofactors and seven metal ions. In the reaction center and light-harvesting antenna proteins, a large number of different molecules (35 chlorophyll a, 11 carotenoid, 2 pheophytin a, 2 plastoquinone, 2 heme, and 1 bicarbonate), as well as four manganese ions, two calcium ions, and an iron ion, are held at precise distances and relative orientations that are required for optimal absorption and conversion of light energy to perform efficient electron transfer.

Science daily describes it as :

http://www.sciencedaily.com/releases/2002/11/021122074236.htm

Work Of Ancient Genetic Engineering ( huh!, didn't know evolution had this miraculous capabilities..... )

During the normal course of PSII function, its proteins become damaged by the electrons zipping through the complex. A sophisticated mechanism exists for recognizing the damaged protein, removing it and replacing it with a freshly made protein. There are lots of details still to be determined as far as how his process works and how it may be regulated. Researchers are also working to tease apart the differences and/or overlap between the de novo assembly pathway and the damage-repair cycle in the Life Cycle of PSII  3)

http://newunderthesunblog.wordpress.com/the-basics/the-light-reactions/photosystem-ii/

Explanations of scientific papers of how it could have evolved is guesswork at best.

http://www.fceqyn.unam.edu.ar/~celular/paginas/Articulos%20Biol%20Cel%202004/Annual%20Reviews/Review%20photosyntesis.pdf

The evolutionary path of type I and type II reaction center apoproteins is still unresolved owing to the fact that a unified evolutionary tree cannot be generated for these divergent reaction center subunits.

http://www.ebi.ac.uk/interpro/potm/2004_11/Page3.htm

The proteins that make up photosystem II are highly diverse in both sequence and structure.  Consequently, these proteins are placed in several different InterPro families, all of which are listed in the table of PSII proteins.  The description below is for the PSII reaction centre protein D1 (also known as PsbA or QB).

https://www.academia.edu/3742566/Photosynthesis_and_the_Origin_of_Life

The early atmosphere of the earth is considered to have been neutral (nitrogen and carbon dioxide). The problem arises of how oxygenic photosynthesis could have evolved under these conditions.There were abundant reducing agents such asferrous ion which made it unlikely that water would be used as an electron donor.

http://creation.com/shining-light-on-the-evolution-of-photosynthesis

PSII produces extremely strong oxidizing agents that can pull electrons out of water, but it is not capable of reducing NADP+ to NADPH. PSI produces extremely strong reducing agents that ultimately do the job of reducing ferredoxin and NADP+. Neither system does anything meaningful apart from the other, which is to say, nothing works unless everything works. The

proponents of evolution must explain :

1. Where the genetic information came from to make PSII, the cofactors, the assembly enzymes, the repair enzymes, how it was assembled in a highly coordinated manner,and how it would have had a survival advantage, without being fully developed,and working in a coordenated way together with the other enzymes and protein complexes. Good luck with that.



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6 Re: Photosystem I and II on Sun 9 Mar 2014 - 13:53

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Photosystem II

Events in the photosystem are as follows:

1. Absorption of a photon by the light harvesting system. The photon is funneled to a reaction center chlorophyll designated P680 . See picture below.

2. Excitation of P680 raises the molecule from the ground state to an excited state at -0.8 volt.

3. The excited P680 is able to quickly transfer an electron from P680 to a lower-energy primary electron acceptor, such as pheophytin a (Ph)

4. The electron is transferred to a series of plastoquinone molecules (Plastoquinone QA and QB) associated with PSII proteins.

5. Two protons are picked up by QB and the reduced plastoquinone, QH2 (plastoquinol), is released into the lipid portion of the thylakoid membrane. The overall reduction of plastoquinone is shown here.

6. Plastoquinol interacts with the membrane-bound cytochrome b6f complex which contains cytochromes and iron-sulfur proteins.

7. The b6f complex donates electrons to a copper protein called plastocyanin (PC). The oxidation of plastoquinone results in release of two protons into the thylakoid lumen.



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7 In pursuit of plant power on Sun 9 Mar 2014 - 20:00

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In pursuit of plant power


the chemistry of nature’s highly complex and efficient photosynthetic apparatus has not even been fully worked out yet, let alone copied! We have periodically reported on the new discoveries in recent years of the brilliant design of the plant photosynthetic apparatus and associated structures. For example, plants have a ‘dimmer switch’, super-sensitive to changes in light conditions, and plants also ‘know’ when to make ‘sunscreen’ when conditions warrant.

One of the major hurdles to solve is the mystery of how plants manage to break apart the water molecule into hydrogen and oxygen without destroying themselves in the process. (Just think of the 1937 Hindenburg disaster where a Zeppelin caught fire, and burned the hydrogen gas into water. To break up water, this amount of energy has to be ‘put back’.)

As a 1999 article in New Scientist, highlighting a limited breakthrough made by Yale University chemists Gary Brudvig and Robert Crabtree, explained:

   “The oxygen is released by splitting water molecules, a reaction that in the lab requires such violence that it would tear any living system apart. So how can plants do it, using only energy from the sun?

   “Try breaking water apart in one go and you’d need a huge blast of energy. To do the job with heat alone, for example, you’d need to raise the temperature of water by thousands of degrees—more than enough to vapourise a geranium.”



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8 Photosystem I on Sun 9 Mar 2014 - 20:04

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Photosystem I

Photosystem I (PS I) (or plastocyanin: ferredoxin oxidoreductase) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known. Louis Duysens first proposed the concepts of photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a cohesive theory of serial photosynthetic reactions. Hill and Bendall’s hypothesis was later justified in experiments conducted in 1961 by Duysens and Witt groups.

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9 New Under The Sun Blog Photosystem II on Sun 9 Mar 2014 - 20:09

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New Under The Sun Blog  Photosystem II



The Photosystem II (PSII) is enzyme at the beginning of the photosynthetic light reactions that splits water into protons and oxygen. Refer to the figure above to orient yourself and PSII within the photosynthetic electron transfer chain.

PSII is by far my favorite enzyme and I have spent most of my research career focused on how it works and how this complex molecular machine gets put together. Light energy is absorbed by antenna complexes and funneled into the PSII complex reaction center P680. The reaction center is a special pair of chlorophyll molecules buried in the heart of this multisubunit membrane protein complex. These chlorophylls are poised in just the right environment relative to other cofactors such that when energy is absorbed by them, charge separation occurs. This means that light causes the movement of electrons (as opposed to energy transfer along antenna systems flowing energetically downhill). When an electron has been separated from the reaction center, it travels through several other cofactors within PSII and finally onto a Plastoquinone molecule (PQ). PQ is a small organic molecule that can move within the thylakoid membrane and carry its electrons over to the next complex in the light reactions (cytochrome b6f). With the light energy causing electrons to be displaced from the reaction center, PSII must replace those electrons from somewhere. It does this by pulling electrons from water molecules. In order to balance the reaction for products and reactants, PSII sequentially pulls four electrons from two water molecules to make oxygen, release 4 protons and make 2 PQH2 molecules (that’s just PQ with 2 electrons). PSII performs this water-splitting reaction using an inorganic manganese-calcium-chloride cluster buried on the lumenal face of the enzyme.



Photosystem II Structure

Here is a very simple cartoon of PSII showing the main functional parts of the enzyme. It is comprised of more than 20 separate proteins, many of which are membrane proteins. It also contains cofactors for capturing light energy and transferring electrons from water to plastoquinone. It can be divided into two functional parts, (1) the electron transfer domain and (2) the water oxidation complex. The portion of the enzyme within the thylakoid membrane contains the components of the electron transfer pathway. The water oxidation complex, where water-splitting occurs, is found on the lumenal side of the complex.

A number of PSII structural models based on x-ray diffraction data have been reported over the last decade with increasing resolution. These models are based on the enzyme from thermophilic cyanobacteria. Each model contains about 20 protein subunits along with a slew of associated cofactors (pigments, ions, metal cofactors, lipid molecules) with a total molecular weight of ~350 kDa per monomer (single unit). This represents a lot of hard work by more than a few talented scientists.

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10 Re: Photosystem I and II on Sun 9 Mar 2014 - 20:13

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http://www.sciencedaily.com/releases/2002/11/021122074236.htm

Photosynthesis is one of the most important chemical processes ever developed by life — a chemical process that transforms sunlight into chemical energy, ultimately powering virtually all the living things and allowing them to dominate the earth. The evolution of aerobic photosynthesis in bacteria is also the most likely reason for the development of an oxygen-rich atmosphere that transformed the chemistry of the Earth billions of years ago, further triggering the evolution of complex life. After decades of research, biochemists now understand that this critical biological process depends on some very elaborate and rapid chemistry involving a series of enormously large and complex molecules a set of complex molecular systems all working together.

the history of photosynthesis development is hard to trace. It's difficult to conceptualize, let alone trace, when explained by a paradigm of purposelessness and mindless direction. One reason for this is the interdependency of different cellular functions. For example, ATP is utilized during photosynthesis. So are by-products of metabolic pathways. If one assumes all related processes (including the Calvin Cycle) evolved prior to photosynthesis, chicken-egg ( irreducible complexity ) dilemnas arise. They also present themselves if the opposite is assumed. The interdependency of basic universal pathways is strong evidence for design.


http://www.sciencedaily.com/releases/2012/02/120221125409.htm

Atmospheric oxygen really took off on our planet about 2.4 billion years ago during the Great Oxygenation Event. At this key juncture of our planet's evolution, species had either to learn to cope with this poison that was produced by photosynthesizing cyanobacteria or they went extinct. It now seems strange to think that the gas that sustains much of modern life had such a distasteful beginning.

So how and when did the ability to produce oxygen by harnessing sunlight enter the eukaryotic domain, that includes humans, plants, and most recognizable, multicellular life forms? One of the fundamental steps in the evolution of our planet was the development of photosynthesis in eukaryotes through the process of endosymbiosis.

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11 assembly of photosystem II on Mon 10 Mar 2014 - 2:35

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Photosystem II (PSII), is a highly multi-subunit pigment protein complex found in cyanobacteria and chloroplasts. Its  composed of

- 17 intrinsic  sub-units
-   3 extrinsic sub-units
-    cofactors
-  35 chlorophylls
-  2   pheophytins
- 11 β-carotenes
- > 20 lipids
- plastoquinones
- 2 heme irons
- 1 non-heme iron
- 4 manganese atoms,
- 4 calcium atoms
- 3 Cl- ions per monomer

Assembly of PSII is highly co-ordinated and proceeds through a number of distinct assembly intermediates. Associated with these assembly complexes are proteins that are not found in the final functional PSII complex. Structural information and possible functions are beginning to emerge for several of these ‘assembly’ factors, notably Ycf48/Hcf136 , Psb27 and Psb28. 1)

PSII is also prone to irreversible damage by sunlight at all light intensities. A PSII repair cycle operates to selectively remove and  replace damaged subunits, primarily the D1 subunit, within PSII thereby helping to maintain PSII activity.

Modular assembly of PSII

PSII seems to be assembled from smaller PSII subcomplexes (or modules) via a series of distinct intermediates.

The cyanobacterial homologue of HCF136/YCF48 is a component of an early photosystem II assembly complex and is important for both the efficient assembly and repair of photosystem II in Synechocystis sp. PCC 6803.  2)  YCF48 was proposed to act as an assembly factor that is transiently bound to PSII assembly intermediate(s), but is not a component of fully assembled monomeric and dimeric core complexes

Some accessory protein factors are components of assembly complexes

A number of auxiliary proteins have been identified that play a role in the assembly/repair or function of PSII, but which are absent in the crystallised complex.

YCF48 (called HCF136 in Arabidopsis thaliana ) binds to the D1 precursor protein (pD1) and promotes formation
of the PSII RC assembly complex








1) http://aob.oxfordjournals.org/content/106/1/1.full
2)http://www.jbc.org/content/283/33/22390.full.pdf



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12 Re: Photosystem I and II on Tue 1 Apr 2014 - 0:56

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Components of PSI:

PsaA/B
PsaC
PsaF/J
PsaI/L
PsaK
chlorophyll a
chlorophyll belonging to the electron transfer system
chlorophyll connecting the electron transfer system to the antenna
Fe4S4 cluster in PsaA/B
Fe4S4 cluster in PsaC

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13 Re: Photosystem I and II on Fri 5 Feb 2016 - 22:39

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Used by plants, algae, and cyanobacteria, the protein photosystem II captures the energy from sunlight and uses it to extract electrons from water molecules. The first eukaryotic photosystem II structure in the PDB reveals an additional protein (in pink) that stabilizes the intricate assembly of photosystem II’s subunits.

Explore this 3D structure of oxygen-evolving photosystem II from a red alga at RCSB PDB:

http://www.rcsb.org/pdb/explore/explore.do?structureId=4YUU

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Evolution of Photosystem II - an enigma - or simply design ? 

http://reasonandscience.heavenforum.org/t1544-photosystem-i-and-ii#5718

The process of photosynthesis is a very complex set of interdependent metabolic pathways,” said Robert Blankenship, professor of biochemistry at Arizona State University 1 This critical biological process depends on some very elaborate and rapid chemistry involving a series of enormously large and complex molecules a set of complex molecular systems all working together.

Photosystem II. Without it, life on earth would cease to exist. How did it emerge? What must be explained in regard of its origin? It catalyzes the thermodynamically most demanding reaction in biological systems, the splitting of water into oxygen and reducing equivalents. It can be divided into two functional parts, (1) the electron transfer domain and (2) the water oxidation complex. The monomer of this bewilderingly complex machinery consists of 20 different protein subunits that bind 77 organic cofactors and seven metal ions. In the reaction center and light-harvesting antenna proteins, a large number of different molecules (35 chlorophyll a, 11 carotenoid, 2 pheophytin a, 2 plastoquinone, 2 heme, and 1 bicarbonate), as well as four manganese ions, two calcium ions, and an iron ion, are held at precise distances and relative orientations that are required for optimal absorption and conversion of light energy to perform efficient electron transfer.

- assembly and degradation of the complex holo-enzyme - molecular machines
- The origin of assembly co-factor proteins, only used for assembly of these molecular complexes
- the origin of  Photosystem II  40 permanent proteins, which must be assembled in the right manner, while other proteins are expressed or associated only in stress conditions or during assembly and/or degradation
- In higher plants and algae, PSII is composed of two moieties: the core complex that contains all the cofactors of the electron transport chain, and the outer antenna, which increases the light-harvesting capacity of the core
- both moieties are essential and irreducible, and the assembly of both depends on highly complex, ordered proceedings which must be correctly encoded in the genome from the beginning.
- molecular super-complexes are often composed of metallic cofactors, in their active site. Photosystem II for example, contains the oxygen-evolving complex in the core structure, that generates the redox potential required to drive water splitting. This Metal-cluster has around it a protein called D1 which provides most of the ligands to the Mn4CaO5. Both, the D1 subunit, and the ligands are essential.

 

- The assembly of this metal cluster is a light-driven process called photoactivation
- It is a highly complex process, requiring for the assembly is an oxidative process that involves removal of electrons  from the Mn ions and the formation of oxo-bridges between the metals of the cluster with the bridging oxygen  atoms.  The photoassembly process was found to have a high complexity and up to now, there is no completed model for this process.

Maintenance of the highly dynamic Mn4CaO5 cluster also requires the delivery of a constant supply of the proper levels of Mn2+ and Ca2+

1. https://uncommondescent.com/intelligent-design/is-photosynthesis-irreducibly-complex/#more-2255

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