Talk:Cytochrome c oxidase
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Article Grading: The following comments were left by the quality and importance raters: (edit · refresh) I have added a crucial reference to the Science paper giving the crystal structure of bovine cytochrome oxidase, and added details to the mechanism of electron transfer and the cycle which reduces oxygen to two waters. I do believe that this article should incorporate some of the details from the article on COX biogenesis, and then the latter deleted. I am not up on the details of this other article and will leave it up to someone else to decide what to move over to this article. GraybeardBiochemist 23:52, 28 October 2007 (UTC) |
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Article Grading: The following comments were left by the quality and importance raters: (edit · history · refresh · how to use this template) I have added a crucial reference to the Science paper giving the crystal structure of bovine cytochrome oxidase, and added details to the mechanism of electron transfer and the cycle which reduces oxygen to two waters. I do believe that this article should incorporate some of the details from the article on COX biogenesis, and then the latter deleted. I am not up on the details of this other article and will leave it up to someone else to decide what to move over to this article. GraybeardBiochemist 23:52, 28 October 2007 (UTC) |
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This sentence (as of 2/8/2007) is not accurate and probably should be rewritten.
"Cyanide, sulfide, azide and carbon monoxide all bind to Cytochrome c Oxidase, thus inhibiting the protein from functioning which results in chemical suffocation of cells."
Cytochrome c oxididase is a ferric heme protein in the resting state. While cyanide binds tightly to ferric proteins, carbon monoxide binds tightly to ferrous heme proteins. While the mechanism of action of CO is to suffocate people by virtue of its action on ferrous heme oxygen transport proteins (i.e. hemoglobin and myoglobin), it's not a large player at the oxidase level.
Cyanide and those agents which bind tightly to CCO cause an immediate halt of aerobic respiration, and because of the buildup of chemical intermediates, will eventually cause the cessation of aerobic and anaerobic processes.
I'm complaining because there is a constant confusion among lay people (and even scientists not familiar with the area) about the mechanism of toxicity in vivo of CO and CN-. They are not the same and people trying to make this topic easier may in fact be obscuring real facts.
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"There are 2 molecules water gained in the Cytochrome C oxidase catalized reaction and not only one!"
Ions are usually written with the number first, then the polarity of charge, i.e. Fe2+ or Fe3+, rather than Fe+2/+3 as is written here. Man I gotta get around to learnin LaTeX 203.129.49.176 20:21, 17 May 2006 (UTC)
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This traces the energy, electrons, and photons when light reverses the respiratory action and act as a fuel cell to generate 4H+ and O2 from H20 to slow the krebs cycle when animals are in bright sunlight.
Normal operation of 3rd pump (CcO): electrons are brought in through the prior pumps that received e- energy from the krebs cycle. At the active pumping site of CcO, 2e- are bought in to attract 2H+ in tunnel A and 2H+ in tunnel B. O2 is spread apart in the iron-based active site so that the 2e- and 2H+ of tunnel B react with one oxygen to form H20. This releases energy that boots the 2H+ in tunnel A on up the gradient to the outside of the membrane. Since the 2e- that were being used to hold the 2H+ in place are now inside the H2O, there is nothing holding 2H+ in the active area. The exact mechanism of this second step of booting up the gradient is not known. Another 2e- are waiting in the wings (a 2 copper site) to react with the other oxygen in the same manner with the same result (H2O and a booting of another 2H+ upwards). Summary: O2+4e+8H => H2O + H2O + 4H+ where 4H+ is the energy gradient increase.
Instead of forming H2O to use electrons and release energy, let's run it in reverse and let light energy breaks apart H2O to release electrons. There are 5 metal atoms in this protien that do everything. 3 are at the active site, 2 irons and a copper. Two more coopers are at a "staging" area for bringing electrons in from the "electron food chain". Only the coppers absorb light strongly in the near-infrared, which can penetrate blood and water. In fact, it appears hemoglobin evolved to specifically allow these frequencies through since they were already being used for energy in bacteria. The copper at the 2-iron main reaction (pumping) site absorbs strongest at 670 nm (red) which is a higher energy level than what the other two coppers absorb at near-infrared). 1/4 or so of the red photon energy is absorbed and splits H2O (fuel cell!) to generate the 2e- that are needed at the reaction site to pull in 2H+ up tunnel A. This creates 2H+ in tunnel "B" from the H2O and the split oxygen is being held in one of the irons. Then the 2e- combine with the 2H+ in tunnel B and the oxygen held in the iron to recreate the H20 which releases the rest of the light energy the system had temporarily stored which boots the tunnel A 2H+ on up the gradient. So no electrons or ANYTHING ELSE comes in or out of the system, but 4H+ are delivered up the gradient thanks to light energy. The other 2 coppers that are outside of the 2 active iron site may also get into the act by absorbing light to push 2 more temporary electrons against an H+ gradient to bring them to the active site, but i don't know if it's necessary for the light-pumping. These wold also be cycled back and forth without using anything but light energy.
Therefore this cyctochrome c oxidase is not merely a respiration-based pump. Its organization and use of copper is not merely an evolutionary left over from bacteria that used light. It is an active photoreceptor providing possibly up to 20% of the energy to people laying out in the sun. Smaller animals with little hair will see the greatest benefit since the light only penetrate a few centimeters in the windo between the absorption of blood and water.
But the story isn't finished: When light has pushed a lot of H+ outside the mitochondrial membrane, the H+ gradient is stronger and there is an increased pull towards the 2 staging coppers to pull e- out of the reaction site. This prevents the 2e- from absorbing back into the H2O which leaves 2H+ in tunnel B. O2 is created if it occurs twice (2H2O + 2 red photons => 4H+, 4e-, and O2). The 2e- in the 2-copper staging area absorbs light energy which might be used to jolt e- back into cytochrome c to be used for the previous pumps to act in reverse, as they are known to do. As the H+ gradient increases, the first 2 pumps that provided e- to our 3rd pump (under food conditions) start wanting to act in reverse (they are basically "passive" pumps known to act equally well in reverse). By providing the 2e- necessary, these pumps will recreate NADH and FADH food energy and deplete some of the H+ gradient and recreate NADH or FADH. The other two pumps do not absorb light energy. CcO is blue in color like some bacteria as a result of the coppers that are absorbing blue's opposing color, red (and infrared).
The increased H+ gradient creates more ATP, and the reversed pumps create more NADH and FADH. This halts the krebs cycle which increases the available pyruvate and therefore glucose so that energy stores are increased for time periods longer than ATP.
Only in recent years has the functioning of CcO been understood well enough to show how it could be run in reverse.
From a PhD thesis:
"Tyrosine residues in photosynthetic plants ...have been shown to ... oxidize H2O to O2 which is the reverse of CcO. Therefore [this tyrosine residue at the center of CcO] is presumed to play an important role in [here comes the blah blah blah that just misses the "reversibility" point] in electron redistribution and electron transfer at the active site" There it is, right under their nose, screaming at them "i am capable of photosythesis". A single red photon hitting the copper that's right next to this tyrosine residue is all that's needed to split the H2O to O2. Right before that paragraph she wrote that the tyrosine residue is "highly conservative" which means "easily reversible"
Scott
[edit] Terminal Electron Acceptor
Perhaps I'm wrong, but I've been educated that the terminal electron acceptor is oxygen (or another inorganic molecule such as Fe3+, NO3-, etc.), and that cytochrome oxidase catalyzes this reaction (not that cytochrome oxidase IS the terminal electron acceptor). In the sense that cytochrome oxidase is the final protein in the ETC to have the electron it is the terminal acceptor, but it is merely passing it off to it's final acceptor, oxygen. Therefore, I think that calling it the terminal electron acceptor is misleading and this should be reworded.
