File:P450cycle.svg
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[edit]The factual accuracy of this diagram or the file name is disputed.
Reason: See https://en.wikipedia.org/wiki/User_talk:Slashme/Archive_6#P450_Mechanism - not completely useless, but should be used with care until it's fixed. For usage see: |
DescriptionP450cycle.svg |
English: ==The P450 catalytic cycle==
1: The substrate binds to the active site of the enzyme, in close proximity to the heme group, on the side opposite to the peptide chain. The bound substrate induces a change in the conformation of the active site, displacing a water molecule from the distal axial coordination position of the heme iron[1] changing the state of the heme iron from low-spin to high-spin[2]. This gives rise to a change in the spectral properties of the enzyme, with an increase in absorbance at 390~nm and a decrease at 420~nm. This can be measured by difference spectrometry and is referred to as the "type~I" difference spectrum (see inset graph in figure). Some substrates cause an opposite change in spectral properties, a "reverse type~I" spectrum, by processes that are as yet unclear. Inhibitors and certain substrates that bind directly to the heme iron give rise to the type~II difference spectrum, with a maximum at 430~nm and a minimum at 390~nm (see inset graph in figure). If no reducing equivalents are available, this complex remains stable, allowing the degree of binding to be determined from absorbance measurements in vitro[3] 2: The change in the electronic state of the active site favours the transfer of an electron from NAD(P)H[4]. This takes place via the electron transfer chain, as described above, reducing the ferric heme iron to the ferrous state. 3: Molecular oxygen binds covalently to the distal axial coordination position of the heme iron. The cysteine ligand is a better electron donor than histidine, with the oxygen consequently being activated to a greater extent than in other heme proteins. However, this sometimes allows the bond to dissociate, the so-called "decoupling reaction", releasing a reactive superoxide radical, interrupting the catalytic cycle[1]. 4: A second electron is transferred via the electron-transport system, reducing the dioxygen adduct to a negatively charged peroxo group. This is a short-lived intermediate state. 5: The peroxo group formed in step 4 is rapidly protonated twice by local transfer from surrounding amino-acid side chains, releasing one mole of water, and forming a highly reactive iron(V)-oxo species[1]. 6: Depending on the substrate and enzyme involved, P450 enzymes can catalyse any of a wide variety of reactions. A hypothetical hydroxylation is shown in this illustration. After the product has been released from the active site, the enzyme returns to its original state, with a water molecule returning to occupy the distal coordination position of the iron nucleus. S An alternative route for mono-oxygenation is via the "peroxide shunt": interaction with single-oxygen donors such as peroxides and hypochlorites can lead directly to the formation of the iron-oxo intermediate, allowing the catalytic cycle to be completed without going through steps 3, 4 and 5[3]. A hypothetical peroxide "XOOH" is shown in the diagram. C: If carbon monoxide (CO) binds to reduced P450, the catalytic cycle is interrupted. This reaction yields the classic CO difference spectrum with a maximum at 450 nm.
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Date | |
Source | M.Sc. Thesis, David Richfield (User:Slashme) |
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Slashme at English Wikipedia When using this image in external works, it may be cited as follows:
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current | 15:07, 22 April 2012 | 9,240 × 6,968 (38 KB) | Slashme (talk | contribs) | {{Information |Description ={{en|1====The P450 catalytic cycle== 1: The substrate binds to the active site of the enzyme, in close proximity to the heme group, on the side opposite to the peptide chain. The bound substrate induces a change in the ... | |
14:49, 22 April 2012 | No thumbnail | 9,240 × 6,968 (38 KB) | Slashme (talk | contribs) | Corrected peroxide shunt arrow. | |
10:34, 5 July 2008 | 9,240 × 6,968 (35 KB) | Slashme (talk | contribs) | {{Information |Description={{en|1===The P450 catalytic cycle== 1: The substrate binds to the active site of the enzyme, in close proximity to the heme group, on the side opposite to the peptide chain. The bound substrate induces a change in the conforma |
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