Cytochrome c: Difference between revisions

== ''Rhodothermus marinus'' cytochrome ''c'' ==

<scene name='Sandbox_Reserved_335/Heme/1'>'Figure 1. The heme group of monoheme cytochrome ''c'' purified from ''Rhodothermus marinus''</scene>

Monoheme cytochromes ''c'' are involved in electron transport chains in both prokaryotes and eukaryotic mitochondria.<ref name=main /> They mediate the transfer of electrons mainly from the ''bc''₁ complexes or their analogs to heme-copper oxygen reductases (HCOs) in the [http://en.wikipedia.org/wiki/Electron_transport_chain electron transport chain] of [http://en.wikipedia.org/wiki/Oxidative_phosphorylation oxidative phosphorylation]. Heme ''c'' containing domains are often found fused to other protein domains such as these HCOs, including the ''caa''₃ oxygen reductases<ref name=main /><ref>PMID:14691678</ref>; these enzymes are membrane-bound and catalyze the reduction of O₂ to water.<ref>PMID:11334784</ref> In addition to being involved in oxidative phosphorylation, monoheme cyt ''c'' has also been seen to participate in the electron transport chain of [http://en.wikipedia.org/wiki/Photosynthesis photosynthesis].<ref name=main /> Cytochrome ''c'' has also been determined to be a major signalling molecule in the apoptotic pathways.

In the electron transport chain (ETC), cyt ''c'' shuttles electrons between the respiratory complexes III and IV; complex III is the cytochrome ''bc''₁ complex and IV is cyt ''c'' oxidase. Initially, the heme iron in cyt ''c'' is in the reduced, Fe³⁺ state; this allows for the uptake of one electron, oxidizing the iron to the Fe²⁺ state.<ref name='etc'>Karp, Gerald (2008). Cell and Molecular Biology (5th edition). Hoboken, NJ: John Wiley & Sons. ISBN 978-0470042175.</ref> The ETC in eukaryotes is quite simple compared to that of prokaryotes (Figure 3). [[Image:Etc.gif |frame|left| |size|300| Figure 3. The electron transport chain of a) eukaryotes as compared to b) prokaryotes.]] In prokaryotic systems, electrons can enter the ETC at a number of places and multiple donors can be in play; however, the underlying transport system remains the same. Electrons are ultimately transferred from donor to various redox complexes including the ''bc''₁ complex and cytochrome ''c'', and finally to a terminal electron acceptor such as molecular oxygen in eukaryotes.<ref name=etc />

The cytochrome oxidase reaction accounts for nearly 90% of all oxygen uptake in most cells.<ref name=etc /> Due to the large role of cytochromes within the ETC, it would be highly detrimental to the cell if any inhibitors were to be present in the organism. Cyanide and azide bind tightly to the cytochrome oxidase complex, halting electron transport and reducing the overall ATP production.<ref name=etc />

In all organisms, cells undergo [http://en.wikipedia.org/wiki/Apoptosis apoptosis], or programmed cell death, by which there is an extrinsic and an intrinsic pathway. The extrinsic pathway involves an immune response by killer lymphocytes, and once the lymphocyte has been bound to the target cell, an apoptotic cascade occurs.<ref name=etc /> The intrinsic pathway includes cyt ''c'', present in the intermembrane space of mitochondria. In this pathway, the presence of an apoptotic stimulus causes cyt ''c'' to be released into the cytosol. Cytochrome ''c'' in the cytosol now can be recognized and bound to various apoptotic factors, activating them and forming the [http://en.wikipedia.org/wiki/Apoptosome apoptosome]. The apoptosome recruits [http://en.wikipedia.org/wiki/Caspase caspases], which are activated and result in a caspase cascade to proceed with apoptosis.<ref name=etc />