Protein cytochromes

All tissues except mature red blood cells are able to manufacture haem for use in the respiratory cytochrome proteins of the electron transport chain. However, the liver is an especially important site of haem synthesis because it (a) is a major organ of erythropoiesis in utero and (b) haem-containing cytochrome-P450 (CYP-450) enzymes play significant roles in hepatic detoxification of drugs, toxins and endogenous waste products (Section 6.4). 

The following description of the electron transfer-proton transport scheme is illustrated in Figure 7.26. First, an electron is transferred from doubly reduced dihydroplastoquinone (PQFI2) to a high potential electron transfer chain that consists of the Reiske iron-sulfur protein and the cytochrome protein containing heme f. Rappaport,Lavergne and co-workers have reported a midpoint potential at pH 7.0 of +355 mV for heme f. These two centers reside on the electropositive (lumen or p) side of the membrane, exterior to the membrane. As a result, two protons are transferred to the aqueous lumen phase. A second electron is transferred from PQH2 sequentially to heme bp. 

A purified cytochrome protein isolated from a bacterial preparation was found to contain 0.376% iron. What can be deduced about the molar mass of the protein ... 

V. General Comparison of Membrane-Integral Diheme Cytochrome Proteins 139... 

Here (BChl)2 stands for the special pair of bacteriochlorophyll molecules, UQ for ubiquinone, and Cyt for the cytochrome protein. Steps 1 and 3 involve excitation of bacteriochlorophyll and transfer of a pair of electrons to a ubiquinone molecule. Steps 2 and 4 restore the special pair to its initial state. Steps 5 and 6 transfer hydrogen ions outside the membrane wall and restore the cytochrome to its reduced form. The net reaction is the light-driven movement of hydrogen ions from inside the cell to outside the cell. 

Favia AD, Cavalli A, Masetti M, Carotti A, Recanatini M. Three-dimensional model of the human aromatase enzyme and density functional parameterization of the iron-containing protoporphyrin IX for a molecular dynamics study of heme-cysteinato cytochromes. Proteins 2006 62 1074-87. 

Buffer pH Temp. Initial concn. cytochrome protein/ml s Vmg protein/3 ml Ref. 

What is the nature of the iron-containing proteins of eiectron transport A number of iron-containing proteins are part of the electron transport chain. In the cytochrome proteins, the iron is bound to a heme group. In other proteins, the iron is bound to the protein along with sulfur. 

The determination of the three-dimensional structure naturally solved most of the structural questions. The pigments (6 porphyrins and 2 quinones) are organized in two branches, A and B, within the protein in almost perfect C2 symmetry, see Fig.l. The donor P is a bacteriochlorophyll (BChl) dimer. Da and Db (i.e. BChl moieties closely related to the A or B branch, respectively), and interacts with the two quinone (Q) acceptors, via a BChl monomer, Ba, and a bacteriopheophytin (BPh) monomer, Oa- is the intermediate acceptor I and transfers the electron to Qa, the primary, and via a high-spin iron to Qb, the secondary acceptor, which communicates with a quinone pool in the lipid membrane. P+, in turn, is re-reduced from an associated heme system on a close-by cytochrome protein. Cytochrome and the quinone pool are not depicted in Fig.l. 

Direct electron transfer via c-type cytochromes proteins on the outer membrane of the bacteria ... 

In the biofilm, the oxidized mediator, M, is reduced by cells at a rate proportional to the utilization of substrate, It is speculated that redox mediators, such as flavins, can interact with outer membrane c-type cytochrome proteins, for example, MtrC and OmcA, receiving electrons from the periplasmic cytochrome MtrA and transferring them to terminal electron acceptors outside of the cell. In this case, it is assumed... 

When a lot of respiration happens, ROS increase enough to kick a heme off one cytochrome protein and move it to an empty catalase protein, which then helps protect the area from stray ROS. 

Electron transport from Q to P700+ is mediated by a quinone, iron-sulfur, and a cytochrome protein complex in the thylakoid membrane. This protein, the cytochrome 6 / complex, is remarkably similar to the cytochrome bc complex of the mitochondrial electron transport chain. 

The RC is made of three polypeptides L, M, H (MW 32,34, and 28 kDa, respectively). L and M make up the core of the reaction center. They carry the cofactors four bacteriochlorophylls, two bacteriopheo-phytins, two quinones of species-dependent chemical nature, and one nonheme Fe + atom. In many species of bacteria, the RC also includes a bound tetraheme c-type cytochrome of about 12 kDa MW (when it is absent, its function of electron donation to P is fulfilled by a soluble c-type cytochrome). Proteins and cofactors are organized, as sketched in Figure 118.1. The RC is endowed with an approximate Cj symmetry in structure that is not found in the function. The primary donor P is a dimer of bacteriochlorophylls, a so-called special pair, in that the two molecules are held in close proximity by the I and M subunits they are in electronic interaction without establishing any chemical link. Recent developments in the structural analysis of RCs dealt with refinements of the structure and with possible structural changes coupled to electron transfer. - ... 

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