Flavin mononucleotide electron transport chain

FIGURE 32-7 Sources of free radical formation which may contribute to injury during ischemia-reperfusion. Nitric oxide synthase, the mitochondrial electron-transport chain and metabolism of arachidonic acid are among the likely contributors. CaM, calcium/calmodulin FAD, flavin adenine dinucleotide FMN, flavin mononucleotide HtT, tetrahydrobiopterin HETES, hydroxyeicosatetraenoic acids L, lipid alkoxyl radical LOO, lipid peroxyl radical NO, nitric oxide 0 "2, superoxide radical. 

Figure 4-13. The electron transport chain and oxidative phosphorylation. Heavy arrows indicate the flow of electrons. Fe-S = iron-sulfur centers FMN = flavin mononucleotide CoQ = coenzyme Q (ubiquinone) Cyt = cytochrome. nH+ indicates that an undetermined number of protons are pumped from the matrix to the cytosolic side. The numbers at the top of the figure correspond to the three major stages of electron transfer described in the text in V B.

The first electron carrier in the electron transport chain is an enzyme that contains a tightly bound coenzyme. The coenzyme has a structure similar to FAD. The enzyme formed by the combination of this coenzyme with a protein is called flavin mononucleotide (FMN). Two electrons and one ion from NADH plus another H ion from a mitochondrion pass to FMN, then to an iron-sulfur (Fe—S) protein, and then to coenzyme Q (CoQ). CoQ is also the entry point into the electron transport chain for the two electrons and two H ions from FADH2. As NADH and FADH2 release their hydrogen atoms and electrons, NAD and FAD are regenerated for reuse in the citric acid cycle. 

There are many carrier molecules for electrons one is called the nicotinamide adenine dinucleotide (NAD ) and another is the flavin adenine dinucleotide (FAD ). The reduced cofactors NADH and FADH2 transfer electrons to the electron transport chain. Flavin mononucleotide receives electrons from NADH and passes them to coenzyme Q through Fe-S systems. Coenzyme Q receives electrons from flavin mononucleotide and FADH2 through Fe-S systems. Cytochromes receive electrons from the reduced form of coenzyme Q. Each cytochrome consists of a heme group, and the iron of the heme group is reduced when the cytochrome receives an electron Fe + Fe " ". At the end of the electron transfer chain, oxygen is reduced to water. 

The energy necessary to generate ATP is extracted from the oxidation of NADH and FADH2 by the electron transport chain, a series of four protein complexes, denoted Complexes I-IV (Fig. 7b). NADH is oxidized by Complex I FADH2 is oxidized by Complex II. Each complex contains multiple redox centers several iron-sulfur proteins and flavin mononucleotide in Complex I, and three iron-sulfur centers and a heme in Complex II. The electrons are then passed to coenzyme Q, which contains an organic redox center. Coenzyme Q transfers the electrons to Complex III. Complex III contains three hemes and... 

Electron transport to O2 occurs via a series of oxidation-reduction steps in which each successive component of the chain is reduced as it accepts electrons and oxidized as it passes electrons to the next component of the chain. The oxidation-reduction components of the chain include flavin mononucleotide (FMN), Fe-S centers, CoQ, and Fe in the cytochromes b, Cj, c, a, and aj. Cu is also a component of cytochromes a and (Fig- 21.5). With the exception of CoQ, all of these electron acceptors are tightly bound to the protein subunits of the carriers. 

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