Iron-containing proteins mediator molecules

The following material focuses on fte reduction of oxygen to water and ATP production, events that take place at the mitochondrial membrane. Energy fuels are electron donors. The coenz)anes NAD" and FAD accept these electrons and mediate their transfer to the respiratory chain. The respiratory chain is a series of iron-containing proteins associated with the mitochondrial membrane. Transfer of four electrons through this series of proteins can result in the reduction of one molecule of oxygen to water. This electron transport chain is shown in Figure 5.2. 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

The absorption of light by P qq then leads to the series of electron transfer reactions of photosystem I. The substance to which the excited-state chlorophyll, P700, gives an electron is apparently a molecule of chlorophyll a this transfer of electrons is mediated by processes that take place in the reaction center. The next electron acceptor in the series is bound ferredoxin, an iron—sulfur protein occurring in the membrane in photosystem I. The bound ferredoxin passes its electron to a molecule of soluble ferredoxin. Soluble ferredoxin in turn reduces an FAD-containing enzyme called ferredoxin-NADP+ reductase. The FAD portion of the enzyme reduces NADP+ to NADPH (Figure 22.6). We can summarize the main features of the process in two equations, in which the notation ferredoxin refers to the soluble form of the protein. 

Iron, Fe a bioelement found in all living cells. The human body contains 4-5 g Fe, of which 75 % is in hemoglobin. In living organisms Fe occurs in the II and III oxidation states in higher animals it is stored bound to protein. It is transported in the blood as a complex with transferrin (see Siderophilins), from which it is transferred enzymatically to metal-ffee porphyrin molecules (see Heme iron). Non-heme iron (see) is also found in a number of compounds, e.g. Iron-sulfur proteins (see). The Fe metabolism of microorganisms is mediated by a group of natural products called Siderochromes (see). 

Complex I, also referred to as the NADH dehydrogenase complex, catalyzes the transfer of electrons from NADH to UQ. The major sources of NADH include several reactions of the citric acid cycle (see pp. 284-287), and fatty acid oxidation (Chapter 12). Composed of at least 25 different polypeptides, complex I is the largest protein component in the inner membrane. In addition to one molecule of FMN, the complex contains seven iron-sulfur centers (Figure 10.2). Iron-sulfur centers, which may consist of two or four iron atoms complexed with an equal number of sulfide ions, mediate 1-electron transfer reactions. Proteins that contain iron-sulfur centers are often referred to as nonheme iron proteins. Although the structure and function of complex I are still poorly understood, it is believed that NADH reduces FMN to FMNH2. Electrons are then transferred from FMNH2 to an iron-sulfur center, 1 electron at a time. After transfer from one iron-sulfur center to another, the electrons are eventually donated to UQ (Figure 10.3). 

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