Alternative electron transport chains

The reason for this is that reoxidation of NADH via the alternative electron transport chain (not coupled to oxidative phosphorylation) liberates heat. 

Porphyrin rings containing iron are also a feature of the cytochromes. Several cytochromes are responsible for the latter part of the electron transport chain of oxidative phosphorylation that provides the principal source of ATP for an aerobic cell (see Section 15.1.2). Their function involves alternate oxidation-reduction of the iron between Fe + (reduced form) and Fe + (oxidized form). The individual cytochromes vary structurally, and their classification (a, b, c, etc.) is related to their absorption maxima in the visible spectrum. They contain a haem system that is covalently bound to protein through thiol groups. 

Organisms with anaerobic mitochondria can be divided into two different types those which perform anaerobic respiration and use an alternative electron acceptor present in the environment, such as nitrate or nitrite, and those which perform fermentation reactions using an endogenously produced, organic electron acceptor, such as fumarate (Martin et al. 2001 Tielens et al. 2002). An example of the first type is the nitrate respiration that occurs in several ciliates (Finlay et al. 1983), and fungi (Kobayashi et al. 1996 Takaya et al. 2003), which use nitrate and/or nitrite as the terminal electron acceptor of their mitochondrial electron-transport chain, producing nitrous oxide as... 

The electron-transport chain should be organized to give alternating (H+ + e ) and pure e carriers. 

H-subunit - heavy (molecular-weight) subunit H - alternative designation for HVATP - protons translocated per ATP synthesized by the electron-transport chain HVe - protons translocated per electron transferred by the electron-transport chain HjA - a general designation for a reduced compound, e.g., HjO, HjS, etc. 

The presence of c-type cytochromes in these organisms came as a surprise in 1954. The discovery by Postgate (4 4) in England and Ishimoto and colleagues in Japan (425) of a functional cytochrome in Desuljovibrio helped to do away with the idea that cytochromes c were confined to oxygen respiration. At the time, Cg (Table XX) was the only cytochrome that could be detected in Desuljovibrio, and this fostered models of short electron transport chains or at least radical alternatives to the standard flavoprotein -> b c a pattern. Since then, other c, b, and... 

The fate of pyruvate depends on the route used for NADH oxidation. If NADH is reoxidized in a shuttle system, pyruvate can be used for other pathways, one of which is oxidation to acetyl-CoA and entry into the TCA cycle for complete oxidation. Alternatively, in anaerobic glycolysis, pyruvate is reduced to lactate and diverted away from other potential pathways. Thus, the use of the shuttle systems allows for more ATP to be generated than by anaerobic glycolysis by both oxidizing the cytoplasmically derived NADH in the electron transport chain and by allowing pyruvate to be oxidized completely to CO2. 

Figure 15.2 Electron carriers and hydrogen carriers by alternating two types of oxida-tion/reduction the electron transport chain can pump H+ across the membrane. This creates a gradient which can be used to drive ATP formation.

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