Anabolism electron carriers

Distinct coenzymes are required in biological systems because both catabolic and anabolic pathways may exist within a single compartment of a cell. The nicotinamide coenzymes catalyze direct hydride transfer (from NAD(P)H or to NAD(P)+) to or from a substrate or other cofactors active in oxidation-reduction pathways, thus acting as two-electron carriers. Chemical models have provided... 

Catabolism is the degradative phase of metabolism in which organic nutrient molecules (carbohydrates, fats, and proteins) are converted into smaller, simpler end products (such as lactic acid, C02, NH3). Catabolic pathways release energy, some of which is conserved in the formation of ATP and reduced electron carriers (NADH, NADPH, and FADH2) the rest is lost as heat. In anabolism, also called biosynthesis, small, simple precursors are built up into larger and more complex... 

NAD-linked dehydrogenases remove two hydrogen atoms from their substrates. One of these is transferred as a hydride ion ( II ) to NAD+ the other is released as H+ in the medium (see Fig. 13-15). NADH and NADPH are water-soluble electron carriers that associate reversibly with dehydrogenases. NADH carries electrons from catabolic reactions to their point of entry into the respiratory chain, the NADH dehydrogenase complex described below. NADPH generally supplies electrons to anabolic reactions. Cells maintain separate pools of NADPH and NADH, with different redox potentials. This is accomplished by holding the ratios of [reduced form]/[oxidized form] relatively high for NADPH and relatively low for NADH. Neither NADH nor NADPH can cross the inner mitochondrial membrane, but the electrons they carry can be shuttled across indirectly, as we shall see. 

Comparing this equation with the equation for the complete oxidation of palmitoyl-CoA (see table 18.1, equation 1), we find major differences in carriers and intermediates. The principal electron carrier in the anabolic pathway is the NADPH-NADP+ system in the catabolic pathway, /3 oxidation, the principal electron carriers are FAD-FADH2 and NAD+-NADH. The second striking difference between the two pathways is that malonyl-CoA is the principal substrate in the anabolic pathway but plays no role in the catabolic pathway. These differences reflect the fact that the two pathways do not share common enzymes. Indeed, in animal cells the reactions occur in separate cell compartments biosynthesis takes place in the cytosol, whereas catabolism occurs in the mitochondria. 

The citric acid cycle is the central metabolic pathway and indirect producer of energy. It receives fuels from the other pathways at many points and generates reduced electron carriers that go into the electron transport chain. It is also involved in anabolism, as many of its intermediates can be drawn off to synthesize other compounds. 

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