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B2. The Chemistry of NAD and FAD

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    132916
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    NAD+ is a derivative of nicotinic acid or nicotinamide.

    Figure: NAD+ is a derivative of nicotinic acid or nicotinamide.

    nicotinacidder.gif

    It and its reduction product, NADH, exists in the cells as interconvertible members of a pool whose total concentration does not vary significantly with time. Hence, if carbohydrates and lipds are being oxidized by NAD+ to produce energy in the form of ATP, levels of NAD+ would begin to fall as NADH rises. A mechanism must be be present to regenerate NAD+ from NADH if oxidation is to continue. As we will see later, this happens in the muscle under anaerobic conditions (if dioxygen is lacking as when you are running a 100 or 200 m race, or if you are being chased by a saber-toothed tiger) when pyruvate + NADH react to form lactate + NAD+.

    PYRTOLACTATE.gif

    Under aerobic conditions (sufficient dioxygen available), NADH is reoxidized in the mitochondria by electron transport through a variety of mobile electron carriers, which pass electrons to dioxygen (using the enzyme complex cytochrome C oxidase) to form water.

    NAD+/NADH can undergo two electron redox steps, in which a hydride is transferred from an organic molecule to the NAD+, with the electrons flowing to the positively charged nitrogen of NAD+ which serves as an electron sink. NADH does not react well with dioxgyen, since single electron transfers to/from NAD+/NADH produce free radical species which can not be stabilized effectively. All NAD+/NADH reactions in the body involve 2 electron hydride transfers.

    Figure: All NAD+/NADH reactions in the body involve 2 electron hydride transfers

    nad1or2e.gif

    FAD (or flavin mononucleotide-FMN) and its reduction product, FADH2, are derivatives of riboflavin.

    Figure: derivatives of riboflavin

    fad.gif

    FAD/FADH2 differ from NAD+/NADH since they are bound tightly (Kd approx 10-7 - 10-11 M) to enyzmes which use them. This is because FADH2 is susceptible to reaction with dioxygen, since FAD/FADH2 can form stable free radicals arising from single electron transfers. FAD/FADH2 can undergo 1 OR 2 electrons transfers.

    Figure: FAD/FADH2 can undergo 1 OR 2 electrons transfers

    fad1or2e.gif

    FAD/FADH2 are tightly bound to enzymes so as to control the nature of the oxidizing/reducing agent that interact with them. (i.e. so dioxygen in the cell won't react with them in the cytoplasm.) If bound FAD is used to oxidize a substrate, the enzyme would be inactive in any further catalytic steps unless the bound FADH2 is reoxidized by another oxidizing agent.


    This page titled B2. The Chemistry of NAD and FAD is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Henry Jakubowski.

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