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Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions. Oxidation and reduction occur in tandem. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions .
The chemical reactions underlying metabolism involve the transfer of electrons from one compound to another by processes catalyzed by enzymes. The electrons in these reactions commonly come from hydrogen atoms, which consist of an electron and a proton. A molecule gives up a hydrogen atom, in the form of a hydrogen ion (H + ) and an electron, breaking the molecule into smaller parts. The loss of an electron, or oxidation , releases a small amount of energy; both the electron and the energy are then passed to another molecule in the process of reduction , or the gaining of an electron. These two reactions always happen together in an oxidation-reduction reaction (also called a redox reaction)—when an electron is passed between molecules, the donor is oxidized and the recipient is reduced. Oxidation-reduction reactions often happen in a series, so that a molecule that is reduced is subsequently oxidized, passing on not only the electron it just received but also the energy it received. As the series of reactions progresses, energy accumulates that is used to combine P i and ADP to form ATP, the high-energy molecule that the body uses for fuel.
Oxidation-reduction reactions are catalyzed by enzymes that trigger the removal of electrons (either one or two) from a substrate (sometimes the removal coincides with the removal of a proton) and transfers then to a second substrate, usually a coenzyme which can temporarily maintain the electrons (and sometimes protons) before transferring then to a second compound. There are two broad classes of coenzymes that work in redox reactions in the cell. The first are those coenzymesm that can carry both electrons and protons, the two most common are nicotinamide adenine dinucleotide (NAD + ) and flavin adenine dinucleotide (FAD) . Their respective reduced coenzymes are NADH and FADH 2 . A third coenzyme, nicotinamide adenine dinucleotide phosphate (NADP + ) is the primary reductant for anabolic reactions and has similar (yet distinct) properties to its unphosphorylated counterpart NAD + . The second general group of enzymes (that contain co-factors such as heme) only carry electrons, and include cytochromes , and Iron-Sulfur (Fe-S) proteins .
Consider a red/ox reaction that requires NAD as a co-enzyme. As the reaction proceeds NAD+ is reduced to NADH. What would happen to the reaction rate and the substrate concentration if the NAD+ pool is fixed (a finite number of molecules in the cell)?
As the reaction proceeds, NAD+ is reduced to NADH, if the NAD+ concentration is fixed, and NADH is not recycled the reaction will stop (or sit at some equilibrium)with no net increase in NADH because the NAD+ pool becomes so low.
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