Biological Redox Reactions

Biological Redox Reactions - Biological Redox Reactions...

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Biological Redox Reactions Bryant Miles The transfer of electrons is equally as important as the transfer of phosphoryl groups. Oxidation is the loss of electrons, reduction is the gain of electrons. It is easy to tell of an organic compound has been oxidized or reduced. If an organic molecule gains molecular oxygen or loses hydrogen is has been oxidized. I.e. H 3 CC H 3 H 2 H 2 2e - + 2 H + H 2 O H 3 H 2 OH H 3 H 3 1/2O 2 H 3 H 2 OH H 2 H 2 H 2 O Note that CH 2 ==CH 2 and CH 3 CH 2 OH are at the same oxidation state. No oxidation-reduction occurs during the interconversion of ethylene into ethanol or vice versa. H 3 H 2 OH H 3 O H 2e - + 2 H + H 3 O H 1/2O 2 H 3 O OH If an organic molecule loses oxygen or gains hydrogen, then is has been reduced. H 3 H 3 H 2 H 2 2e - + 2 H + H 3 H 3 H 3 H 2 OH 2e - + 2 H + + H 2 O H 3 H 2 OH H 3 O H 2e - + 2 H + H 3 O H H 3 O OH 2e - + 2 H + + H 2 O
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Electrons are transferred from one molecule to another in four different ways. 1. Directly as electrons . I. e. Fe 2+ + Cu 2+ ± Fe 3+ + Cu + 2. As Hydrogen atoms . The hydrogen atom contains a proton, H + and an electron, e - . AH 2 A + 2e - + 2H + 3. As a Hydride ion :H - . The hydride ion has two electrons and is highly reactive. In biological systems it is directly transferred to NAD-linked dehydrogenases. 4. Through the direct combination with oxygen. Molecular oxygen combines with organic reactants to oxidize hydrocarbons to alcohols, aldehydes to acids ect. I.e. R—CH 3 + 1/2O 2 ± R—CH 2 OH In biological systems, oxidation is often coincident with the loss of hydrogen, dehydrogenation . The enzymes that catalyze these oxidations are called dehydrogenases . Oxidation-reduction reactions (Redox reactions) must occur together. Electrons are transferred from the reducing agent to the oxidizing agent such that the reducing agent is oxidized and the oxidizing agent is reduced. It is convenient however to describe the electron transfer reaction as two half reactions, one for the oxidation of the reduced species and one for the reduction of the oxygen species. I.e. The oxidation of the ferrous ion by the cupric ion, Fe 2+ + Cu 2+ ± Fe 3+ + Cu + Can be described by the two half reactions: (1) Fe 2+ ± Fe 3+ + e - (2) Cu 2+ + e - ± Cu + A half reaction consists of an electron donor and its conjugate electron acceptor. In the first half- reaction shown above, Fe 2+ is the electron donor and Fe 3+ is the conjugate electron acceptor.
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This note was uploaded on 04/11/2010 for the course BICH bich 411 taught by Professor Bryantmiles during the Spring '10 term at Texas A&M.

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Biological Redox Reactions - Biological Redox Reactions...

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