Redox Reactions in Metabolism Standard reduction potentials coenzymes in metabolism and pyruvate dehydrogenase Bioc 460 Spring 2008 Lecture 27 Miesfeld NADH Acetyl CoA Vitamins are organic compounds in nature that were discovered through dietary deficiency diseases such as beriberi Redox reactions in living cells provide metabolic energy using NAD NADH The PDH reaction uses a ball and chain mechanism to generate acetyl CoA Key Concepts in Redox Metabolism Reduction potentials are a measurement of electron affinity Compounds with a very high affinity for electrons are oxidants e g O 2 and have a positive reduction potential E 0 Very strong reductants are compounds that readily give up electrons e g NADH and have a negative reduction potential E 0 Electrons flow from reductants to oxidants electrons flow toward compounds with higher E values Coenzymes are organic compounds that provide reactive chemical groups to enzymes many coenzymes were discovered as vitamins through the study of dietary deficiency diseases Most coenzymes such as nicotinamide adenine dinucleotide NADH and thiamin pyrophosphate TPP are noncovalently associated with enzymes The pyruvate dehydrogenase PDH complex is a mitochondrial metabolic machine that converts pyruvate to acetyl CoA in a favorable reaction G 33 4 kJ mol The PDH reaction is in the mitochondrial matrix and captures decarboxylation energy in the form of NADH Redox reactions transfer electrons Redox reactions oxidation reduction in the citrate cycle are a form of energy conversion involving the transfer of electron pairs from organic substrates to the carrier molecules NAD and FAD The energy available from redox reactions is due to differences in the electron affinity of two compounds and is an inherent property of each molecule based on molecular structure Coupled redox reactions consist of two half reactions 1 an oxidation reaction loss of electrons 2 a reduction reaction gain of electrons Conjugate redox pairs Compounds that accept electrons are called oxidants and are reduced in the reaction whereas compounds that donate electrons are called reductants and are oxidized by loss of electrons Each half reaction consists of a conjugate redox pair represented by a molecule with and without an electron e Fe2 Fe3 is a conjugate redox pair in which the ferrous ion Fe2 is the reductant that loses an e during oxidation to generate a ferric ion Fe3 the oxidant Fe2 Fe3 eSimilarly the reductant cuprous ion Cu can be oxidized to form the oxidant cupric ion Cu2 plus an e in the reaction Cu Cu2 e Conjugate redox pairs Two half reactions are combined to form a redox reaction For example the transfer of an e from from Fe2 the reductant to Cu2 the oxidant to form Fe3 and Cu Fe2 Fe3 eCu2 e Cu Fe2 Cu2 Fe3 Cu The Fe was oxidized and the Cu was reduced in a redox reaction in which the e was the shared intermediate This Fe Cu redox reaction takes place within the cytochrome c oxidase complex in the electron transport system of the inner mitochondrial membrane Aerobic respiration is the transfer of electrons from glucose to O2 to form CO2 and H2O The more electrons a carbon atom has available to donate the more reduced less oxidized it is Hydrogen is less electronegative than carbon and therefore electrons in CH bonds are considered owned by the carbon Oxygen is more electronegative than carbon and the electrons in C O and C O bonds are all owned by the oxygen atom Redox reactions in the citrate cycle involve the transfer of e pairs to generate NADH and FADH2 The reduction of NAD to NADH involves the transfer of a hydride ion H which contains 2 e and 1 H and the release of a proton H into solution NAD 2 e 2 H NADH H In contrast FAD is reduced by sequential addition of one hydrogen 1 e and 1 H at a time to give the fully reduced FADH2 product FAD 1 e 1 H FADH 1 e H FADH2 Enzymes that catalyze biochemical redox reactions are strictly called oxidoreductases however since most oxidation reactions involve the loss of one or more hydrogen atoms they are often called dehydrogenases Reduction potential E is a measure of the electron affinity of a given redox pair Biochemical standard reduction potentials E are determined under standard conditions using an electrochemical cell that measures the relative e affinity of a test redox pair compared to the hydrogen half reaction Two half cells are connected by a galvanometer which measures the flow of electrons between two electrochemical cells An agar bridge between the two half cells allows ions to flow and balance the charge to keep the electron circuit intact Fe3 has a higher e affinity than H Standard reduction potentials are expressed as half reactions and written in the direction of a reduction reaction Redox pairs with a positive E have a higher affinity for electrons that redox pairs with a negative E Electrons move from the redox pair with the lower E more negative to the redox pair with the higher E more positive The hydrogen half reaction is set as the standard with a E 0 Volts The amount of energy available from a coupled redox reaction is defined as E By convention the E of a coupled redox reaction is determined by subtracting the E of the oxidant e acceptor from the E of the reductant e donor using the following equation E E e acceptor E e donor The E for a coupled redox reaction is proportional to the change in free energy G as described by the equation n is the number of e G nF E If E 0 then the reaction is favorable since G will be negative A coupled redox reaction is favorable when the reduction potential of the e acceptor is more positive than that of the e donor Calculating the G for a citrate cycle oxidation reaction using the E of the half reactions The oxidative decarboxylation of isocitrate by the enzyme isocitrate dehydrogenase in the third reaction of the citrate cycle Isocitrate NAD ketoglutarate CO2 NADH H Using the E values from the table with the half reactions as reductions NAD H 2 e NADH E 0 32 V ketoglutarate CO2 2 e 2 H isocitrate E 0 38 V And now calculate E considering that NAD is the e acceptor and isocitrate is the e donor electrons move from low E to higher E E E e acceptor E e donor E 0 32 V 0 38 V 0 06 V Another way to get the same answer If it makes more sense to you to write the two half reactions in the direction of the overall net reaction then simply reverse the E value for the isocitrate oxidation and add the two E values together Writing each half reaction in the direction of the net
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