Chapter 5 - Part 2-1.pdf

Oxidative Phosphorylation in the Formation of ATP Oxidation Reduction Potentials Redox potential is the potential for a species to acquire or to donate electrons At pH 7, the standard redox potential is indicated by Eo’ rather than Eo. Those couples whose reducing agents are better donors of electro...

Oxidative Phosphorylation in the Formation of ATP Oxidation Reduction Potentials Redox potential is the potential for a species to acquire or to donate electrons At pH 7, the standard redox potential is indicated by Eo’ rather than Eo. Those couples whose reducing agents are better donors of electrons are assigned more negative redox potentials (NAD+ NADH couple is -0.32) Acetaldehyde is a stronger reducing agent than NADH , Acetate-acetaldehyde couple has a standard redox potential of -0.58V. The couples whose oxidizing agents are better electron acceptors than NAD + have greater affinity for electrons and have more positive redox potentials. Redox potential of some reaction couples 97 Oxidative Phosphorylation in the Formation of ATP Electron Transport Five of the reactions from glycolysis to TCA cycle is catalyzed by dehydrogenases, enzymes that transfer pairs of electrons from substrates to coenzymes (such as NAD+ and FAD): – Coenzymes: are organic molecules that bind loosely to the active site of an enzyme and aid in substrate recruitment. – Cofactors: a non-protein compound (inorganic) that is needed for an enzymes biological activity Pyruvate dehydrogenase 1.Citrate synthase 2.Aconitase 3.Isocitrate dehydrogenase 4. -ketoglutarate dehydrogenase 5.Succinyl-CoA synthetase 6.Succinate dehydrogenase 7.Fumarase 8.Malate dehydrogenase Four of these reactions generate NADH and one produces FADH2 High-energy electrons associated with NADH or FADH2 are transferred through a series of specific electron carriers that constitute the 98 electron-transport chain of the inner mitochondrial membrane What are cofactors and coenzymes Enzyme Inorganic (Metal ions) Non protein component Cofactors Organic (Helper molecules) Not tightly bound with enzyme and release after catalysis the catalysis called coenzyme Mg+2 Holoenzyme Holoenzyme Tightly bound organic factor is called a prosthetic group Apoenzyme Coenzymes are derived from vitamins 99NADH, NADPH, FAD etc. Examples: Q) Name the active form of enzyme Holoenzyme Q)Name the inactive form of enzyme Apoenzyme 100 Select True/ False Without coenzymes or cofactors, enzymes cannot catalyze reactions effectively. True What do you called an enzyme when it loses its cofactor? apoenzyme 101 Oxidative Phosphorylation in the Formation of ATP Types of Electron Carrier • In the ETC, there are 5 types of membrane-bound electron carriers: • Flavoproteins • Cytochromes • Copper atoms • Ubiquinone • Iron-sulfur proteins • All their redox centres are prosthetic groups, except for ubiquinone • Most of these carriers are parts of larger complexes in the ETC 102 Oxidative Phosphorylation in the Formation of ATP Types of Electron Carrier 1. Flavoproteins are polypeptides bound to one of two prosthetic groups: – Flavin adenine dinucleotide (FAD) or – Flavin mononucleotide (FMN) – Prosthetic group is derived from Vit B2 (riboflavin) – FMN and FAD are capable of accepting and donating two protons and two electrons. Major flavoproteins of the mitochondria are NADH dehydrogenase of the electron-transport chain and succinate dehydrogenase of the TCA cycle. H+ + eH+ + eQuinone Semiquinone Hydroquinone 103 Fig 5.12 a Oxidative Phosphorylation in the Formation of ATP Types of Electron Carrier 2. Cytochromes contain heme prosthetic groups bearing Fe or Cu metal ions. The iron atom of a heme undergoes reversible transition between the Fe3+ and Fe2+ oxidation states as a result of the acceptance and loss of a single electron There are three distinct cytochrome types (a, b, and c) present in the electron-transport chain, which differ from one another by substitutions within the heme group Cytochrome c is a soluble protein in the intermembrane space 104 Fig 5.12 b Oxidative Phosphorylation in the Formation of ATP Types of Electron Carrier 4. Ubiquinone (coenzyme Q) is a lipid-soluble molecule. Each ubiquinone is able to accept and donate two electrons and two protons The partially reduced molecule is the free radical ubisemiquinone, and the fully reduced molecule is ubiquinol ( UQH 2 ). H+ + eUbiquinone H+ + eUbisemiquinone Ubiquinol Fig 5.12 c 105 Types of Electron Carrier 1.Flavoproteins (accept and donate 2e- and 2 H+) – derived from Vit B2 2. Cytochromes (accept and donate 1e- ) – soluble protein in the intermembrane space 3. Copper atoms (accept and donate 1e- ) 4.Ubiquinone (accept and donate 2e- and 2 H+) – lipid soluble molecule 5. Iron-sulfur proteins (accept and donate 1e- ) – iron atoms are linked 106 Oxidative Phosphorylation in the Formation of ATP How Did We Figure Out the Sequence of Events of the ETC? The specific sequence of carriers that constitute the electron-transport chain was worked out using a variety of inhibitors that blocked electron transport at specific sites along the route. After an inhibitor was added to cells, the oxidation state of the various electron carriers in the inhibited cells was determined. Thus, by identifying reduced and oxidized components in the presence of different inhibitors, the sequences of the carriers could be determined. Reduced state Oxidized state Fig 5.15: Inhibitors to determine the ETC carrier sequence 107