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Unformatted text preview: of the enzyme’s active site ensures that none of the substrate is diverted to nonproductive side­reactions. No wasteful by products are formed Compare that with your experience in Organic Chem lab where a 30­50% yield is substantial Recall that E can distinguish between isomers & D,L forms. Prosthetic Groups & Coenzymes Prosthetic Groups & Coenzymes Enzymes may contain small non­protein molecules that participate in catalysis but are not themselves substrates p 93 Prosthetic groups Bind tightly to enzyme Typically bind permanently by covalent bonds Example: heme group in cytochromes Coenzymes Loosely bound to enzymes Single coenzyme molecule may associate with a number of different enzymes Most derivatives of vitamins: NAD+/NADH, a derivative of niacin, is a good example Fructose Bisphosphate Aldolase Fructose Bisphosphate Aldolase Catalytic Cycle of Enzyme Catalytic Cycle of Enzyme Oxidation­Reduction Oxidation­Reduction o o o o o o Oxidation involves the removal of an electron(s) from a substance Reduction is defined as the addition of an electron(s) Frequently, biochemical reactions involve not just the transfer of electrons, but of whole hydrogen atoms A hydrogen atom (H) consists of an electron plus a proton When its electron is removed, the hydrogen atom becomes a proton (or a hydrogen ion H+) o H H+ + e­ (oxidation of hydrogen) Oxidation of ferrous iron o Fe+2 Fe+3 + e­ (oxidation of ferrous iron) Remembering the flow Remembering the flow Remember the term “Oil Rig”: O xidation I s L oss o R eduction I s G ain Electron Donors & Acceptor Electron Donors & Acceptor • • Oxidation­Reduction reactions involve electrons being donated by an electron donor and being accepted by an electron acceptor. Hydrogen gas can be oxidized to release electrons and hydrogen ions: • H2 ⇒ 2 e­ + 2 H+ This is the oxidation half of the reaction 1/2 reaction implies that, for any oxidation reaction to occur, a reduction reaction is also necessary • • Electron Donors & Acceptors Electron Donors & Acceptors • H2 oxidation can be coupled to the reduction of many substances including O2 in a second reaction:[ ½ O2 + 2e­ + 2H+ 2H+ + O­ 2 • 1/2 O2 + 2 e­ + 2 H+ ⇒ 2O H This is the reduction 1/2 reaction Putting the two half­reactions together yields the following overall balanced reaction: • H2 + 1/2 O2 ⇒ 2O H • • • In these types of reactions, we refer to the substance oxidized, in this case H2, as the electron donor, and the substance reduced, in this case O2, as the electron acceptor Reduction Potentials Reduction Potentials o o o o o o o Substances vary in their tendency to become oxidized or to become reduced This tendency is expressed as the reduction potential (E0´) The E0’ is in units of volts (V)[0 std conditions; ’ means pH = 7] By convention, reduction potentials are expressed for half reactions written as reductions Thus, oxidized form + e­ ⇒ reduced form Since the pH of the cytoplasm is neutral (or nearly so), standard conditions are at pH=7.0 Example of Hydrogen oxidation coupled to oxygen reduction: next slide Example: H2 oxidation coupled to Example: H O2 reduction o Gi...
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