Topic 9 INTRODUCTION TO CATABOLIC PATHWAYS OXIDATION REDUCTION REACTIONS lecture 13 OBJECTIVES 1 Understand the fundamental differences between ATP production by substrate level phosphorylation and chemiosmotic synthesis 2 Understand the concept of metabolic pathway and the free energy changes that take place 3 Know what a redox reaction is the importance of electronegativity in the transfer of electrons and the free energy changes that take place in redox reactions ATP is the universal energy coupling agent in biological systems fig 9 2 active transport against concentration gradients motility and biosynthesis It has been estimated that up to 70 of ATP hydrolyzed is used for the active transport of inorganic ions in resting quiescent organisms However during intense locomotory activity the bulk of ATP hydrolysis is due to contractile proteins fig 6 10 ATP is regenerated by catabolic pathways which breakdown metabolic fuels such as carbohydrates and fats into smaller molecules Some of the energy present in C C bonds is trapped in the form of ATP The overall process of catabolic ATP production is known as cellular respiration aka cellular energy metabolism metabolic pathway a series of linked enzyme catalyzed reactions which are usually under precise control enz 1 enz 2 enz 3 enz 4 enz 5 A B C D E F fuel compound smaller C fragments GA free energy content of A GF f e content of F GA GF DG GF GA very negative value therefore some of the lost free energy can be trapped in the form of ATP Net reaction xADP A F xATP the amount of ATP generated is dependent on the magnitude of the DG Normally ATP ADP the process of cellular respiration maintains high ATP s because the ATP synthesis is directly coupled to the rate of ATP hydrolysis due to the fact that the catabolic pathways are precisely regulated Key chemical reactions important to cellular respiration ATP synthesis 1 1 substrate level phosphorylation fig 9 7 pyruvate kinase is an enzyme that catalyzes the reaction phoshpoenolpyruvate ADP pyruvate ATP the DG for this reaction is very negative some of the free energy is trapped as ATP 2 chemiosmotic ATP synthesis here a very high concentration gradient of H is generated we ll see how soon It is very thermodynamically favorable for the protons to flow down their concentration gradient but the membrane will not permit this However if a H channel is present it can allow the protons to flow downhill and in the process trap energy to form ATP Oxidation Reduction Reactions redox reactions Redox reactions involve one substance losing an electron and the other substance gaining an electron as shown by the simple equation from your text Xe Y X YeWe can say the following about the reaction 1 Xe has been oxidized it lost an electron to X it is the reducing agent reductant 2 Y has been reduced it gained an electron to Ye it is the oxidizing agent oxidant What determines the direction of transfer of electrons electronegativity an index of how readily an atom attracts electrons 1 it is a function of the number of electrons in the outer shell and the distance of the outer shell from the nucleus 2 atoms differ in their electronegativity order of electronegativity O N C H oxygen is one of the most electronegative of all elements It has room for only two electrons 3 electrons are transferred from less electronegative to more electronegative atoms order of electron flow H C N O 4 redox reactions are accompanied by a decrease in free energy DG negative Some of this energy can be trapped 2 Fig 9 3 methane oxidation burning yields energy in the form of heat Methane is oxidized oxygen is reduced to water Biological redox reactions p 156 of text gross chemical equation for the complete catabolism of carbohydrate by cellular respirationC6H12O6 6 O2 6CO2 6H2O energy heat ATP C6H12O6 is oxidized to carbon dioxide oxygen is reduced to water oxidation in this case results in addition of oxygen and removal of electrons and hydrogen reduction involves gaining of electrons and hydrogen In cellular respiration there are a number of enzyme catalyzed redox reactions these reactions involve the participation of a coenzyme such as NAD NADH which becomes reduced or oxidized depending on the direction of the reaction see fig 9 4 Electrons are shuttled back and forth in the process of oxidizing or reducing carbon compounds as shown in your textC OH NAD C O NADH H NAD is reduced in this reaction and C OH is oxidized Xe Y X Ye H H Reduced coenzyme like NADH is very electropositive the opposite of electronegative as well shall see in biological systems reduced coenzymes transfer their electrons to a series of more electronegative compounds which ultimately results in the generation of ATP fig 9 5 3
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