Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. The Organic Chemistry of Metabolism Chapter 20 Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction We have now studied the typical reactions of the major classes of organic compounds and the structure and reactions of carbohydrates and lipids We now apply this background to the study of the organic chemistry of metabolism -oxidation of fatty acids glycolysis Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Five Key Participants Five compounds participating in these and a great many other metabolic pathways are: ATPADPAMP ATP, ADP, and AMP are universal carriers of phosphate groups NAD + /NADH FAD/FADH 2 NAD + /NADH and FAD/FADH 2 are coenzymes involved in oxidation/reduction of metabolic intermediates Coenzyme Coenzyme: a low-MW, nonprotein molecule or ion that binds reversibly to an enzyme, functions as a second substrate, and is regenerated by further reaction Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Adenoside Triphosphate ATP is the most important of the compounds involved in the transfer of phosphate groups Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Adenosine Triphosphate Hydrolysis of the terminal phosphate of ATP gives ADP and phosphate in glycolysis, the phosphate acceptors are -OH groups of glucose and fructose Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. NAD + /NADH Nicotinamide adenine dinucleotide (NAD + ) is a biological oxidizing agent Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. NAD + /NADH NAD + is a two-electron oxidizing agent, and is reduced to NADH Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. NAD + /NADH we will discuss two types of oxidations involving NAD + Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Oxidation by NAD + Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. FAD/FADH 2 Flavin adenine dinucleotide (FAD) is also a biological oxidizing agent Riboflavin Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. FAD/FADH 2 we discuss one type of oxidation involving FAD, namely oxidation of the hydrocarbon chain of a fatty acid Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Oxidation by FAD Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Oxidation by FAD Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Fatty Acids and Energy Fatty acids in triglycerides are the principle storage form of energy for most organisms carbon chains are in a highly reduced form the energy yield per gram of fatty acid oxidized is greater than that per gram of carbohydrate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Oxidation of Fatty Acids There are two major stages in the oxidation of fatty acids activation of the free fatty acid in the cytoplasm and its transport across the inner mitochondrial membrane -oxidation -Oxidation -Oxidation: a series of four enzyme-catalyzed reactions that cleaves carbon atoms two at a time, from the carboxyl end of a fatty acids Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Activation of Fatty Acids Begins in the cytoplasm with formation of a thioester formation of the thioester is coupled with the hydrolysis of ATP to AMP and pyrophosphate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Activation of Fatty Acids activation involves reaction with ATP Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Activation of Fatty Acids and then reaction with coenzyme A Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. -Oxidation Reaction 1: oxidation of a carbon-carbon single bond to a carbon-carbon double bond Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. -Oxidation Reaction 2: hydration of the carbon-carbon double bond; only the R-enantiomer is formed Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. -Oxidation Reaction 3: oxidation of the -hydroxyl group to a carbonyl group Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. -Oxidation Reaction 4: cleavage of the carbon chain by a reverse Claisen condensation Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. -Oxidation mechanism of the reverse Claisen condensation Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. -Oxidation this series of reactions is then repeated on the shortened fatty acyl chain and continues until the entire fatty acid chain is degraded to acetyl-CoA Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis GlycolysisGlycolysis: a series of 10 enzyme-catalyzed reactions by which glucose is oxidized to two molecules of pyruvate glycolysis is a 4-electron oxidation; it occurs in two separate 2-electron oxidation steps Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 1 Reaction 1: phosphorylation of -D-glucose Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 2 Reaction 2: isomerization of glucose 6- phosphate to fructose 6-phosphate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 2 this isomerization is most easily seen by considering the open-chain forms of each monosaccharide it is one keto-enol tautomerism followed by another Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 3 Reaction 3: phosphorylation of fructose 6- phosphate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 4 Reaction 4: cleavage of fructose 1,6- bisphosphate to two triose phosphates Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 4 reaction 4 is a reverse aldol reaction an intermediate is an imine formed by the C=O group of fructose 1,6-bisphosphate and an -NH 2 group of the enzyme catalyzing this reaction Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 4 reverse aldol reaction gives two three-carbon fragments, one as an imine Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 4 hydrolysis of the imine gives dihydroxyacetone phosphate and regenerates the -NH 2 group of the enzyme Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 5 Reaction 5: isomerization of triose phosphates Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 6 Reaction 6: oxidation of the -CHO group of glyceraldehyde 3-phosphate the -CHO group is oxidized to a carboxyl group which is in turn converted to a carboxylic-phosphoric mixed anhydride the oxidizing agent, NAD +, is reduced to NADH Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 6 We divide this reaction into three steps step 1: formation of a thiohemiacetal Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 6 step 2: oxidation of the thiohemiacetal by NAD + Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 6 step 3: conversion of the thioester to a mixed anhydride Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 7 Reaction 7: transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 8 Reaction 8: isomerization of 3-phosphoglycerate to 2-phosphoglycerate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 9 Reaction 9: dehydration of 2-phosphoglycerate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 10 Reaction 10: phosphate transfer to ADP stage 1: transfer of the phosphate group Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis - Rexn 10 stage 2: enolization to pyruvate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Glycolysis Summing these 10 reactions gives the net equation for glycolysis Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Fates of Pyruvate Pyruvate does not accumulate in cells, but rather undergoes one of three enzyme-catalyzed reactions, depending of the type of cell and its state of oxygenation reduction to lactate reduction to ethanol oxidation and decarboxylation to acetyl-CoA A key to understanding the biochemical logic behind two of these fates is to recognize that glycolysis needs a continuing supply of NAD + if no oxygen is present to reoxidize NADH to NAD +, then another way must be found to do it Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Lactate Fermentation In vertebrates under anaerobic conditions, the most important pathway for the regeneration of NAD + is reduction of pyruvate to lactate Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Pyruvate to Lactate while lactate fermentation does allow glycolysis to continue, it increases the concentration of lactate and also of H + in muscle tissue, as seen in this balanced half-reaction when blood lactate reaches about 0.4 mg/100 mL, muscle tissue becomes almost completely exhausted Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Pyruvate to Ethanol Yeasts and several other organisms regenerate NAD + by this two-step pathway decarboxylation of pyruvate to acetaldehyde reduction of acetaldehyde to ethanol Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Pyruvate to Acetyl-CoA Under aerobic conditions pyruvate undergoes oxidative decarboxylation the carboxylate group is converted to CO 2 the remaining two carbons are converted to the acetyl group of acetyl-CoA Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Pyruvate to Acetyl-CoA Oxidative decarboxylation of pyruvate to acetyl- CoA is considerably more complex than the previous equation suggests In addition to NAD + (from the vitamin niacin) and coenzyme A (from the vitamin pantothenic acid), it also requires FAD (from the vitamin riboflavin) thiamine pyrophosphate (from thiamine, B1) lipoic acid Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. Pyruvate to Acetyl-CoA Copyright 2000 by John Wiley & Sons, Inc. All rights reserved. End Chapter 20 The Organic Chemistry of Metabolism