Chapter 23 Metabolism and Energy Production Lecture Presentation Chapter 23 Metabolism and Energy Production Karen C. Timberlake

Chapter 23 Metabolism and Energy Production Exercise physiologists work with athletes as well as patients who have been diagnosed with diabetes, heart disease, pulmonary disease, or other chronic disabilities or diseases. Often these patients have been prescribed exercise as a form of treatment, and they have been referred to an exercise physiologist.

Chapter 23 Readiness Core Chemistry Skills Writing Equations for Hydrogenation, Hydration, and Polymerization (12.7) Classifying Enzymes (20.2) Identifying Important Coenzymes in Metabolism (22.2) Identifying the Compounds in Glycolysis (22.4)

23.1 The Citric Acid Cycle The citric acid cycle is a series of reactions that connects the intermediate acetyl CoA from the catabolic pathways in stage 2 with electron transport and the synthesis of ATP in stage 3. Learning Goal Describe the oxidation of acetyl CoA in the citric acid cycle.

23.1 The Citric Acid Cycle The citric acid cycle (stage 3) operates under aerobic conditions. oxidizes the two-carbon acetyl group in acetyl CoA to CO2. produces reduced coenzymes NADH and FADH2. is named for the six-carbon citrate ion from citric acid (C6H8O7), a tricarboxylic acid, formed in the first reaction. is also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle. Core Chemistry Skill Describing the Reactions in the Citric Acid Cycle

Citric Acid Cycle Overview In the citric acid cycle, six carbons move through the citric acid cycle, producing oxaloacetate and 2CO2. each turn contains four oxidation reactions producing the reduced coenzymes NADH and FADH2. one GTP (converted to ATP in the cell) is produced during the citric acid cycle.

Citric Acid Cycle Overview In the citric acid cycle, eight reactions oxidize acetyl CoA from pyruvate or fatty acids, producing CO2 and the high-energy compounds FADH2, NADH, and GTP. Reactions involved in the citric acid cycle include condensation, dehydration, hydration, oxidation, reduction, and hydrolysis.

Stages of Catabolism

Reaction 1: Formation of Citrate In the first reaction of the citric acid cycle, citrate synthase catalyzes the condensation of an acetyl group (2C) from acetyl CoA with oxaloacetate (4C) to yield citrate (6C) and coenzyme A. the energy to form citrate is provided by the hydrolysis of the high-energy thioester bond in acetyl CoA.

Reaction 2: Isomerization In reaction 2 of the citric acid cycle, citrate rearranges to isocitrate, a secondary alcohol. aconitase catalyzes the dehydration of citrate (tertiary alcohol) to yield cis-aconitate, followed by a hydration that forms isocitrate (secondary alcohol).

Reaction 3: Oxidation, Decarboxylation In reaction 3, isocitrate undergoes decarboxylation by isocitrate dehydrogenase. One carbon is removed by converting a carboxylate group (COO−) to CO2. The dehydrogenase removes hydrogen ions and electrons, used to reduce NAD+ to NADH and H+.

Reaction 4: Decarboxylation, Oxidation In reaction 4, catalyzed by α-ketoglutarate dehydrogenase, α-ketoglutarate (5C) undergoes decarboxylation to yield (4C) succinyl CoA. oxidation of the thiol group (— SH) in HS — CoA provides hydrogen that is transferred to NAD+ to form a second molecule of NADH and H+.

Reaction 5: Hydrolysis In reaction 5, catalyzed by succinyl CoA synthetase, hydrolysis of the thioester bond in succinyl CoA yields succinate and HS — CoA. energy from hydrolysis is transferred to the condensation of phosphate and GDP forming GTP, a high-energy compound similar to ATP.

Reaction 6: Hydrolysis In reaction 6, catalyzed by succinate dehydrogenase, succinate is oxidized to fumarate, a compound with a C = C bond. 2H lost from succinate are used to reduce the coenzyme FAD to FADH2.

Reaction 7: Hydration In reaction 7, catalyzed by fumarase, water is added to the double bond of fumarate to yield malate, a secondary alcohol.

Reaction 8: Oxidation In reaction 8, catalyzed by malate dehydrogenase, the hydroxyl group in malate is oxidized to a carbonyl group, yielding oxaloacetate. oxidation provides hydrogen ions and electrons for the reduction of NAD+ to NADH and H+.

Summary, Citric Acid Cycle In the citric acid cycle, an acetyl group bonds with oxaloacetate to form citrate. two decarboxylations remove two carbons as two CO2. four oxidations provide hydrogen for three NADH and one FADH2. a direct phosphorylation forms GTP (ATP).

Regulation of the Citric Acid Cycle The reaction rate for the citric acid cycle increases when low levels of ATP or NAD+ activate isocitrate dehydrogenase. decreases when high levels of ATP or NADH inhibit citrate synthetase (first step in cycle).

Study Check How many of each of the following are produced in one turn of the citric acid cycle? A. _ CO2 B. _ NADH C. _ FADH2 D. _ GTP

Solution How many of each of the following are produced in one turn of the citric acid cycle? A. 2 CO2 B. 3 NADH C. 1 FADH2 D. 1 GTP