University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2020
Views:

380

Pages:

13
Author:

Ruben Owens

Chapter 9 Aerobic Metabolism I: The Citric Acid Cycle Overview n Live processes - series of oxidation-reduction reactions ¨ ¨ ¨ n Ingestion of proteins, carbohydrates, lipids Provide basic building blocks for major molecules Produces energy Aerobic metabolism I ¨ Citric Acid Cycle – series of reactions that release chemical energy stored in acetyl-CoA n Acetyl-CoA derived from pyruvate Chapter 9: Overview §Citric acid cycle §Two-carbon fragments oxidized to form CO2 §NAD+/FAD reduced to NADH/FADH2 §Electron transport chain §Transfer of electrons from NADH/FADH2 to electron carriers §Terminal acceptor O2 §Oxidative phosphorylation §Energy released forms proton gradient §Drives ATP synthesis Figure 9.1 Overview of Aerobic Metabolism Section 9.1: Oxidation-Reduction Reactions Figure 9.3 Reduction of Pyruvate by NADH §Redox reactions – electron transfer between an electron donor (reducing agent) & electron acceptor (oxidizing agent) §Many redox reactions have both an electron (e-) and a proton (H+) transferred §Conversion of pyruvate and NADH to lactate and NAD+ (shown above) is under anaerobic conditions Section 9.1: Oxidation-Reduction Reactions §Half-reactions of redox reactions Cu loses e-, electron donor Cu+ ß à Cu2+ + eFe gains e-, electron acceptor Fe3+ + e- ß à Fe2+ Figure 9.4 An Electrochemical Cell §Reduction potential – tendency for gaining electrons §Standard reduction potentials (Eo) relative to standard hydrogen electrode; 2H+ + 2e- à H2 Eo = 0 §Biochemical reference half-reaction is 2H+ + 2e- à H2 Eo = -0.42 §Lower Eo - lower affinity for electrons; higher Eo - greater affinity Section 9.1: Oxidation-Reduction Reactions §DEº′ - difference in reduction potential between donor and acceptor under standard conditions §Relationship between standard reduction potentials (DEº′) and standard free energy (DGº′) is: DGº′ = -nF DEº′ § n - # electrons transferred, § F - Faraday constant (96,485 J/V•mol) §NADH and FADH2 - redox coenzymes, high-energy electron carriers Section 9.1: Oxidation-Reduction Reactions §Nicotinic Acid §nicotinamide adenine dinucleotide (NAD) §Oxidized NAD+; reduced NADH §NAD+ involved in catabolic reactions §nicotinamide adenine dinucleotide phosphate (NADP) §Oxidized NADP+; reduced NADPH) §NADP+ involved in biosynthetic reactions Figure 9.5 Nicotinamide Adenine Dinucleotide (NAD) Section 9.1: Oxidation-Reduction Reactions §Redox Coenzymes: Riboflavin (vitamin B2) §flavin mononucleotide (FMN) §flavin adenine dinucleotide (FAD) §Flavoproteins – FMN/FAD tightly bound prosthetic groups to enzymes §function in a diverse class of redox enzymes; dehydrogenases, oxidases, hydroxylases Figure 9.6 Flavin Coenzymes From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press Section 9.1: Oxidation-Reduction Reactions Figure 9.7 Electron Flow and Energy §Photosynthesis to Aerobic Metabolism §Photosynthesis captures energy in chemical bonds §Aerobic respiration releases bond energy §Energy captured by mitochondrial ETC §Energy transferred from NADH to O2 ½O2 + NADH + H+ à H20 + NAD+ (-220 kJ/mol) Section 9.2: Citric Acid Cycle §Citric acid cycle -harvests energy from acetyl group of acetyl- CoA §Acetyl is derived from catabolism of carbohydrates (e.g., pyruvate), lipids, and some amino acids §Coenzyme A is an acyl carrier molecule §CAC net reaction: Acetyl-CoA + 3NAD+ + FAD + GTP + Pi + H2O à 2CO2 + 3NADH + FADH2 + CoASH + GTP + 2H+ Section 9.2: Citric Acid Cycle §Coenzyme roles in a variety of biosynthetic reactions Section 9.2: Citric Acid Cycle §Conversion of Pyruvate to Acetyl-CoA §Pyruvate dehydrogenase complex converts pyruvate to acetyl-CoA §Highly exergonic (DGº′ = -33.5 kJ/mol) §Net reaction: Pyruvate + NAD+ + CoASH à acetyl-CoA + NADH + CO2 + H+ Section 9.2: Citric Acid Cycle Pyruvate dehydrogenase complex §pyruvate dehydrogenase - thiamine pyrophosphate (TPP) coenzyme, decarboxylation §dihydrolipoyl transacetylase – lipoic acid coenzyme; converts HETPP into acetyl-CoA §dihydrolipoyl dehydrogenase – reoxidizes reduced lipoamide Conversion Steps: 1. Pyruvate loses CO2. hydroxyethylTPP formed; coenzyme – thiamine pyrophosphate 2. Active lipoic acid bound to dihydrolipoyl transacetylase forming HETPP 3. Hydroxyethyl group oxidized & transferred to reduced lipoamide 4. Hydroxyethyl group converted to acetyl group forming acetyl lipoamide 5. Acetyl lipoamide reduced to dihyrolipoamide; acetyl group transferred to sulfhydryl group of CoA – acetyl-CoA 6. Dihyrolipoamide oxidized to lipamide Section 9.2: Citric Acid Cycle Intermediate decarboxylates produces HETPP TPP attacks pyruvate Advantages: 1. Reaction efficiency 2. Regulatory control efficiency Resonance-stablized carbanion attacks lipoamide Enzyme converts hydroxyethyl group Into acetyl group Nucleophilic attack yields acetyl-CoA Regulatory control: § Allosteric effectors Reoxidation of reduced & covalent lipoamide modification § Inhibited: hi ATP, pyruvate, CoASH, NAD+