Bioenergetics PDF

Unit II The Importance of Energy Changes and Electron Transfer in Metabolism CHEM 141: Biochemistry II KFL ‘25 Topic Outline A. Energy Balance G. Firefly Bioluminescence Cycle B. The Nature of Metabolism H. Flow of Elect...

Unit II The Importance of Energy Changes and Electron Transfer in Metabolism CHEM 141: Biochemistry II KFL ‘25 Topic Outline A. Energy Balance G. Firefly Bioluminescence Cycle B. The Nature of Metabolism H. Flow of Electrons in the Living Things are Unique Biological System Thermodynamic Systems I. Biological Redox C. Biochemical Reactions J. Coenzymes in Biological Five General Categories Redox Reactions D. Adenosine Triphosphate (ATP) NAD, NADP E. ATP Hydrolysis FMN, FAD F. Nucleophilic Reactions of ATP K. Coupling of Production and Use of Energy 2 Energy Balance By virtue of the Law of Conservation of Energy: Energy intake = internal heat produced + work + storage Heat and work are not merely accounted in the mechanical work of motion but also during anabolic processes (energy-requiring) Stored chemical energy can be used to drive endergonic reactions 3 The Nature of Metabolism Metabolism is the sum total of all biochemical reactions that take place in an organism. Catabolism is an oxidative process that releases energy; Anabolism is a reductive process that requires energy. Catabolism and anabolism are separate pathways; they are not simply the reverse of each other. 4 The Nature of Metabolism Living Things are Unique Thermodynamic Systems 5 Biochemical Reactions Five General Categories I. C-C bond making or breaking II. Internal rearrangements, isomerizations, and eliminations III. Free-radical reactions IV. Group transfers V. Oxidation-reductions 6 Biochemical Reactions C-C bond making or breaking 7  Cholesterol biosynthesis precursor Biochemical Reactions C-C bond making or breaking 8 Biochemical Reactions Internal Rearrangements, Isomerizations, and Eliminations 9 Biochemical Reactions Internal Rearrangements, Isomerizations, and Eliminations 10 Biochemical Reactions Free-radical reactions 11 Biochemical Reactions Group-transfer reactions transfer of acyl, glycosyl, and phosphoryl groups from one nucleophile to another is common in living cells. 12 Chymotrypsin reaction 13 Biochemical Reactions Group-transfer reactions attachment of a good leaving group to a metabolic intermediate to “activate” the intermediate for subsequent reaction. inorganic orthophosphate (Pi) and inorganic pyrophosphate (PPi) 14 Biochemical Reactions Oxidation-Reduction reactions Carbon atoms in biomolecules can exist in five oxidation states 15 Adenosine triphosphate (ATP) ATP serves a special role in cells as the energy currency that links catabolism and anabolism The donation of energy from ATP generally involves the covalent participation of ATP that usually converts ATP to ADP and Pi or, in some reactions, to AMP and 2 Pi. 16  ATP Hydrolysis 1 The charge separation that results from hydrolysis relieves electrostatic repulsion among the four negative charges on ATP. 17 ATP Hydrolysis Suppose that the standard free energy of the conversion of compound A into compound B However, A can be converted into B under these conditions if the reaction is coupled to the hydrolysis of ATP 18 ATP Hydrolysis A thermodynamically unfavorable reaction sequence can be converted into a favorable one by coupling it to the hydrolysis of a sufficient number of ATP molecules in a new reaction 19  ATP Hydrolysis ATP provides energy by group transfers, not by simple hydrolysis The contribution of ATP to a reaction is often shown as a single step but is almost always a two-step process. 20  Nucleophilic Reactions of ATP Any of the three P atoms (α, β, or γ) may serve as the electrophilic target for nucleophilic attack Adenylylation – nucleophilic attack at the α-position of ATP displaces PPi and transfers adenylate (5’-AMP) as an adenylyl group 21 Firefly Bioluminescence Cycle 22  Flow of Electrons in the Biological System The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms ATP consumption: Resting – 40 kg/day Strenuous Exertion – 0.5 kg/min; 60kg/2-hr run 23 Flow of Electrons in the Biological System The oxidation of carbon fuels is an important source of cellular energy 24 Biological Redox Fats are a more efficient fuel source than carbohydrates, such as glucose, because the carbon in fats is reduced. 25 Biological Redox Living cells have an analogous biological “circuit” Released electrons from glucose oxidation are transferred to O2 – driven by a force proportional to the difference in electron affinity, the electromotive force, emf. 26 Biological Redox Complete oxidation of glucose: ΔG’° = -2,840 kJ/mol Oxidation of glucose involves a series of controlled reactions Electrons removed in these oxidation steps are transferred to coenzymes specialized for carrying electrons, such as NAD+ and FAD+ 27 Coenzymes in Biological Redox Reactions Electrons released from redox reactions are channeled to nucleotide coenzymes (electron carriers). NAD, NADP, FMN, and FAD are water-soluble coenzymes that undergo reversible oxidation and reduction in many of the electron-transfer reactions of metabolism. NAD and NADP move readily from one enzyme to another; while FMN and FAD are usually very tightly bound to the enzymes, called flavoproteins, for which they serve as prosthetic groups. 28 Coenzymes in Biological Redox Reactions Nicotinamide adenine dinucleotide NADH and NADPH act with dehydrogenases as soluble electron carriers 29 Coenzymes in Biological Redox Reactions Half-reactions for the nucleotide cofactors: The oxidized form of nucleotide accepts a hydride ion (:H-); the second proton removed from the substrate is released to the aqueous solvent. NADPH is used almost exclusively for reductive biosyntheses, whereas NADH is used primarily for the generation of ATP. 30 Coenzymes in Biological Redox Reactions Recycling of Coenzyme No net production or consumption of NAD+ or NADH; the coenzymes function catalytically and are recycled repeatedly without a net change in the concentration of NAD+ + NADH. 31 Coenzymes in Biological Redox Reactions Dietary Deficiency of Niacin Causes Pellagra Niacin deficiency, which affects all the NAD(P)- dependent dehydrogenases. “Three Ds”: dermatitis, diarrhea, dementia 32 Coenzymes in Biological Redox Reactions FMN consists of the structure above the dashed line on the FAD (oxidized form). The flavin nucleotides accept two hydrogen atoms (two electrons and two protons) when FAD or FMN accepts only one hydrogen atom, the semiquinone, a stable free radical, forms. 33 Coupling of Production and Use of Energy 34