Metabolism i-chp-15-bioc-361-version-oct-2012 - Intro

Chapter 15The Importance of Energy Changes and

Electron Transfer in Biology

Mary K. CampbellShawn O. Farrellhttp://academic.cengage.com/chemistry/campbell

Paul D. Adams • University of Arkansas

The Nature of Metabolism

• Metabolism:Metabolism: the chemical reactions of biomolecules. It is the biochemical basis of life processes• catabolism:catabolism: the breakdown of larger molecules into

smaller ones; an oxidative process that releases energy

• anabolism:anabolism: the synthesis of larger molecules from smaller ones; a reductive process that requires energy

A Comparison of Catabolism and Anabolism• Metabolism is the sum total of the chemical reactions

of biomolecules in an organism

Summary

• In catabolism, large molecules are broken down to smaller products, releasing energy and transferring electrons to acceptor molecules of various sorts. The overall process is one of oxidation.

• In anabolism, small molecules react to give rise to larger ones; this process requires energy and involves acceptance of electrons from a variety of donors. The overall process is one of reduction

Coenzymes used in Biologically important Redox Reactions• Conversion of ethanol to acetaldehyde is a two-

electron oxidation

NAD+/NADH: An Important Coenzyme

• Nicotinamide adenine dinucleotide (NAD+) is an important coenzyme

• Acts as a biological oxidizing agent• The structure of NADH is comprised of a

nicotinamide portion. It is involved in the reaction. It is a derivative of nicotinic acid

• NAD+ is a two-electron oxidizing agent, and is reduced to NADH

The Structures and Redox States of the Nicotinamide Coenzymes

FAD/FADH2

• Flavin adenine dinucleotide (FAD) is also a biological oxidizing agent

• Protons, as well as, electrons are accepted by FAD

The Structures of Riboflavin, Flavin Mono-nucleotide (FMN), and Flavin Dinucleotide (FAD)

Coupling of Production and Use of Energy

• The coupling of energy-producing and energy-requiring reactions is a central theme in the metabolism of all organisms

• Energy cannot be used directly, must by shuttled into easily accessible forms of chemical energy

• “High Energy” bonds- bonds that require or release convenient amounts of energy, depending on the direction of the reaction

• ATP is essential high energy bond-containing compound

• Phosphorylation of ADP to ATP requires energy• Hydrolysis of ATP to ADP releases energy

The Phosphoric Anhydride Bonds in ATP are “High Energy” Bonds

ATP

• 4 (-) charges on ATP and 3 on ADP, therefore ATP is less stable.

• Why is ATP less stable, charge-wise, than ADP?• Energy must be expended to put on additional

negative charge on ADP• Also, entropy loss when ADP is phosphorylated

because there is a potential loss of resonance hybridization of inorganic phosphate (Pi) upon phosphorylation of ADP to ATP

Loss of a Resonance-Stabilized Phosphate Ion in Production of ATP

ATP Hydrolysis Decreases in Electrostatic Repulsion• Marked decrease in

electrostatic repulsion of -phosphate of GDP upon hydrolysis of ATP to ADP

Role of ATP as Energy Currency

Summary

• Hydrolysis of ATP to ADP releases energy

• In the coupling of biochemical reactions, the energy released by one reaction, such as ATP hydrolysis, provides energy for another

Coenzyme A in Activation of Metabolic Pathways• A step frequently encountered in metabolism is

activation• activationactivation:: the formation of a more reactive

substance • A metabolite is bonded to some other molecule and

the free-energy change for breaking the new bond is negative.

• Causes next reaction to be exergonic

Two Ways of Looking at Coenzyme A

• Coenzyme A (CoA-SH) contains units of 2-2-mercaptoethylaminemercaptoethylamine, pantothenic acid, and 3’,5’-ADP

The Hydrolysis of Acetyl-CoA

• The metabolically active form of a carboxylic acid is the corresponding acyl-CoA thioester, in which the thioester linkage is a high-energy bond

The Role of Electron Transfer and ATP Production in metabolism

Summary

• Metabolic pathways proceed in many stages, allowing for efficient use of energy

• Many coenzymes, particularly coenzyme A(CoA) play a crucial role in metabolism