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

Standard States for Free-Energy Changes

• Standard states• for pure solids and liquids, _____________________• for gases, the gas at _____________________• for solutions, a concentration __________________

• For the reaction

We can rewrite the equation that relates the G for the reaction under any conditions to the free-energy change under standard conditions (G˚)

Standard States for Free-Energy Changes (Cont’d)

• When the reaction is at equilibrium, G = _____

• If we can determine the concentration of reactants and products at equilibrium, we can determine Keq and, from it, the change in free energy for conversion of one mole of reactant to product(s)

Modified Standard State for Biochemical Rxns

• Standard free energy change, G°, assumes a molar concentration of 1 M• if [H+] = 1 M, then pH = _____• but the pH in most cells is near the neutral range

• For biochemical reactions, we define a different standard state for the concentration of H+

• standard state for [H+] = _____________________• this modified standard state is given the symbol

• SummaryThe usual thermodynamic standard state implies that the system involved is at pH=0, which is seldom, if ever, found in living things. The modified standard state explicitly states that the system is at pH=7

Biochemical Connections: Thermodynamics

The Nature of Metabolism

• _____________:_____________: the chemical rxns of biomolecules • Metabolism Metabolism is the biochemical basis of• _____________:_____________: the breakdown of larger molecules

into smaller ones; an oxidative process that releases energy

• _____________:_____________: 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 biochemical reactions in an organism

The Role of Oxidation and Reduction in Metabolism

• Oxidation-Reduction reactions are those in which electrons are transferred from a donor to an acceptor• ________:________: the loss of electrons; the substance that

loses the electrons is called a ________ agent________ agent• ________:________: the gain of electrons; the substance that

gains the electrons is called an ________ agent________ agent

• Carbon in most reduced form- ______________________• Carbon in most oxidized form- ___________ ___________

(final product of catabolism

Summary

• In ________________, 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 ________________, 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 Rxns

• 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 moiety. 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

Structures & Redox States of Nicotinamide Coenzymes

FAD/FADH2

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

• Protons and electrons are accepted by FAD

The Structures of Riboflavin, Flavin Mononucleotide (FMN), and Flavin Dinucleotide (FAD)

Coupling of Production and Use of Energy

• The coupling of ________-____________ and ________-____________ 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 require or release convenient amounts of energy, depending on the direction of the reaction

• ______ is an essential high energy bond-containing compound

• Phosphorylation of ADP to ATP _________ energy• Hydrolysis of ATP to ADP _________ energy

The Phosphoric Anhydride Bonds in ATP

“High Energy” Bonds are in red

ATP

• Four – charges on ATP and 3 on ADP, therefore ATP is ______________ ______________ .

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

___________________________________________• Also, entropy loss when ADP is phosphorylated

because there is a potential loss of ______________ ______________ of inorganic phosphate (P i) on phosphorylation of ADP to ATP

Loss of a Resonance-Stabilized Phosphate Ion

…in Production of ATP

ATP Hydrolysis Decreases Electrostatic Repulsion

• Marked decrease in ________________ _______________ 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

CoA in Activation of Metabolic Pathways

• A step frequently encountered in metabolism is activation• activationactivation:: the formation of a ___________________

substance • A metabolite is bonded to some other molecule and

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

• Causes next reaction to be ______________

Two Ways of Looking at Coenzyme A

Coenzyme A (CoA-SH) contains pantothenic acid, 3’,5’-ADP and 2-mercaptoethylamine2-mercaptoethylamine

The Hydrolysis of Acetyl-CoA

The metabolically active form of a carboxylic acid is the acyl-CoA thioester with a high-energy thioester bond

The Role of Electron Transfer & ATP Production in Metabolism

Summary

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

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