The importance of energy changes and electron transfer in

metabolism

Chapter 15

The Nature of Metabolism

• Metabolism: the sum total of the chemical reactions of biomolecules in an organism. It is the biochemical basis of life processes.– Catabolism: the breakdown of larger molecules

into smaller ones; an oxidative process that releases energy

– 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

The Role of Oxidation and Reduction in Metabolism

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

that loses the electrons is called a reducing agent– reduction: the gain of electrons - the substance

that gains the electrons is called an oxidizing agent

Summary

• In Catabolism - large molecules are broken down to smaller products, releasing energy and transferring electrons to acceptor molecules of various sorts - 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 - Reduction

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 - 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

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

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– activation: 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-mercaptoethylamine, 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

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

What are standard states?

• Standard states

– for pure solids and liquids-the pure substance

– for gases, the gas at a pressure of 1 atm

– for solutions, a concentration of 1 mol/L

Standard States for Free-Energy Changes

• 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˚)

A Modified Standard State for Biochemical Applications

• Standard free energy change, G°, assumes a concentration of 1 M– if [H+] = 1 M, then pH = 0– 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+] = 10-7 M, pH = 7.0– this modified standard state is given the symbol G°’

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

• This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and does not discriminate on the following basis:

• against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political affiliation or belief; and

• against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or his or her participation in any WIA Title I-financially assisted program or activity.

Disclaimer• This workforce solution was funded by a grant awarded under the

President’s Community-Based Job Training Grants as implemented by the U.S. Department of Labor’s Employment and Training Administration. The solution was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership. This solution is copyrighted by the institution that created it. Internal use by an organization and/or personal use by an individual for non-commercial purposes is permissible. All other uses require the prior authorization of the copyright owner.