1. The Importance of Energy Changes and Electron Transfer in Metabolism
Chapter Fifteen
The Importance of Energy Changes and Electron Transfer in Metabolism

2. Standard States for Free-Energy Changes
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
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˚)

3. 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°’
Summary
The 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

4. Biochemical Connections: Thermodynamics

5. The Nature of Metabolism
Metabolism: the chemical reactions of biomolecules. 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

6. A Comparison of Catabolism and Anabolism
• Metabolism is the sum total of the chemical reactions of biomolecules in an organism

7. 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
Carbon in most reduced form- alkane
Carbon in most oxidized form- CO2 (final product of catabolism

8. Standard States for Free-Energy Changes (Cont’d)
When the reaction is at equilibrium, G = 0
• 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)

9. 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

10. Coenzymes used in Biologically important Redox Reactions
Conversion of ethanol to acetaldehyde is a two-electron oxidation

11. 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

12. The Structures and Redox States of the Nicotinamide Coenzymes

13. FAD/FADH2
Flavin adenine dinucleotide (FAD) is also a biological oxidizing agent
Protons, as well as, electrons are accepted by FAD

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

15. 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

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

17. 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

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

19. ATP Hydrolysis Decreases in Electrostatic Repulsion
Marked decrease in electrostatic repulsion of -phosphate of GDP upon hydrolysis of ATP to ADP

20. Organophosphates Important in Producing Energy

21. Role of ATP as Energy Currency

22. 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

23. 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

24. Two Ways of Looking at Coenzyme A
Coenzyme A (CoA-SH) contains units of 2-mercaptoethylamine, pantothenic acid, and 3’,5’-ADP

25. 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

26. The Role of Electron Transfer and ATP Production in metabolism

27. 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