1. The importance of energy changes and electron transfer in metabolism
Chapter 15

2. 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

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

4. 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

5. 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

6. 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

7. The Structures and Redox States of the Nicotinamide Coenzymes

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

9. 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

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

11. 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

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

13. Role of ATP as Energy Currency

14. 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

15. 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

16. Two Ways of Looking at Coenzyme A
Coenzyme A (CoA-SH) contains units of 2-mercaptoethylamine, pantothenic acid, and 3’,5’-ADP
The structure of coenzyme A and space filling model

17. 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
Plays imp role in citric acid cycle

18. 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

19. What are 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
All substances involved in reaction are in their standard states is pure substance itself. We use standard conditions for any process as the basis for comparing reactions

20. 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˚)
R is gas constant and T is absolute temperature. The value of delta G under a given set of conditions depends on value of delta G degree and on concentration of reactants and products.

21. 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°’

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

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

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