prentice hall c2002chapter 101 major pathways in cells metabolic fuels three major nutrients...

Prentice Hall c2002 Chapter 10 1

Major Pathways in Cells

• Metabolic fuels

Three major nutrients consumed by mammals: (1) Carbohydrates - provide energy(2) Proteins - provide amino acids for protein

synthesis and some energy(3) Fats - triacylglycerols provide energy and

also lipids for membrane synthesis

Prentice Hall c2002 Chapter 10 2

• Overview of catabolic pathways

Prentice Hall c2002 Chapter 10 3

Catabolism produces compounds for energy utilization

• Three types of compounds are produced that mediate the release of energy

(1) Acetyl CoA

(2) Nucleoside triphosphates (e.g. ATP)

(3) Reduced coenzymes (NADH, FADH2, QH2)

Prentice Hall c2002 Chapter 10 4

Reducing Power

• Electrons of reduced coenzymes flow toward O2

• This produces a proton flow and a transmembrane potential

• Oxidative phosphorylation is the process by which the potential is coupled to the reaction: ADP + Pi ATP

Prentice Hall c2002 Chapter 10 5

Compartmentation of metabolic processes

Prentice Hall c2002 Chapter 10 6

Thermodynamics and Metabolism

• Free-energy change (G) is a measure of the chemical energy available from a reaction

G = Gproducts - Greactants

• H = change in enthalpy

• S = change in entropy

A. Free-Energy Change

Prentice Hall c2002 Chapter 10 7

• Both entropy and enthalpy contribute to G

G = H - TS

(T = degrees Kelvin)

-G = a spontaneous reaction in the direction written

+G = the reaction is not spontaneous

G = 0 the reaction is at equilibrium

Relationship between energy and entropy

Prentice Hall c2002 Chapter 10 8

Prentice Hall c2002 Chapter 10 9

The Standard State (Go) Conditions

• Reaction free-energy depends upon conditions

• Standard state (Go) - defined reference conditions

Standard Temperature = 298K (25oC)

Standard Pressure = 1 atmosphere

Standard Solute Concentration = 1.0M

• Biological standard state = Go’ or G’o

Standard [H+] = 10-7 M (pH = 7.0) rather than 1.0M (pH = 1.0); [H2O]=55.5M; [Mg2+]=1mM

Prentice Hall c2002 Chapter 10 10

B. Equilibrium Constants and Standard Free-Energy Change

• For the reaction: A + B C + D

Greaction = Go’reaction + RT ln([C][D]/[A][B])

• At equilibrium: Keq = [C][D]/[A][B] and Greaction = 0, so that:

Go’reaction = -RT ln Keq

Prentice Hall c2002 Chapter 10 11

C. Actual Free-Energy Change Determines Spontaneity of Cellular Reactions

• When a reaction is not at equilibrium, the actual free energy change (G) depends upon the ratio of products to substrates

• Q = the mass action ratio

G = G’o + RT ln Q

Where Q = [C]’[D]’ / [A]’[B]’

Prentice Hall c2002 Chapter 10 12

Prentice Hall c2002 Chapter 10 13

The Free Energy of ATP

• Energy from oxidation of metabolic fuels is largely recovered in the form of ATP

Prentice Hall c2002 Chapter 10 14

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• Hydrolysis of ATP

Prentice Hall c2002 Chapter 10 18

Complexes between ATP and Mg2+

Prentice Hall c2002 Chapter 10 19

ATP is an “energy-rich” compound

• A large amount of energy is released in the hydrolysis of the phosphoanhydride bonds of ATP (and UTP, GTP, CTP)

• All nucleoside phosphates have nearly equal standard free energies of hydrolysis