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


• Overview of catabolic pathways


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)


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


Compartmentation of metabolic processes


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


• 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


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


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


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]’


The Free Energy of ATP

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


• Hydrolysis of ATP


Complexes between ATP and Mg2+


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

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

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