Metabolism Is the Sum of Cellular Reactions

Metabolism Is the Sum of Metabolism Is the Sum of Cellular ReactionsCellular Reactions

• Metabolism - the entire network of chemical reactions carried out by living cells

• Metabolites - small molecule intermediates in the degradation and synthesis of polymers

• Catabolic reactions - degrade molecules to create smaller molecules and energy

• Anabolic reactions - synthesize molecules for cell maintenance, growth and reproduction

Anabolism and catabolismAnabolism and catabolism

Metabolic Pathways Are Metabolic Pathways Are Sequences of ReactionsSequences of Reactions

• Metabolism includes all enzyme reactions

• Metabolism can be subdivided into branches

• The metabolism of the four major groups of biomolecules will be considered:

CarbohydratesLipidsAmino AcidsNucleotides

Forms of metabolic pathwaysForms of metabolic pathways

(a) Linear (b) Cyclic

(c) Spiral pathway (fatty acid biosynthesis)

Forms of metabolic pathwaysForms of metabolic pathways

Metabolic Pathways Are Metabolic Pathways Are RegulatedRegulated

• Metabolism is highly regulated to permit organisms to respond to changing conditions

• Most pathways are irreversible

• Flux - flow of material through a metabolic pathway which depends upon:

(1) Supply of substrates(2) Removal of products(3) Pathway enzyme activities

Feedback inhibitionFeedback inhibition

• Product of a pathway controls the rate of its own synthesis by inhibiting an early step (usually the first “committed” step (unique to the pathway)

Feed-forward activationFeed-forward activation

• Metabolite early in the pathway activates an enzyme further down the pathway

Covalent modification for Covalent modification for enzyme regulationenzyme regulation

• Interconvertible enzyme activity can be rapidly and reversibly altered by covalent modification

• Protein kinases phosphorylate enzymes (+ ATP)

• Protein phosphatases remove phosphoryl groups

• The initial signal may be amplified by the “cascade” nature of this signaling

Regulatory role of a protein kinase, Regulatory role of a protein kinase, amplification by a signaling cascadeamplification by a signaling cascade

Major Pathways in CellsMajor 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

Reducing PowerReducing 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

Thermodynamics and MetabolismThermodynamics and Metabolism

A. Free-Energy Change

• 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

Relationship between energy and entropy Relationship between energy and entropy

• 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

The Standard State (The Standard State (GGoo) Conditions) 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’

Standard H+ concentration = 10-7 (pH = 7.0) rather than 1.0M (pH = 1.0)

Equilibrium Constants and Equilibrium Constants and Standard Free-Energy ChangeStandard 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

Actual Free-Energy Change Determines Actual Free-Energy Change Determines Spontaneity of Cellular Reactions 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 = Go’ + RT ln Q

Where Q = [C]’[D]’ / [A]’[B]’

• Hydrolysis of ATP

ATP is an ATP is an “energy-rich” “energy-rich” compoundcompound

• 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

Energy of phosphoanhydridesEnergy of phosphoanhydrides

(1) Electrostatic repulsion among negatively charged oxygens of phosphoanhydrides of ATP

(2) Solvation of products (ADP and Pi) or (AMP and PPi) is better than solvation of reactant ATP

(3) Products are more stable than reactants There are more delocalized electrons on ADP, Pi or AMP, PPi than on ATP

Glutamine synthesis requires Glutamine synthesis requires ATP energyATP energy

Phosphoryl-Group TransferPhosphoryl-Group Transfer

• Phosphoryl-group-transfer potential - the ability of a compound to transfer its phosphoryl group

• Energy-rich or high-energy compounds have group transfer potentials equal to or greater than that of ATP

• Low-energy compounds have group transfer potentials less than that of ATP

Transfer of the phosphoryl Transfer of the phosphoryl group from PEP to ADPgroup from PEP to ADP

• Phosphoenolpyruvate (PEP) (a glycolytic intermediate) has a high P-group transfer potential

• PEP can donate a P to ADP to form ATP

Structures of PC and PAStructures of PC and PA

Nucleotidyl-Group TransferNucleotidyl-Group Transfer

• Transfer of the nucleotidyl group from ATP is another common group-transfer reaction

• Synthesis of acetyl CoA requires transfer of an AMP moiety to acetate

• Hydrolysis of pyrophosphate (PPi) product drives reaction to completion

Synthesis of acetyl CoASynthesis of acetyl CoA

Synthesis of acetyl CoASynthesis of acetyl CoA

Thioesters Have High Free Thioesters Have High Free Energies of HydrolysisEnergies of Hydrolysis

• Thioesters are energy-rich compounds

• Acetyl CoA has a Go’ = -31 kJ mol-1

Succinyl CoA Energy Can Succinyl CoA Energy Can Produce GTPProduce GTP

Reduced Coenzymes Conserve Energy Reduced Coenzymes Conserve Energy from Biological Oxidations from Biological Oxidations

• Amino acids, monosaccharides and lipids are oxidized in the catabolic pathways

• Oxidizing agent - accepts electrons, is reduced

• Reducing agent - loses electrons, is oxidized

• Oxidation of one molecule must be coupled with the reduction of another molecule

Ared + Box Aox + Bred

Diagram of an electrochemical cellDiagram of an electrochemical cell

• Electrons flow through external circuit from Zn electrode to the Cu electrode

Standard reduction potentials Standard reduction potentials and free energyand free energy

• Relationship between standard free-energy change and the standard reduction potential:

Go’ = -nFEo’

n = # electrons transferred

F = Faraday constant (96.48 kJ V-1)

Eo’ = Eo’electron acceptor - Eo’

electron donor

Reduction PotentialsReduction Potentials

Cathode (Reduction)Half-Reaction

Standard PotentialE° (volts)

Li+(aq) + e- -> Li(s) -3.04

K+(aq) + e- -> K(s) -2.92

Ca2+(aq) + 2e- -> Ca(s) -2.76

Na+(aq) + e- -> Na(s) -2.71

Zn2+(aq) + 2e- -> Zn(s) -0.76

Cu2+(aq) + 2e- -> Cu(s) 0.34

O3(g) + 2H+(aq) + 2e- -> O2(g) + H2O(l) 2.07

F2(g) + 2e- -> 2F-(aq) 2.87

Actual reduction potentials Actual reduction potentials ((E)E)

• Under biological conditions, reactants are not present at standard concentrations of 1 M

• Actual reduction potential (E) is dependent upon the concentrations of reactants and products

E = Eo’ - (RT/nF) ln ([Aox][Bred] / [Ared][Box] )

Electron Transfer from NADH Electron Transfer from NADH Provides Free EnergyProvides Free Energy

• Most NADH formed in metabolic reactions in aerobic cells is oxidized by the respiratory electron-transport chain

• Energy used to produce ATP from ADP, Pi

• Half-reaction for overall oxidation of NADH:

NAD+ + 2H+ + 2e- NADH + H+ (Eo’ = -0.32V)

Example

Suppose we had the following voltaic cell at 25o C:

Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s)What would be the cell potential under these conditions?

Example

Suppose we had the following voltaic cell at 25o C:

Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s)What would be the cell potential under these conditions?

Ag+ + e- ---> Ag0 E0red = + 0.80 v

Cu+2 + 2e- ----> Cu0 E0red = + 0.337 v

Example: Biological SystemsExample: Biological Systems

Both NAD+ and FAD are oxidizing agents

N

NH2

O

R

N

NH2

O

R

N

N

N

N

R

H

O

O N

HN

NH

N

R

H

O

O

2H 2e

H , 2e

,

NAD NADH

FAD FADH2

The question is which would oxidize which?

NAD + FADH2 NADH + FAD + H

OR

FAD + NADH + H NAD + FADH2

Which one of the above is the spontaneous reaction?

in which G is negative

To be able to answer the question

We must look into the “electron donation” capabilities of NADH and FADH2

i.e. reduction potentials of NADH and FADH2

NAD + 2H NADH + H Eo ' = -0.32 eV

+ 2HFAD FADH2

2e+

+ 2e Eo ' = -0.22 eV

Eo’ = Eo’electron acceptor - Eo’

electron donor

Remember,

NAD + 2H NADH + H Eo ' = -0.32 eV

+ 2HFAD FADH2

2e+

+ 2e Eo ' = -0.22 eV

For a spontaneous reaction Eo ’ must be positive

Therefore,Therefore,

NAD + 2H NADH + H Eo ' = -0.32 eV

+ 2HFAD FADH2

2e+

+ 2e Eo ' = -0.22 eV

rearrange

NADH + H NAD + 2H 2e+ Eo ' = +0.32 eV

+ 2HFAD FADH2+ 2e Eo ' = -0.22 eV

Add the two reactionsNADH + H+FAD FADH2 + NAD Eo ' = 0.10 eV

NADH + H+FAD FADH2 + NAD Eo ' = 0.10 eV

electronacceptor

electrondonor