Metabolism Is the Sum of Cellular 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 catabolism

Metabolic Pathways Are Sequences 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: Carbohydrates Lipids Amino Acids Nucleotides

Forms of metabolic pathways (a) Linear(b) Cyclic

(c) Spiral pathway (fatty acid biosynthesis) Forms of metabolic pathways

Metabolic Pathways Are Regulated 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 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 activation Metabolite early in the pathway activates an enzyme further down the pathway

Covalent modification for enzyme 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, amplification by a signaling cascade

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

Reducing Power Electrons of reduced coenzymes flow toward O 2 This produces a proton flow and a transmembrane potential Oxidative phosphorylation is the process by which the potential is coupled to the reaction: ADP + P i ATP

Thermodynamics and Metabolism A. Free-Energy Change Free-energy change (  G) is a measure of the chemical energy available from a reaction  G = G products - G reactants  H = change in enthalpy  S = change in entropy

Relationship between energy and entropy Both entropy and enthalpy contribute to  G  G =  H - T  S (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 (  G o ) Conditions Reaction free-energy depends upon conditions Standard state (  G o ) - defined reference conditions Standard Temperature = 298K (25 o C) Standard Pressure = 1 atmosphere Standard Solute Concentration = 1.0M Biological standard state =  G o’ Standard H + concentration = (pH = 7.0) rather than 1.0M (pH = 1.0)

Equilibrium Constants and Standard Free-Energy Change For the reaction: A + BC + D  G reaction =  G o’ reaction + RT ln([C][D]/[A][B]) At equilibrium: Keq = [C][D]/[A][B] and  G reaction = 0, so that:  G o’ reaction = -RT ln K eq

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]’

Hydrolysis of ATP

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

Energy of phosphoanhydrides (1) Electrostatic repulsion among negatively charged oxygens of phosphoanhydrides of ATP (2) Solvation of products (ADP and P i ) or (AMP and PP i ) is better than solvation of reactant ATP (3) Products are more stable than reactants There are more delocalized electrons on ADP, P i or AMP, PP i than on ATP

Glutamine synthesis requires ATP energy

Phosphoryl-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 group 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 PA

Nucleotidyl-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 (PP i ) product drives reaction to completion

Synthesis of acetyl CoA

Thioesters Have High Free Energies of Hydrolysis Thioesters are energy-rich compounds Acetyl CoA has a  G o’ = -31 kJ mol -1

Succinyl CoA Energy Can Produce GTP

Reduced Coenzymes Conserve Energy 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 A red + B ox A ox + B red

Diagram of an electrochemical cell Electrons flow through external circuit from Zn electrode to the Cu electrode

Standard reduction potentials and free energy Relationship between standard free-energy change and the standard reduction potential:  G o’ = -nF  E o’ n = # electrons transferred F = Faraday constant (96.48 kJ V -1 )  E o’ = E o’ electron acceptor - E o’ electron donor

Reduction Potentials Cathode (Reduction) Half-Reaction Standard Potential E ° (volts) Li + (aq) + e - -> Li(s)-3.04 K + (aq) + e - -> K(s)-2.92 Ca 2+ (aq) + 2e - -> Ca(s)-2.76 Na + (aq) + e - -> Na(s)-2.71 Zn 2+ (aq) + 2e - -> Zn(s)-0.76 Cu 2+ (aq) + 2e - -> Cu(s)0.34 O 3 (g) + 2H + (aq) + 2e - -> O 2 (g) + H 2 O(l)2.07 F 2 (g) + 2e - -> 2F - (aq)2.87

Actual reduction potentials (  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 =  E o’ - (RT/nF) ln ([A ox ][B red ] / [A red ][B ox ] )

Electron Transfer from NADH Provides 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, P i Half-reaction for overall oxidation of NADH: NAD + + 2H + + 2e - NADH + H + (E o’ = -0.32V)

Example Suppose we had the following voltaic cell at 25 o 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 25 o 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 - ---> Ag 0 E 0 red = v Cu e > Cu 0 E 0 red = v

Example: Biological Systems Both NAD + and FAD are oxidizing agents

The question is which would oxidize which? OR 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 FADH 2 i.e. reduction potentials of NADH and FADH 2

 E o’ = E o’ electron acceptor - E o’ electron donor Remember, For a spontaneous reaction  E o ’ must be positive

Therefore, rearrange Add the two reactions

electron acceptor electron donor