Biochem- 14

Estimate the energy content of Acetyl CoA (CH3COS-CoA) using the energy value of -220kJ/mol for reduced bonds A. ΔG'° = 880 kJ/mol B. ΔG'° = - 1100 kJ/mol C. ΔG'° = -880 kJ/mol D. ΔG'° = 1100 kJ/mol E. ΔG'° = -660 kJ/mol

C Acetyl CoA (CH3COS-CoA) has: 3 x C-H bonds Hence 3 x -220 = -660 kJ/mol 1 x C-C bonds. Hence 1 x -220 = -220 kJ/mol Total estimated energy = -880 kJ/mol

Which of the following half-reactions has the greatest reduction potential (i.e. can act as the better reducing agent) compared with all the others in the list. A. NO3- + 2H+ + 2e- ---> NO2- + H2O E'o = 0.421V B. Fumarate2- + 2H+ +2e- ——> succinate2- E'o = 0.031V C. NAD+ + H+ + 2e- ——> NADH E'o = -0.320V D. O2 + 2H+ + 2e- ——> H2O2 E'o = 0.295V

C Electrons flow from lower E'o values to higher E'o values. For a molecule to be a reducing agent it would need to give up its electrons (be oxidised) to other molecules with a higher E'o value. Please refer to the tables of reduction potentials for half reactions in redox reactions

All of the following contribute to the large, negative, free-energy change upon hydrolysis of "high-energy" compounds (e.g. ATP) except A. electrostatic repulsion in the reactant. B. stabilization of products by ionization. C. low activation energy of forward reaction. D. stabilization of products by solvation. E. stabilization of products by extra resonance forms.

C Enzymes (and other catalysts) lower the activation energy required by a reaction to progress from reactants to products. It does not contribute to the overall Free Energy changes in a reaction. The other responses do help stabilize the large Free Energy and can be seen in ATP hydrolysis

Oxidation of glucose is as follows: C6H12O6 + 6O2 —> 6CO2 + 6H2O Using the relationship ΔG = Δ - TΔS we can estimate qualitatively if ΔG for the reaction is negative even without any values for ΔH or ΔS . We can do this because: A. glucose oxidation is endothermic +ΔH B. glucose oxidation decreases entropy -ΔS C. glucose oxidation increases entropy +ΔS D. glucose oxidation is exothermic -ΔH

C,D Glucose oxidation is exothermic so -ΔH negative value glucose oxidation increases entropy +ΔS positive value T is positive ΔG = -ΔH (negative value) - TΔS (positive value) = negative value + negative value = negative ΔG (-ΔG)

The phosphorylation of glucose is an endergonic reaction as follows: Glucose + Pi = glucose 6-phosphate + H2O (where Pi = phosphate). ΔG'o = +13.8 kJ/mol The hydrolysis of ATP is an exergonic reaction as follows: ATP + H2O = ADP + Pi (where Pi = phosphate). ΔG'o = -30.5 kJ/mol These two reactions can be summed to give the overall reaction as follows: Glucose + ATP = glucose 6-phosphate + ADP Calculate the overall standard Gibbs Free Energy value for the summed (coupled) reactions. A. +44.3 kJ/mol B. +16.7 kJ/mol C. -44.3 kJ/mol D. -16.7 kJ/mol

D We can add the standard free energies of the coupled reactions. Both reactions proceed in the directions as written from left to right. Consequently, we do not need to change the sign of the standard free energy values. -30.5 + 13.8 = -16.7 kJ/mol

ATP is known as the 'energy currency' of the cell and needs to be relatively reactive for this role. However, it can remain in a bottle on the shelf (usually in a refrigerator) for many months unchanged. This is because ATP has a relatively high free energy of hydrolysis.

FALSE ATP does have a relatively high free energy of hydrolysis but this is not what prevents ATP from hydrolysing whilst sitting on the shelf. ATP has a relatively high activation energy which does prevent the hydrolysis of ATP whilst sitting on the shelf. The activation energy determines how fast or how slow a reaction will go. In this case the activation is sufficiently high enough to make ATP hydrolysis a slow reaction without any catalyst.

In a coupled reaction we can add the ΔG'o values for each reaction to get the 'overall' ΔG'o values for the combined reaction.

TRUE This is an important concept in coupling an exergonic reaction with an endergonic reaction. The energy released from the exergonic reaction can be used to 'drive' the endergonic reaction. The resulting free energy of the combined reactions must be negative for the reaction to occur spontaneously.

ATP is known as the 'energy currency' of the cell because like money it is the intermediate in the transaction of 'value' between different 'entities'.

True The value here is energy and ATP is the intermediate that transfers energy between the different 'entities', where the entities are reactions that produce energy and reactions that use energy

An oxidation reaction is where a molecule, atom, or ion will: A. lose electrons B. gain electrons C. gain hydroxide D. gain hydrogens

a

FAD transfers electrons by A. accepting two electrons and two hydrogen ions in its structure B. accepting one hydroxyl group and one hydrogen atom in its structure C. accepting only one electron in its structure D. accepting only one hydride ion in its structure

a FAD can accept one electron and one hydrogen ion to form FADH It can also accept two electrons and two hydrogen ions to form FADH2

A carbon is reduced if A. The number of bonds to more electronegative atoms (e.g. O, N, F, Cl, I or S) decreases B. The number of hydrogen atoms bonded to a carbon decreases C. The number of bonds to more electronegative atoms increases D. The number of hydrogen atoms bonded to a carbon increases

a, d

A carbon is oxidised if: A. The number of hydrogen atoms bonded to a carbon decreases B. The number of bonds to more electronegative atoms (e.g. O, N, F, Cl, I or S) increases C. The number of bonds to more electronegative atoms (e.g. O, N, F, Cl, I or S) decreases D. The number of hydrogen atoms bonded to a carbon increases

a,b

Reduction potentials can be used to: A. track the number of electrons stored or transferred B. estimate the Free Energy of an oxidation-reduction reaction C. estimate the enthalpy of ATP hydrolysis D. estimate the amount of volts in hydrogen atoms

a,b Reduction potentials are a way of tracking the number of electrons stored or transferred Reduction potentials determine the Free Energy of an oxidation/reduction reaction Reduction potentials do not measure the 'volts' in a hydrogen atom but do use the reduction of a hydrogen ion as a standard. H+ + e- = 1/2H2 Eo = 0 Volts Standard = 1M concentrations of oxidant and reductant at pH = 0 or pH = 7 for biological reactions (E'o)

Which of the following are potential mechanisms for coupling exergonic reactions with endergonic reactions? A. Chemical coupling B. Sequential reactions C. electrons D. enzyme

a,b,c,d

A reduction reaction is where a molecule, atom, or ion will: A. gain hydrogens B. gain electrons C. gain hydroxide D. lose electrons

b

Exergonic reactions can be used to 'drive' endergonic reactions. This statement means that: A. A reaction with positive Free Energy (+ΔG) can be coupled to provide energy for a reaction with negative Free Energy (-ΔG) B. A reaction with negative Free Energy (-ΔG) can be coupled to provide energy for a reaction with positive Free Energy (+ΔG) C. A reaction with positive enthalpy (+ΔH) can be coupled to provide energy for a reaction with negative enthalpy (-ΔG) D. A reaction with negative enthalpy (-ΔH) can be coupled to provide energy for a reaction with positive enthalpy (+ΔG)

b The terms'exergonic' and 'endergonic' refer specifically to changes in Gibbs Free Energy (ΔG) and not changes in enthalpy (ΔH) or entropy (ΔS). A reaction with negative Free Energy (-ΔG) - exergonic - can be coupled to provide energy for a reaction with positive Free Energy (+ΔG) - endergonic reaction. Exergonic reactions can be used to 'drive' endergonic reactions - if coupled A coupled reaction needs: (1) a common intermediate of the reactions that are coupled (2) a 'place' for the energy transfer to occur Somewhere to provide a mechanism for the transfer of energy from exergonic to endergonic reaction! An enzyme!

The Gibbs Free energy change for ATP hydrolysis is large and negative in part because: A. there is increased electrostatic repulsion among four negative charges of phosphate after hydrolysis B. the products (ADP and phosphate) are more soluble in water than ATP C. the terminal anhydride bonds in ATP are 'stronger' compared with the bonds in the products. D. the products (ADP and phosphate) are relatively more stable

b, d the terminal anhydride bonds are 'weaker' and contribute to the instability of ATP compared with the products. electrostatic repulsion among four negative charges of phosphate is relieved by charge separation after hydrolysis the products are relatively more stable Phosphate is resonance stabilised ADP2- can ionize to ADP3- the products (phosphate and ADP) are more soluble in water than ATP

Exergonic reactions can be used to 'drive' endergonic reactions. This process requires: A. a transport protein that couples the two reactons B. a common intermediate shared by the reactions that are coupled C. a mechanism for the transfer of energy from the endothermic reaction to the exothermic reactions D. a mechanism for the transfer of energy from the exergonic reaction to the endergonic reaction

b,d Exergonic reactions can be used to 'drive' endergonic reactions - if coupled A coupled reaction needs: (1) a common intermediate of the reactions that are coupled (2) a mechanism for the energy transfer

The free energy for ATP hydrolysis in vivo is actually greater than the standard free energy change of -30 kJ mol-1 because: A. it can easily be formed from other nucleotide triphosphates. B. All of the above C. of the actual concentrations of ATP and its hydrolysis products in cells. D. it can participate in phosphoryl group transfers. E. it has stronger electrostatic repulsion.

c

The hydrolysis of ATP has a large negative ΔG'°; nevertheless the molecule is stable in solution. This stability is due to: A. resonance stabilization of products. B. entropy stabilization of products C. the hydrolysis reaction having a large activation energy. D. ionization of the phosphates in the products. E. the hydrolysis reaction being exergonic.

c A large negative free energy indicates that the reaction will proceed spontaneously from reactants to products in the direction written. In this case ATP + H2O ---> ADP + Pi A large negative free energy also indicates that the products of a reaction are probably more stable than the reactants. Thus, the products ADP and Pi are relatively more stable than the reactant ATP. However, ATP is seen to be stable and does not rapidly progress to products. This can only be due to a high energy of activation of ATP. That is, ATP molecules have to possess a kinetic energy large enough to overcome the initial energy needed for activation of ATP before it can spontaneously form the products ADP and Pi. The rate of a reaction is determined by the kinetic energy of the reactant molecules and the size of the energy (activation) required to activate reactants into a transition state complex.

All of the following contribute to the large, negative, free-energy change upon hydrolysis of "high-energy" compounds (e.g. ATP) except: A. electrostatic repulsion in the reactant. B. stabilization of products by ionization. C. low activation energy of forward reaction. D. stabilization of products by solvation. E. stabilization of products by extra resonance forms.

c Enzymes (and other catalysts) lower the activation energy required by a reaction to progress from reactants to products. It does not contribute to the overall Free Energy changes in a reaction. The other responses do help stabilize the large Free Energy and can be seen in ATP hydrolysis.

The free energy of ATP hydrolysis essentially comes from A. breaking the high energy terminal phosphoanhydride bond B. water interacting with ADP C. forming ADP and phosphate D. water interacting with phosphate

c The net energy comes from the formation of bonds in the products minus the energy needed for breaking bonds in ATP.

Calculate (with 2 significant figures) the standard Gibbs Free Energy available for the reaction: Pyruvate + NADH + H+ ——> lactate + NAD+ Given the following information: Pyruvate + 2H+ + 2e- ——> lactate E'o = -0.19V NAD+ + H+ + 2e- ——> NADH E'o = -0.32V Faradays constant (F) = 96485 J V-1 mol-1 A. -25,000 kJ/mol B. +98 kJ/mol C. -25 kJ/mol D. -98 kJ/mol E. +25 kJ/mol F. -98,000 kJ/mol

c The reaction NAD+ + H+ + 2e- ——> NADH E'o = -0.32V occurs in the reverse direction in the overall reaction and so we must reverse the sign of the reduction potential: + 0.32V ΔE'o = -0.19 + 0.32 = 0.13V ΔG'o = -nFΔE'o Two electrons are transfered so n=2 F = Faradays constant = 96485 J V-1 mol-1 ΔG'o = -2 x 96485 x 0.13 = -250861 J/mol = -25 kJ/mol (2 sig fig)

The Free Energy contained in one mol of glucose (C6H12O6), assuming -220kJ/mol for each reduced bond, is: A. +2.640 kJ/mol B. +2640 J/mol C. -2200 kJ/mol D. -2640 kJ/mol E. -2.200 J/mol

d Each glucose molecule contains five C-C and seven C-H reduced bonds which can be oxidised to release energy. The amount of potential energy is calculated by multiplying the number of reduced bonds by the estimated amount of -220kJ/mol. Thus: 5 C-C bonds = 5 x -220 kJ/mol = -1100 kJ/mol 7 C-H bonds = 7 x -220 kJ/mol = -1540 kJ/mol Total estimated energy generated from one mole of glucose = -2640kJ/mol

NAD+ is a molecule that: A. is in the reduced state B. has an overall positive charge C. has no net charge D. has an overall negative charge

d NAD+ the 'plus' sign here is only meant to indicate that it is the oxidised form of the molecule where the reduced form is NADH. In fact NAD+ has an overall negative charge.

Biochemical reactions are more likely to proceed if the reaction has an increase in enthalpy (△H) and a decrease in entropy (△S).

false

The tendency of a metabolic reaction to proceed is due to the free energy of both the reactants and products as well as the change in randomness of that reaction

true