Biochemical Thermodynamics Pt 2

Question 1

Spontaneity and free energy: Endergonic, exergonic, and spontaneous

Answer 1

Thus, if reaction is just spontaneous,
i.e. Go = 0, then Keq = 1

• If Go < 0, then Keq > 1: Exergonic
• If Go > 0, then Keq < 1: Endergonic
• You may catch me saying “exoergic”
  and “endoergic” from time to time:
  these mean the same things.

Question 2

Change in free energy indicates
that the…

Answer 2

reaction could be used to
perform useful work

• If Go < 0, we can do work
• If Go > 0, we need to d

Question 3

What kind of work? (free energy)

Answer 3

Movement (flagella, muscles)

• Chemical work:

– Transport molecules against concentration
gradients

– Transport ions against potential gradients

• To drive otherwise endergonic reactions…

– by direct coupling of reactions

– by depletion of products

Question 4

Coupled reactions

Answer 4

Often a single enzyme catalyzes 2
reactions, shoving them together:

reaction 1, A  B: Go
1 < 0

reaction 2, C  D: Go
2 > 0

• Coupled reaction:

A + C  B + D: Go
C
= Go
1 + Go
2

• If Go
  C
  < 0,
  then reaction 1 is driving reaction 2!

Question 5

How else can we couple ?

Answer 5

Concentration of product may play a
role

• As we’ll discuss in a moment, the
  actual free energy depends on Go
  and on concentration of products and
  reactants
• So if the first reaction withdraws
  product of reaction 2 away,
  that drives the equilibrium of reaction 2
  to the right

Question 6

Free-energy changes for…

Answer 6

chemical reactions
can be added and subtracted to give free-energy changes for other chemical reactions.

Question 7

The free energy of a favorable chemical
reaction can be…

Answer 7

used to make an
unfavorable reaction happen.

Question 8

Often coupled reactions involve…

Answer 8

withdrawal of a product from
availability

• If that happens,
  [product] / [reactant] shrinks,
  the second term becomes negative,
  and G < 0 even if Go >

Question 9

How to solve energy problems
involving coupled equations

• General principles:

Answer 9

– If two equations are added, their
energetics add

– An item that appears on the left and
right side of the combined equation
can be cancelled

Question 10

The coupling of reactions to cause a non-spontaneous
reaction to occur is …

Answer 10

very important in biochemical systems.

Many reactions essential to life do not occur spontaneously.
In the human body these reactions can be made to occur by
coupling them with reactions that are spontaneous.
The energy required to drive non-spontaneous processes is
obtained primarily from the catabolism of foods.

Question 11

Standard free energy change, G°, assumes…

Answer 11

a
concentration of 1 M

• if [H+
  ] = 1 M, then pH = 0

– but the pH in most cells is near the neutral range

Question 12

For biochemical reactions, we define a different standard
state for…

Answer 12

concentration of H+

– standard state for [H+
] = 10-7 M, pH = 7.0

– this modified standard state is given the symbol G°’

Question 13

ATP is the…

Answer 13

“free energy currency” of life.
Its usefulness depends on its intermediate
phosphate transfer potential and its
metastability.

Question 14

The capacity of cells to carry out ATP-driven
reactions depends on…

Answer 14

the relative
concentration of ATP

• the driving force
  for the ATP hydrolysis reaction

Question 15

Energy charge -

Answer 15

energy state of the cell

Question 16

Adenosine Triphosphate:

Answer 16

• ATP readily available in cells
• Derived from catabolic reactions
• Contains two high-energy phosphate bonds
  that can be hydrolyzed to release energy:

Question 17

Any time we wish to drive a reaction that has
Go < +30 kJ/mol, we can…

Answer 17

couple it to ATP
hydrolysis and come out ahead

Question 18

If the reaction we want has
Go < +60 kJ/mol we can…

Answer 18

couple it to
ATP  AMP and come out ahead

Question 19

So ATP is a…

Answer 19

convenient source of energy
— an energy currency for the cell

Question 20

Think of store of ATP
as a roll of quarters:

Answer 20

Vendors don’t give change

• Use one quarter for some reactions,
  two for others
• Inefficient for buying $0.35 items

Question 21

Other high-energy compounds: (coins)

Answer 21

Creatine phosphate: ~ $0.40

• Phosphoenolpyruvate: ~ $0.35
• So for some reactions, they’re more
  efficient than ATP

Question 22

Actual G of a reaction is related to…

Answer 22

the
concentrations / activities of products and
reactants:

G = Go + RT ln [products]/[reactants]

• If all products and reactants are at 1M, then
  the second term drops away; that’s why we
  describe Go
  as the standard free energy

Question 23

Why is there a limitation i.e., a maximum amount of useful work that can be
obtained from a chemical reaction?

Answer 23

More useful work out means less heat out. Remember that
the liberation of heat by a reaction may be crucially important
in ensuring that the reaction causes the entropy of the
universe to increase (2nd Law). If too much of the heat
liberated is converted to useful work the reaction may lose
its spontaneity and stop altogether!
The value of ΔG for the reaction tells us just where this limit is.
That is why G is called “free energy”. ΔG tells us how much
energy is “free” for conversion into useful work.

Question 24

The equilibrium constant K can be calculated
from the…

Answer 24

standard state free energy change.