Bioenergetics slides2 Add bookmark #5

1. Free Energy
• The energy that cells can and must use is free
energy, described by the Gibbs free-energy
function G
– This allows to predict the direction of chemical
reactions,
– exact equilibrium position,
– amount of work they can perform at constant
temperature and pressure
• Acquisition of free energy
– Heterotrophic cells …..from nutrient molecules,
– photosynthetic cells …….from solar radiation.

2. Equilibrium Constant
• At the equilibrium concentrations of reactants
and products, rates of the forward and reverse
reactions are exactly equal.
• The concentrations of reactants and products at
equilibrium define the equilibrium constant, Keq
• In the general reaction,
• the equilibrium constant is given by

3. • at non equilibrium state, the tendency to move toward
equilibrium represents a driving force, the magnitude of
which can be expressed as the free-energy change for the
reaction, ∆G.
• Under standard conditions (298K or 25oC), when reactants
and products are initially present at 1 M conc., the force
driving the system toward equilibrium is defined as the
standard free-energy change, ∆Go. (the standard state for
reactions that involve [H] 1 M, or pH 0.
• Most biochemical reactions, occur near pH 7; both the pH
and the concentration of water (55.5 M) are essentially
constant.
• Physical constants based on this biochemical standard state
are called standard transformed constants. (∆G’o and K’eq).
• these transformed constants are referred as standard free-
energy changes.

4. • Both ∆G’o and K’eq are constant characteristics for each
reaction.
• The standard free-energy change of a chemical reaction is
simply an alternative mathematical way of expressing its
equilibrium constant.
• When
K’eq=1 then ∆G’o=0
K’eq>1 then ∆G’o=-ve
K’eq<1 then ∆G’o=+ve
A small change in ∆G’o responds with a great change in K’eq

5. Chemical Logic and Common
Biochemical Reactions
• All types of chemical reactions don’t take place in
living things.
• biochemical reactions are mainly determined by
– (1) their relevance to that particular metabolic System
– (2) their rates.
• A relevant reaction is one that makes use of an
available substrate and converts it to a useful
product. However, sometimes this may not occur.
• Some chemical transformations are too slow to
contribute to living systems even with the aid of
powerful enzymes.

6. Types of Reactions in living cells
• Most cells have the capacity to carry out thousands
of specific, enzyme-catalyzed reactions.
• Five general categories:
– (1) reactions that make or break carbon–carbon bonds
– (2) internal rearrangements, isomerizations, and
eliminations;
– (3) free-radical reactions;
– (4) group transfers;
– (5) oxidation-reductions.

7. • Two basic chemical principles.
• 1st , a covalent bond consists of a shared pair
of electrons, the bond can be broken in two
general ways.
– In homolytic cleavage, each atom leaves the bond
as a radical, with one unpaired electron.
– In heterolytic cleavage, one atom retains both
bonding electrons.

9. • The second basic principle is that many
biochemical reactions involve interactions
between nucleophiles and electrophiles.

11. Reactions That Make or Break Carbon–Carbon
Bonds
• Heterolytic cleavage of a C⎯C bond yields a
carbanion and a carbocation.
• Conversely, the formation of a C⎯C bond involves
the combination of a nucleophilic carbanion and
an electrophilic carbocation.
• But both intermediates are unstable and
energetically inaccessible. Unless chemical
assistance is provided in the form of functional
groups containing electronegative atoms (O, N)
which facilitate the formation of carbanion and
carbocation intermediates

12. Carbanion formation
• carbonyl group
• imine (CPNH2) group
• delocalize electronscan be further enhanced
by a general acid catalyst or by a metal ion
such as Mg2+

13. Examples of Carbanion formation

14. carbocation intermediate
• in reactions where formation or cleavage C⎯C
bonds is generated by the elimination of a very
good leaving group, such as pyrophosphate

16. Internal Rearrangements, Isomerizations, and
Eliminations
• Redistribution of electrons results in alterations of many
different types without a change in the overall oxidation
state of the molecule.
• different groups in a molecule may undergo oxidation-
reduction but no net change in oxidation state
• cis-trans rearrangement; or change in double bond
position.
• A simple transposition of a C=C bond occurs during
metabolism of oleic acid.
• elimination reaction is the loss of water from an alcohol,
resulting in the introduction of a C=C bond

18. Free-Radical Reactions
• These include: isomerizations that make use
of adenosylcobalamin (vitamin B12) or S-
adenosylmethionine, which are initiated with
a 5-deoxyadenosyl radical, certain radical-
initiated decarboxylation reactions; some
reductase reactions; and some rearrangement
reactions.

19. The biosynthesis of heme in Escherichia coli includes a decarboxylation step

20. Group Transfer Reactions
• The transfer of acyl, glycosyl, and phosphoryl
groups from one nucleophile to another is
common in living cells.
• The chymotrypsin reaction is one example of acyl
group transfer
• Glycosyl group transfers involve nucleophilic
substitution at C-1 of a sugar ring.

21. • Phosphoryl group transfers play a special role in
metabolic pathways
• a good leaving group to a metabolic intermediate
to “activate” the intermediate for subsequent
reaction.
• Among the better leaving groups in nucleophilic
substitution reactions are inorganic
orthophosphate and inorganic pyrophosphate;
• esters and anhydrides of phosphoric acid.

22. • The large family of enzymes that catalyze
phosphoryl group transfers with ATP as donor
are called kinases (Greek kinein, “to move”).
Hexokinase, for example, “moves” a
phosphoryl group from ATP to glucose.

23. Oxidation-Reduction Reactions
• Carbon atoms can exist in five oxidation
states,

24. • Transitions between these states are of crucial
importance in metabolism
• In many biological oxidations, a compound loses
two electrons and two hydrogen ions
• these reactions are commonly called
dehydrogenations and the enzymes that catalyze
them are called dehydrogenases
• In some, biological oxidations, a carbon atom
becomes covalently bonded to an oxygen atom.
• The enzymes that catalyze these oxidations are
generally called oxidases or, if the oxygen atom is
derived directly from molecular oxygen (O2),
oxygenases.

25. • Every oxidation must be accompanied by a
reduction, in which an electron acceptor
acquires the electrons removed by oxidation.
• Every oxidation must be accompanied by a
reduction, in which an electron acceptor
acquires the electrons removed by oxidation.
• Oxidation reactions generally release energy.
• Most living cells obtain the energy needed for
cellular work by oxidizing metabolic fuels such
as carbohydrates or fat.