bioenergetics (1)
TRANSCRIPT
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BIOCHEMISTRY
JULIUS P. MARIO, RMT, MSci
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ENERGY
Capacity to do work
unlike matter, it is known and recognized byits effects
cannot be seen, touched, smelled or weighed
constant in the universe
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TYPES OF ENERGY
Radiant = solar energy
Thermal = associated with random motion of
atoms and moleculesChemical = stored within the structural units of
chemical substances
= can be released, stored or convertedto other energy forms
Potential = available by virtue of an objects
position
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ENERGY CHANGES
chemical reactions absorb or release energy,
generally in the form of heat
HEAT
transfer of thermal energy between two bodies
that are at different temperatures
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THERMOCHEMIS
TRY
Study of heat change in chemical reactions
SYSTEM AND SURROUNDING
System = specific part of the universe that is of
interest to us
Surrounding = the rest of the universe outside the
system
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TYPES OF SYS
TEM
OPEN = can exchange mass and energy with its
surroundings
CLOSED = allows the transfer of energy but not
mass
ISOLATED = does not allow the transfer of
either mass or energy
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EXOTHERMIC PROCESS
= any process that gives off heat
= transfers thermal energy to the surroundings= total energy of products is less than the total
energy of the reactants
2H2 (g) + O2 (g) 2H2O(M) + energy
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ENDOTHERMIC PROCESS
= any process that requires heat to be applied to
the system by the surroundings
= total energy of reactants is less than the total
energy of products
energy + 2HgO(s) 2Hg(M) + O2 (g)
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THERMODYNAMICS
scientific study of the interconversion of heat
and other kinds of energy
study of the changes in the state of a system
provide useful guidelines for understanding the
energetics and directions of processes
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STA
TE OF
THE SYS
TEM
the values of all relevant macroscopic
properties, for examples, composition, energy,
temperature, pressure and volume
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SIGN CONVENTIONS FOR WORK & HEA
T
PROCESS SIGN
work done by the system on the surroundings -
work done on the system by the surroundings +
heat absorbed by the system from the
surroundings (endothermic) +
heat absorbed by the surroundings from
the system (exothermic) -
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LAWS OFTHERMODYNAMICS1ST LAW
= energy cannot be created nor destroyed but can be
converted from one form to another
2ND LAW
= the entropy of the universe increases in a spontaneous
process and remains unchanged in an equilibrium
process
3RD LAW
= entropy of a perfect crystalline substance is zero at the
absolute zero temperature
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Reaction Rates and Chemical Equilibrium
Chemical kinetics = study of reaction rates
Reaction rate = change in concentration of a
reactant or product per unit time
molecular collisions - effective collisions
activation energy - minimum energy necessary
for the reaction to happen
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Depends on
1. Speed of colliding objects
2. Angle of approach = head-on is best3. Must have proper orientation
energy collision of activation energy - no
reactionH2O(M) + HCl(g) H3O
+(aq) + Cl-(aq)
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Activation energy & Energy Diagrams
Activation E
E of productsE of reactants
E or rxn
Progress of reaction
energy
Transition state
Downhill reactions - exothermic
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Activation energy & Energy Diagrams
Activation E E of products
E of reactants
E or rxn
Progress of reaction
energy
Transisiton state
Uphill reactions - endothermic
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Transition state - top of the energy hill
- one or more original bonds are
partially broken or
- one or more bonds (new) may
be in the process of formation
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Activation energy - energy hill
- if low, faster reaction
- if high, slower reaction
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Factors Affecting Rates of Reaction
A. Nature of reactants for solid = surface area
affects for gases, pressure- rate ions in
aqueous solution - instantaneous
B. Concentration
direct relationship between rate and
concentration
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C. Temperature
= a 10oC rise in temp, reaction rate doubles
= has two reasons:
1.temp - more rapid movement of
molecules
- more probability of collision
2. Different distribution of speeds - more
effective collisions than total number of
collisions
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D. presence of catalyst - alternative pathway by
providing surface for molecules to meet
heterogeneous - separate phase from reactant
homogeneous - same phase as the reactant
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Once equilibrium is reached, the following equation is
valid
K = [C]c[D]d the equilibrium expression
[A]a[B]b
CO(g) + H2O(g) CO2(g) + H2(g)
K = [CO
2][H2] [x] = x in mol/L[CO][H2O] x = liquid or gaseous
Ab equilibrium, the concentration of CO2 x concentration
of H2 & divided by the concentrations of H2O & CO isa constant, K.
Universal custom is to write them with Products on top and
Reactants below.
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Write the equilibrium expression for the reaction
SO3(g) + H2O(M) H2SO4(g)
O2(g) + 4Cl2(g) 2Cl2O5(g)
2NH3(g) N2(g) + 3H2(g)
I2(g) + H2(g) 2HI(g)
PCl3 + Cl2 PCl5
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Le Chateliers Principle
When a reaction equilibrium, the forward and
reverse reactions take place at the same rate,
and the concentrations of all components do
not change as long as we dont do anything to
the system.
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If an external stress is applied to a system in
equilibrium, the system reacts in such a way as to
partially relieve that stress.
A. Addition of a Reaction Component
HCl - catalyst-doesnt affect equi
CH3COOH + C
2H
5OH CH
3COOC
2H
5+ H
2O
acetic acid ethyl acetate
= results to equilibrium shift
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B.Removal of a Reaction Component
reaction rate but not easy to do
NO MATTER WHAT HAPPENS TO THE
INDIVIDUAL CONCS, THE VALUES OF
THE EQUILIBRIUM CONSTANT (K)REMAINS UNCHANGED.
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C. Change in Temperature
The effect of a ( T on a reaction that has
reached equilibrium depends on whether the
rxn is exothermic (gives off heat) or
endothermic (required heat).2H2(g) + O2(g) 2H2O(M) + 137,000 cal
temp + heat
heat as product, its addition pushesthe equilibrium to the opposite
siteto the left
H2 & O2; & H2O
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in T drives an exo rxn towards reactants (left)
in T drives an exo rxn towards product (right
for endo, opposite is true)
= changes both the position of equilibrium but also
the value of K, (equi contant)
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D.Change in pressure= influences the equilibrium only if one or more of
the components of the reaction is a gas
N2O4(g) 2NO2(g)
one mole of gas 2 moles of gas(reactant) (product)
= in P shifts the equi in the direction that will the
moles in the gas phase and hence the pressure.
With the above, the shift is to the left.
An increase in pressure shifts the reaction toward
the side with fewer moles of gas (otherwise if
theres in P).
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E. Effect of catalyst
= the rates of both the forward and reverse
reactions to the same extent.
: addition of catalyst has no effect on the positionof equilibrium.
However, adding catalyst to a system not yet at equi
caused it to reach equi faster than it would without
the catalyst.
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G and Go
G = Gibbs free energy
= energy available to do work
G = change in free energy
G < 0 spontaneous in the forward reaction
G > 0 nonspontaneous; spontaneous in theopposite direction
G = 0 at equilibrium; no net change
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Go = standard free-energy of reaction
= free-energy change for a reaction when
it occurs under standard-state conditions,
when reactants in their standard states are
are converted to products in their standardstates.
GAS = 1 atm pressure
LIQUID = pure liquid
SOLID = pure solid
ELEMENTS => Go f= 0
SOLUTION = 1 molar concentration
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Free energy is a useful thermodynamic function
for understanding enzymes
Some of the principles of thermodynamics:
To fully understand how enzymes operate, we need to
consider two thermodynamic properties of the reaction:
(1) the free-energy difference ((G) between theproducts and reactants and
(2) the energy required to initiate the conversion of
reactants to products.
The former determines whether the reaction will be
spontaneous, whereas the latter determines the rate of
the reaction. Enzymes affect only the latter.
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The Free-Energy Change Provides Information
About the Spontaneity but Not the Rate of a
Reaction
The free-energy change of a reaction ((G) tells us if the
reaction can occur spontaneously:
1. A reaction can occur spontaneously only if(G is
negative. Such reactions are said to be exergonic.2. A system is at equilibrium and no net change can take
place if(G is zero.
3. A reaction cannot occur spontaneously if(G is positive.
An input of free energy is required to drive such areaction. These reactions are termed endergonic.
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Two additional points need to be emphasized.
=the (G of a reaction depends only on the freeenergy of the products (the final state) minus thefree-energy of the reactants (the initial state)
= the (G of a reaction is independent of the path(or molecular mechanism) of the transformation.The mechanism of a reaction has no effect on(G.
For example, the (G for the oxidation of glucoseto CO2 and H2O is the same whether it occurs bycombustion in vitro or by a series of enzyme-catalyzed steps in a cell.
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=the (G provides no information about the rate of a
reaction
A negative (G indicates that a reaction can occur
spontaneously, but it does not signify whether it will
proceed at a perceptible rate.
The rate of a reaction depends on the free energy of
activation ((G), which is largely unrelated to the (G of
the reaction.
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The Standard Free-Energy Change of a Reaction
Is Related to the Equilibrium Constant
As for any reaction, we need to be able todetermine (G for an enzyme-catalyzed reactionin order to know whether the reaction isspontaneous or an input of energy is required.
To determine this important thermodynamicparameter, we need to take into account thenature of both the reactants and the products aswell as their concentrations.
Consider the reaction
A + B C + D
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The (G of this reaction is given by
[C][D]
(G = (Go + RT ln [A][B]
where:
(Go is the standard free-energy change
R is the gas constant
T is the absolute temperature,
and [A], [B], [C] and [D] are the molar concentrations (more precisely, the
activities) of the reactants
(Go is the free-energy change for this reaction under
standard conditions, that is, when each of the reactants
A, B, C and D is present at a concentration of1.0 M (for
a gas, the standard state is usually chosen to be 1
atmosphere).
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Relation between (Go and Keqat 25oC
(Go
eq kcal mol-1
kJ/mol-1
10-5
6.82 28.53
10-4
5.46 22.84
10-3
4.09 17.11
10-2 2.73 11.4210
-11.36 5.69
1 0 0
10 -1.36 -5.69
102
-2.73 -11.42
10
3
-4.09 -17.1110
4-5.46 -22.84
105
-6.82 -28.53
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The overall free-energy change for a chemically
coupled series of reactions is equal to the sum of thefree-energy changes of the individual steps.
A - B + C Go = +5 kcal/mol
B - D Go = -8 kcal/mol
______________________________________
A - C + D Go = -3 kcal/mol
A thermodynamically unfavorable reaction can be
driven by a thermodynamically favorable reaction to
which it is coupled.
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Standardfree energies ofhydrolysis of
some phosphorylated compounds
Phosphoenolpyruvate -14.8 kcal/mol
1,3-bisphosphoglycerate -11.8 kcal/mol
Creatine phosphate -10.3 kcal/mol
ATP to ADP -7.3 kcal/mol
Glucose-1-phosphate -5.0 kcal/mol
Pyrophosphate (PPi) -4.6 kcal/mol
Glucose-6-phosphate -3.3 kcal/mol Glycerol-3-phosphate -2.2 kcal/mol