small a digression before talking about Enzymes
Thermodynamics
2nd Law of Thermodynamics -- for any spontaneous process, the Entropy (disorder) of the universe, Suniverse, must increase. This is very difficult to measure, however ===> we need a criterion of spontaneity which applies to our system (organism).
which we exist, more or less) the criterion is the change in
At constant Temperature and Pressure (conditions under Gibbs Free Energy = G
It is determined by the Entropy change of the system, S, and by the change in Enthalpy, H, or heat content of the system (the energy exchanged with the surroundings) if H < 0 --> the reaction is exothermic and heat is given off H > 0 --> the reaction is endothermic and heat is absorbed
G = H - TS
What does G Depend Upon? aA + bB cC + dD
Given the Following Chemical Reaction The Free Energy Change is:
and
G = H - TS [C ]c [ D]d o G = G + RT ln [ A]a [ B]b
A reaction can be spontaneous even if:
Go > 0
-- G can still be < 0 if [C] and [D] are small or [A] and [B] are large
S < 0 --if H is << 0 H > 0 --if S is >> 0
1
Equilibrium
At Equilibrium there is no net change in free energy
==>
G = 0
[C]c [D]d [C]c [D]d = K eq and 0 = G + RT ln a [B] b [A]a [B]b [A]
o
Therefore...
G = RT ln K eq
o
Figure 6.8: ATP and Hydrolysis of ATP Consider the reaction ATP + H2O ADP + Pi
ATP has more free energy than ADP and Pi The free energy change for this reaction, G, is less than 0 and the reaction is favorable, i.e. it is Exergonic.
Note: Pi is an abbreviation for phosphate ions.
2
ATP Provides Energy for:
Mechanical Work: Muscle contraction, flagella and cilia movement etc. Transport Work: Pumping ions and molecules across membranes against a concentration gradient Chemical Work: Coupling energy from ATP to Endergonic reactions to make them go
ATP drives cellular work for
Fig. 9-2: A review of how ATP drives cellular work.
3
See Figure 6.7: Energy coupling by phosphate transfer
Glutamate Ammonia
Glutamine
2 + 2
2 2
Go = +3.4 kcal/mole
1
This reaction is catalyzed by an enzyme in two steps
2 2
2 2
2
2 2 2 2
Energetically, this can be described as the sum of the following two reactions
Glu + NH3 ----> Glu-NH2 + H2O ATP + H2O ----> ADP + Pi Glu + NH3 + ATP ----> Glu-NH2 + ADP + Pi
Go = +3.4 kcal/mole
Go = -7.3 kcal/mole Go = - 3.9 kcal/mole
4
Figure 6.9: The ATP Cycle
Figure 6.11: Activation Energy Initiates Reactions
5
Figure 6.14: Enzymes reduce the energy of activation, Ea, or the energy barrier of a reaction.
Characteristics of Enzymes
Increase rates (alter kinetics) -- Increase the rate of approach to equilibrium but dont change G Specificity--Enzymes bind specific molecules and not others Induced Fit--Enzyme adapts to bind Substrate and bend it
into the transition state whose structure is intermediate between the structures of the substrate and the product.
pH Optimum--since enzymes are composed of aa's, their structures and therefore activity (rate at which they catalyze reactions) depend upon the pH which can drastically alter their structures. Temperature--rates of enzyme catalyzed reactions increase with temperature (as do ...