Biology 1002B Lecture Notes - Lecture 6: Hyperthermophile, Reagent, Non-Competitive Inhibition

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18 Apr 2012
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Enzymes (4.4-4.5)
What Enzymes Do And Do Not Do
The laws of thermodynamics tell us if a process is spontaneous
or not. They do not tell us the speed of the reaction
o A reaction can be spontaneous yet very very slow
Speed up a spontaneous reaction
Cannot provide energy therefore can not induce an endergonic reaction
Although enzymes lower the activation energy of a reaction, they do not alter the
G of the reaction. The free energy of the reactants and products is the same;
the only difference is the path the reaction takes.
Enzymes DO speed up the rate of spontaneous reactions by lowering activation
energy. Enzymes DO NOT supply free energy to a reaction. Therefore, enzymes
CANNOT make an endergonic reaction proceed spontaneously.
Exergonic Reaction
ΔG is negative-- EA Activation Energy
o Chemical reactions require bonds to break and
new bonds to be formed. For bonds to be broken,
they must first be strained (made less stable) so
that bond breakage can occur. To get reacting
molecules into a more unstable state requires a
small input of energy. Even if the reaction is
spontaneous the reaction will not actually start
until a small boost of energy is added. This initial
energy is called the activation energy and also
explains why spontaneous reactions are not rapid.
o Activation energy is basically the amount of energy
needed to get to transition state
o Activation energy represents a kinetic barrier
(kinetic means speed) as it is a barrier that
prevents the reaction from going fast.
o Where do reactions get activation energy from?
Molecules taking part in chemical reactions are in
constant motion (at temperatures above zero) and
periodically reacting molecules may gain enough energy to reach
transition state. If a significant number of reactant molecules reach the
transition state then the free energy that is released can be enough to get
the remaining reactants to the transition state. This is rare under normal
conditions.
o Chemists usually use heat to provide the energy needed for reactant
molecules to get to the transition state and thus speed up the rate of
reaction. This is problematic in biology: High temperatures destroy the
structural components of cells, particularly proteins and can result in cell
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death. Also, an increase in temperature would speed up all possible
chemical reactions in a cell, not just the specific reactions that are part of
metabolism.
Thermodynamically unstable
o kinetically stable (rate at which it reacts on its own is very slow even
though it is spontaneous)
Transition state
o Higher energy state
o Bonds able to break more easily
Enzymes Lower Activation Energy
Enzymes are Catalysts: Chemical agents that speed up the rate of a reaction
without itself taking part in the reaction (They are regenerated at the end, are not
consumed). Enzymes are a group of proteins that are the most common
biological catalysts.
Enzymes can speed up the rate of a reaction 1012-1020 times
Activation energy required to get to transition state is lowered in presence of
enzymes, since enzymes change the path of a reaction to one with a lower
activation energy.
Rate of a reaction is proportional to the number of reactant molecules that can
acquire the necessary energy to get to the transition state.
Enzymes increase the number of molecules that are able to get to the transition
state.
Important to remember enzymes do not change G (starting and final free
energy are the same)
Life Needs Enzymes
o Cannot increase temperature, pressure etc. to drive reactions, explained
above.
o Enzymes enable you to increase the rate of reactions without raising the
temperature
Enzyme Structure
Enzymes much larger then substrate
Enzymes must be flexible (see point 5)
Substrate only interacts with very specific
part of enzyme called the active site.
Active site only apparent when enzyme
in its native conformation, cannot be told
by looking at primary sequence
Conformation (shape) of enzyme
changes upon substrate binding to active site (induced
fit) (enzyme wraps around it)
Binds to its substrate, forming an enzyme-substrate
complex
Enzyme substrate complex then undergoes catalysis, to
make enzyme and product
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