Biology Lecture No. 6: Enzymes
Wednesday January 25 , 2012
-Spontaneous reactions proceed to completion without any energy input. They are driven by enthalpy,
entropy or both. However, spontaneous reactions do not reveal their given rate of reaction. They can
literally occur for as long or as fast as possible.
-This is why catalysts are used in nature, in order to complete reactions (that are otherwise slow) at a
quicker rate. For example, OMP-enzyme decarboxylation occurs 20 milliseconds with a catalyst.
Without it, it takes 78 million years for that reaction to reach completion.
-Enzymes are a prominent class of proteins, of which the cell contains thousands, which can increase the
rate of spontaneous reactions.
-In endergonic (or non-spontaneous) reactions, enzymes cannot be of much use as they do not provide
energy. Such reactions would proceed with the aid of ATP or another external energy source.
Energy Profile Of An Exergonic Reaction:
-In spontaneous exergonic reactions, where ∆G of the products is negative, the free energy required for
bonds to be broken usually comes from molecular motion. This point at which bonds break is defined as
the transition state. This must be reached by the reaction in order to create products with higher free
energy than the substrate.
- The activation energy (E a represents a kinetic barrier as only few molecules can acquire the transition
state. This explains why some spontaneous reactions take a while to reach completion. An example of
this is the breakdown of propane which is thermodynamically unstable, yet kinetically very stable. Thus,
the reaction takes very long to complete. A spark can easily give more molecules the ability to reach the
Enzymes & Activation Energy:
-Enzymes lower the activation energy and thus make it easier for more of the substrate to reach the
transition state. It may change the pathway of the reaction, but the end-product is still the same.
-Therefore, it can be said that the rate of reaction is proportional to the number of molecules that
acquire the transition state.
-As most biological molecules cannot easily cope with high temperature conditions, life evolved
enzymes in order to catalyze reactions without the need for heat. Enzyme Structure:
-Proteins that are flexible have an absolute advantage over those that are not, this is especially the case
with enzymes. The conformation of the enzyme changes upon substrate binding, which induces a
change in the enzyme’s structure, thus promoting catalysis. This change in conformation is known as
-The specific region where the substrate binds is known as the active site and is where catalysis occurs.
The rate of catalysis, for a typical enzyme, is approximately 1000 catalytic cycles per second. Catalytic
rate can be measured from either the rate at which the substrate leaves, or the rate at which the
product is made.
Lowering Activation Energy:
-Enzymes can lower activation energy in one of three ways:
Precise Orientation Of Substrates – Without enzyme activity, the probability that two substrates
will interact correctly is unlikely. An enzyme increases the likelihood of ideal substrate
interaction and thus, the formation of products.
Charge Interactions – Perhaps the active site has specific charges that aid in reestablishing the
charges which are present in the product.
Conformational Strain – The substrate molecule might have to be strained in order to acquire
the correct conformation of the product.
-All of the above are forms of transition states that the substrate must undergo for products to be
synthesized. The enzyme simply recreates this transition state through various means without requiring
so much free energy.
-Enzyme kinetics descri