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Introduction to enzymes - Biochemistry
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BIOCHEMISTRY – LECTURE SIX PART ONEintroduction to enzymes
The first law of thermodynamics is that energy cannot be created or destroyed –
it can be converted from one form to another. The second law of
thermodynamics is that entropy of an isolated system will always increase –
entropy measurement of disorder or randomness in a system. Ordered = low
entropy, disordered = high entropy. Gibbs free energy, in relation to reactants
and products, indicates the energy available in those molecules to do work. If the
free energy in the products is less than that in the products (change is negative)
then the reaction is likely to occur.
In the exergonic reaction the products have less potential energy than the
reactants. Energy from this reaction would be released. The change in Gibbs free
energy in this system is negative. In the endergonic reaction the products have
more potential energy than the reactants. It would take an input of energy to
form the products from the reactants. The change in Gibbs free energy in this
system is positive. In the exergonic reaction there is a hump. This hump
represents the activation energy. The activation energy is the energy needed to
allow the reaction to happen (otherwise it would have happened spontaneously
already).
A catalyst is a substance which increases the rate of a chemical reaction, without
being used up or permanently changed in the reaction. This is what enzymes do.
They do not change the final equilibrium of reactant and products, just increase
the speed at which the equilibrium is reached. They do this by lowering the
activation energy required for the reaction to occur. The products and reactants
are the same and the change in free energy is therefore the same. It used to be
said that enzymes acted as a lock and key, the substrate had to be a perfect fit
for the enzyme. The new model is the induced fit model, where the binding of
the substrate induces a change in shape in the enzyme.
Enzyme (E) and substrate (S) bind to form enzyme-substrate complex (ES). The
substrate is converted into the product. The product is still bound to the enzyme
in an enzyme-product complex (EP). The enzyme and product part ways, leaving
the enzyme and the product separate (E+P). The higher the concentration of
substrate, the faster the rate of product formation, up to the point where all
active sites of the enzymes are occupied. This is known as V max. Different
enzyme/substrate pairs will have different initial velocities depending on the
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