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Enzymes
 Enzymes are protein catalysts that increase the rate
of chemical reactions by providing an alternative
pathway for the reaction .
 This pathway involves increasing the free energy of
activation of the reaction.
 Enzymes are not changed in the overall catalytic
process.
 All enzymes are of a protein nature .
 Most enzymes have a globular structure.
 Enzymes are found in all tissues and fluids of the
body.
 They can be intracellular and extracellular.
Some enzymes require chemical groups or molecules
other than proteins for enzyme activity , when the
chemical needed is an ion or metal it is called a
cofactor , when it is a small organic molecule it is
called coenzyme such as vitamins
If the additional group needed for the reaction binds
tightly and permanently it is called a prosthetic group.
 Apo enzyme
(Inactive enzyme)
form)
+ coenzyme
Holo enzyme
(active enzyme)
Property of enzymes
 Solubility: water soluble
 Molecular weights range from 10000-several hundred thousand
 Absorb light in the UV range. Maximum absorption at 280nm due
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to aromatic a.a
Enzymes are charged molecules. Charge on enzyme depends on
pH of the solution. Positively charged below pI, and negatively
charged above pI.
They have distinctive pI .
Enzymes are proteins thus when exposed to heat or other
denaturating agents they lose their native conformation and
consequently lose their catalytic activity.
Denaturing agent include:
Extreme change in pH
8M urea
Heavy metals
Radiations
Detergents
Enzyme specificity
Enzymes are very specific towards the reaction they catalyse and the
substrate they react upon.
Enzyme specificity can be classified into the following;
1-Absolute specificity; Here the enzyme reacts with one substrate
only , such as the glucokinase enzyme which acts on glucose only.
2- Group specificity; Herethe enzyme acts upon a group of
substrates(reactants) that share a common functional group,
Such as the polyphenoloxidase enzymes which act on di and tri hydroxy
phenols.
3- Bond specific; The enzyme identifies a specific bond and acts upon
it .
4-sterio-isomer specific ; The enzyme identifies a specific isomer and
acts only upon it. It Is capable of differentiating between L- and Disomers of a compound.
Enzymes
 Enzyme Nomenclature; Enzymes are named by
adding “ase”
to the end of substrates name for example,
glucosidase, urease, sucrase
 OR,the enzyme is named indicating the substrate
acted upon and the type of reaction that it catalyzes
:
 e.g: GlutamicOxaloaceticTransaminase (GOT),
where Glutamic and Oxaloacetic acid are the
substrates and the type of reaction is a transfer
reaction which involves the transfer of an amino
group.
How do enzymes increase the rate of
chemical reactions
 Enzymes are released unchanged and thus can be reused repeatedly
which explains why only small amounts of enzyme is needed in
biological systems.
 For any chemical reaction to occur the reactants should reach a high
energy state called the transition state.
 The activation energy of a reaction is the energy needed to transfer
all the reactant molecules in one mole to the top of the energy
barrier (transition State) at standard
conditions.
So the activation
Energy is simply the
Minimum energy needed
For the reaction to occur.
Enzymes
 Enzymes increase the rate of chemical reactions by decreasing the
activation energy of the reaction.
 Enzymes decrease the activation energy by forming the ES complex ,
which has higher energy than the reactants thus it needs less energy
to reach the transition state .

Free energy
Enzymes
Enzymes
Specificity hypothesis;
1- Lock and key hypothesis; The enzyme active site should have a
conformation complementary to that of the substrate, just like the
key and lock are complementary to each other.
Enzymes
 Lock and Key Analogy: lock = enzyme, key = substrate.
Enzymes
 Lock and Key Analogy: lock = enzyme, key = substrate.
Enzymes
Induced Fit Theory
Enzyme is not rigid, changes shape with substrate.
Induced Fit Theory
Enzyme is not rigid, changes shape with substrate.
Enzymes
 Induced- fit theory: The enzyme changes shape on binding
to the substrate, so that the conformation of substrate and
enzyme active site is complementary only after binding.
When the substrate binds to the enzyme it induces a
change in the enzyme conformation the enzymes active site
is then moulded into a precise conformation that is
complementary to that of the substrate .
 This explains how enzymes can react upon a group of
substrates that share some chemical similarities.