27. Lock and key hypothesis
• Enzyme are very specific to the substrate they act on because they
have particular shape / configuration into which substrate with
complementary shape fit in exactly as the key fit into the lock, thus
the lock (enzyme) and key (substrate) hypothesis.
• When an enzyme / substrate complex is formed, the substrate
activated into forming the product of the reaction. Once formed, the
product no longer fit into the active site and escape into the
surrounding medium leaving the active site free to receive other
substrate molecule.
28. Types / classes of enzymes
• There are six types/kinds of enzymes. Different types of enzymes
have different classifications based on the kind of reactions they
catalyze.
• These six types of enzymes are as follows: oxidoreductases,
transferases, hydrolases, lyases, isomerases, and ligases. Hydrolases
are the most common type, followed by oxioreductases and
transferases. They account for over half of the known enzymes.
29. Oxioreductases
• Oxidoreductases catalyze oxidation or reduction reactions. These reactions
involve the transfer of electrons from one molecule (the reductant) to
another (the oxidant).
• These reactions are vital to life for their role in essential metabolic
processes like glycolysis, which occurs in nearly every organism on the
planet.
• The most common name used is a dehydrogenase and sometimes
reductase will be used. An oxidase is referred to when the oxygen atom is
the acceptor.
Transferases
• The transferase enzymes catalyze the transfer of a functional group (such
as methyl) from one molecule to another.
• The first molecule is called the donor and the second molecule is called the
acceptor.
• These transfer processes are some of the most basic and vital reactions in
life. Example: Hexokinase used in glycolysis.
30. Hydrolases
• The hydrolases bring about hydrolysis: this is the breaking of chemical
bonds with the addition water.
• There is a wide variety of identified hydrolases, over 200 of them, from
those that break down proteins to those that cleave ester bonds and more.
• Exohydrolase enzymes cut the molecules at the end of the chain, and
endohydrolase enzymes do so in the middle of the chain.
• For example: Chymotrypsin
Lyases
• Lysis reactions – those that generate a double bond – are brought about by
lyase enzymes. Lysis reactions are the kind of elimination reactions that are
not hydrolytic or oxidative.
• The lyases are also sometimes called synthase enzymes.
• For example: Fructose bisphosphate aldolase used in converting fructose
1,6-bisphospate to G3P and DHAP by cutting C-C bond
31. Isomerases
• The isomerase enzymes catalyze structural changes within a molecule –
this just brings about a change in shape since there is only one substrate
and one product with nothing gained or lost.
• For example: phosphoglucose isomerase for converting glucose 6-
phosphate to fructose 6-phosphate
Ligases
• Ligation is brought about by ligase enzymes. Ligation occurs when two
substrates are joined together.
• Chemical potential energy is usually required for this reaction to occur, so it
is often paired with the hydrolysis of a diphosphate bond.
• DNA ligase – which catalyzes the ligation or repair of breaks in DNA – is an
example of a vital enzyme in this category.
32. Course work
• Induced fit hypothesis
• Enzyme inhibitors
• Enzyme regulation
• Co-factors and co-enzymes
