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Class notes for chapter 4 enzymes
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Enzymes Enzyme Action
Factors Affecting Enzyme Activity
Enzyme Inhibitors
Cofactors, Coenzymes and Prosthetic Groups
Enzyme action
, Enzymes are biological catalysts. They are globular proteins that interact with substrate
molecules causing them to react at much faster rates without the need for harsh
environmental conditions.
They catalyse metabolic reactions – both in individual cells and in entire organisms. They
can also affect both the structure and function of organisms.
Enzyme action can be either intracellular or extracellular.
Intracellular Enzyme Example – Catalase
Hydrogen Peroxide is the toxic by-product of several cellular reactions. If left to
build up, it can kill cells.Catalase is an enzyme that works inside cells to catalyse
the break down of hydrogen peroxide to harmless oxygen and water.
Extracellular Enzyme Example – Amylase and Trypsin
Amylase and Trypsin both work outside cells in the human digestive system.
Amylase is found in saliva. It’s secreted into the mouth by cells in the salivary
glands. It catalyses the hydrolysis of starch into maltose in the mouth.
Trypsin catalyses the hydrolysis of peptide bonds. It is produced by cells in the
pancreas and secreted into the small intestine.
All enzymes have an active site, which has a specific shape. The active site is the part of the
enzyme that the substrate molecules bind to. The specific shape of the active site is
determined by the protein’s tertiary structure.
For the enzyme to work, the substrate must fit into the active site. If the substrate shape
doesn’t match the active site, the reaction won’t be catalysed. This means that enzymes
usually only work with very substances – normally only one.
Enzymes reduce activation energy. If two substrate molecules need to be joined, attaching
to the enzyme holds them together, reducing any repulsion between the molecules so that
they can bond more easily. If the enzyme is catalysing a breakdown reaction, fitting into the
active site puts a strain on the bonds in the substrate. This means that the bonds can break
more easily.
Enzyme – substrate fit hypotheses
There are two main hypotheses which explain how an enzyme fits with a substrate in order
to catalyse a reaction. These are the ‘Lock and Key’ model and the ‘Induced Fit’ model.
The Lock and Key
model:
The Induced Fit model:
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