Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
a) the role of enzymes in catalysing reactions that affect metabolism at a cellular and whole
organism level. To include the idea that enzymes affect both structure and function.
Enzymes as biological catalysts
• Enzymes speed up chemical reactions by acting as biological catalysts.
• A catalyst is a substance that speeds up chemical reactions without being used up itself.
• Biological catalysts are those found within living organisms
• They catalyse metabolic reactions - both at a cellular level (i.e. respiration) and for the organism
as a whole (i.e. digestion in mammals)
• Enzymes can affect structure ion an organism, enzymes are involved in the production of
collagen, an important structural protein in connective tissues of animals.
• As well as function like respiration.
Enzyme action is either:
• Intracellular - within cells
• or, extracellular - outside of cells
b) the role of enzymes in catalysing both intracellular and extracellular reactions. To include
catalase as an example of an enzyme that catalyses intracellular reactions and amylase and
trypsin as examples of enzymes that catalyse extracellular reactions.
Examples - Intracellular Enzyme
Catalase:
• Catalyse is an enzyme that works inside of cells to catalyse the breakdown of hydrogen peroxide
to harmless oxygen and water
• Hydrogen peroxide is the toxic build up of several cellular reactions. If it is left to build up, it can
lead to the death of cells.
Example - Extracellular Enzyme
Amylase and Trypsin:
• Both work outside of the cells in the human digestive system.
• Amylase is found within the saliva. Its 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 - turning big polypeptides into smaller ones
(which then get broken down into amino acids by other enzymes). trypsin is produced by cells in
the pancreas and secreted into the small intestine.
c) The mechanism of enzyme action. (c) the mechanism of enzyme ac on
To include the tertiary structure, specificity, active site, lock and key hypothesis, induced-fit
hypothesis, enzyme-substrate complex, enzyme- product complex, product forma on and lowering
of activation energy.
Enzyme Structure
Enzymes are globular proteins.
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,Active site:
• The active site is where the substate molecule bind to the enzyme
• The active site has a specific shape, which is determined by the enzymes tertiary structure.
Complimentary shaped substrates
• For the enzyme to work correctly the substate must fit into the active site, and so have the
correct complimentary shape.
• If the substrate doesn't have the correct shape, then the reaction will not be catalysed
Specificity:
• Enzymes are specific they will only bind to specific, complimentary shaped substrates.
• When a substrate binds to the active site then it is called the Enzyme-Substrate Complex
How enzymes speed up reactions:
In a chemical reaction, a certain amount of energy is required before the reaction will start. This is
called the activation energy - it is often provided as heat energy.
Enzymes reduce the amount of energy required to reach this activation energy by proving an
alternative pathway.
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, When a substance binds to an enzymes active site, an enzyme substrate complex is formed - its
this that lowers the activation energy:
Heres why:
1) If two substance molecules need to be joined, attaching to the enzyme holds them closer
together, reducing any repulsion between the molecules so they can bond more easily
2) If the enzyme its catalysing a breakdown reaction, fitting into the active site puts strain on the
bonds in the substance. This strain means that the substance breaks down more easily.
Models of enzyme action:
The lock and Key Model:
• Enzymes are a bit tricky, they only work with substrates that fit their active site. Early scientists
came up with the idea of the lock and key model.
• This is where the substrate fits into the enzyme the same way a key fits into a lock.
• The active site and substrate have a complementary shape.
• Scientists realised that the lock and key model didn't give the full story.
• The enzyme and substrate do have to fit together in the first place, but new evidence shows that
they enzyme-substrate complex changed shape slightly to complete the fit.
• This locks the substrate even tighter into the active site.
The Induced Fit Model
• The induced fit model helps to explain why enzymes are so specific and only bond to one
particular substrate.
• The substrate not only has to fit into the active site but it also has to make the active site change
shape in the right way too.
• This is a prime example of how a widely accepted theory can change when evidence comes
along.
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