Download ENZYMES • Enzymes are catalysts Catalyst: A chemical agent that

ENZYMES
Enzymes are catalysts
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Catalyst: A chemical agent that speeds up a reaction without being
consumed in the process
• Does not affect the free energy change ( G) of the reaction
• Does not affect an equilibrium since it usually speeds up the forward
and reverse reactions equally
• Function by lowering the activation energy of the reaction, therefore
allowing reactants to more easily become products
•
Increased temperature also increases the rate of reaction, however,
high temperatures denature proteins and kill cells
Substrate: The reactant that an enzyme acts on
• Enzymes bind to their substrate and while joined, convert the
substrate to the product
• Each enzyme is specific for a particular substrate and will often not
work with isomers
Active site: The location where the substrate binds to an enzyme
Induced fit model: After a substrate binds to the active site, the enzyme
changes shape slightly to better accommodate the substrate
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Weak bonds with the R-groups of the amino acids in the active site
hold the substrate in place
Enzymes lower the activation energy by binding substrates together
in the correct orientation and by applying stress to the substrate’s
bonds, reducing the amount of thermal energy that must be
absorbed to achieve transition state
Active sites can also provide suitable microenvironments for
particular reactions (e.g. can provide alternate pH levels depending
on the amino acids present in the active site)
Cofactors
• Non-protein enzyme helper
• May bind to active site or bind with the substrate
• May be inorganic, (e.g. zinc or copper ions), or it may be an organic
molecule
o If organic, it is commonly called a coenzyme
• Most vitamins are coenzymes or provide raw materials for the
construction of coenzymes
Environmental Effects on Enzyme Activity
Substrate Levels
• As levels increase, so too does the reaction rate
• Enzymes can become saturated
Temperature
• As temperature increases, reaction rate will increase
• Above a certain point, increase in temperature causes enzyme
denaturation
o
• Most human enzymes have a temperature optimum of around 37 C
pH
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Enzymes also have a pH optimum
o E.g. Digestive enzymes (pepsin pH 2,
trypsin pH 8)
Extreme pH’s may cause denaturation
Controlling Enzyme Function
1. Enzyme Inhibitors
• Chemicals that interfere with enzyme function
• Usually temporary but could be permanent (e.g. hydrogen cyanide –
humans, penicillin – bacteria)
Competitive inhibitors
• Fit into the active site and block the substrate
Noncompetitive inhibitor
• Bind to the enzyme and cause a conformation change that alters
the active site
2. Allosteric Regulation
• Enzyme shape is altered to speed up or slow down enzyme function
• Essential for metabolic control
Allosteric site
• Receptor site remote from the active site that binds with a
regulator that can either inhibit or stimulate an enzyme’s activities
• Usually present in proteins with quaternary structure
• Activator – A substance that stabilizes the protein and keeps the
active sites available
• Allosteric inhibitor – Stabilizes the inactive form of the enzyme
o Works like a noncompetitive inhibitor
3. Feedback Inhibition
• Occurs when the product of a reaction pathway acts as an inhibitor
of the pathway
• Prevents the cell from wasting chemical resources to synthesize
more products than necessary
Feedback inhibition
A
Enzyme 1
B
Enzyme 2
C
Enzyme 3
D
End product
4. Enzyme Production and Location
• Cells can control metabolic pathways by restricting the production
and location of enzymes and enzyme complexes
o Enzymes responsible for cellular respiration are in the
mitochondria so respiration can be controlled by limiting the
movement of reactants into the mitochondria