Post on 05-Jan-2016
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Factors Influencing Enzyme Action
Substrate
Active site
Enzyme Enzyme-substratecomplex
Substrate
Active site
Enzyme Enzyme-substratecomplex
Enzyme reactions
enzyme + substrate
enzyme-substrate complex
enzyme + products
Factors that affect enzyme rates of reaction
• An enzyme’s rate of reaction is measured by the amount of substrate converted to a product per minute.
• An enzyme’s activity can be affected by:1. General environmental factors, such as
temperature and pH.2. Cofactors and coenzymes.3. Enzyme and substrate concentration which affects
the saturation of the active site.4. Inhibitors.
Optimal temperature fortypical human enzyme
Optimal temperature forenzyme of thermophilic (heat-tolerant bacteria)
Temperature (°C)
Optimal temperature for two enzymes
0 20 40 60 80 100
Rate
of r
eacti
onOptimal pH for pepsin(stomach enzyme)
Optimal pHfor trypsin(intestinalenzyme)
pH
Optimal pH for two enzymes
0
Rate
of r
eacti
on
1 2 3 4 5 6 7 8 9 10
• What is the effect of temperature on enzyme function?
• What is the effect of pH on enzyme function?
1. Environmental Factors• Each enzyme has an optimal temperature in
which it can function.• Each enzyme has an optimal pH in which it can
function.• The wrong temperature of pH can denature an
enzyme, which means the enzyme proteins shape is destroyed.
2. Cofactors and Coenzymes• Cofactors are nonprotein inorganic enzyme
helpers.– For example, metal ions like iron in
hemoglobin hold oxygen.• Coenzymes are organic cofactors.
– For example, vitamins like niacin which serves as an electron acceptor.
Example of Cofactor Enzyme Activation:
Binding of one oxygen to hemoglobin activates other sites to enhance oxygen binding at the
three other sites.
3a. Enzyme Concentration• What does the
graph tell us about the effect of increasing [enzyme]?
Rate
of R
eacti
on
Enzyme Concentration
• Increasing the [enzyme] means the rate of reaction keeps increasing.
3b. Substrate Concentration
• Increasing [substrate] only increases the reaction rate until all the enzyme active sites are in use.
• What does the graph show us about [substrate]?
4. Enzyme Inhibitors• Competitive inhibitors bind to the active site of
an enzyme, competing with the substrate.
• Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective.
• Noncompetitive inhibition is also called allosteric regulation.
LE 8-19Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitor
Noncompetitive inhibition
A substrate can normally bind to the
active site of an enzyme.
A competitive Inhibitormimics the substrate,
competing for the active site.
A noncompetitive inhibitor bindsto the enzyme away from theactive site, altering the shape
of the enzyme so that itsactive site no longer functions.
AZT and AIDS• AZT is the first retroviral
drug approved to fight the HIV virus.
• It is a competitive inhibitor for the enzyme reverse transcriptase.
Allosteric Regulation of Enzymes• Allosteric regulation is the term used to describe cases
where an enzyme’s function is affected by the binding of a regulatory molecule at a different site than the active site.
• Allosteric regulation may either inhibit or stimulate an enzyme’s activity.
• Usually the end product of a long series of reactions that make a metabolic pathway inhibits an enzyme near the beginning of the pathway.
• This way when the [product] is high the chemical reactions leading to the product are slowed down.
Allosteric Regulation
Homework
• Read MHR p 41 – 50• Do p 54 # 1 – 11, 13• Adapt and redraw the diagram on p43 to
illustrate an anabolic reaction instead of a catabolic reaction.
• Note: anabolic = synthesiscatabolic = hydrolysis or
breakdown
Feedback Inhibition
Active siteavailable
Initial substrate(threonine)
Threoninein active site
Enzyme 1(threoninedeaminase)
Enzyme 2
Intermediate A
Isoleucineused up bycell
Feedbackinhibition Active site of
enzyme 1 can’tbindtheoninepathway off
Isoleucinebinds toallostericsite
Enzyme 3
Intermediate B
Enzyme 4
Intermediate C
Enzyme 5
Intermediate D
End product(isoleucine)
Substrates
Enzyme
Products
Substrates enter active site; enzymechanges shape so its active siteHolds the substrates (induced fit).
Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.
Active site acts on substrates
Substrates areconverted intoproducts.
Products arereleased.
Activesite is
availablefor two new
substratemolecules.
Quick Review ofInduced Fit Model of Enzyme Action
Enzyme-substratecomplex