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Limiting factors
Factors affecting the rate of reactions
Substrate concentration
Pro
duct
form
atio
n
Substrate conc. limiting
Enzyme limiting
EFFECT OF SUBSTRATE CONC. ON REACTIONS
More substrate more chance of a collision between substrate and enzyme
↑substrate ↑number of reactions
Enzyme concentration
More enzyme more chance of a collision
↑ Reaction rate
Enzyme activity will not increase
If enzymes are limiting, reaction rate will drop off, but enzymes are re-usable, so the reaction will continue
Enzyme concentration
Rat
e of
rea
ctio
n
Reaction rate increases rapidly.
Enzyme conc. is limiting
Reaction rate levels off or stops.
eg substrate runs out
EFFECT OF ENZYME CONC. ON REACTIONS
Enzyme cofactors
• May be – Organic co-enzymes, usually vitamins eg vit B– Activating ions eg Na+, K+, Ca2+, Mg2+
• complete or alter the enzyme active site
• increase enzyme activity (enzyme molecules work more effectively)
• Without the cofactors the enzyme will not work, or will work too slowly.
Cofactor concentration
Rat
e of
rea
ctio
n
Rate of reaction increases rapidly.
Cofactor conc. is limiting
Rate of reaction levels off.
Not enough enzyme for the co-factors to attach to – enzymes
are limiting
EFFECT OF COFACTOR CONC. ON REACTIONS
Inhibitors
Competitive inhibitors bind to the enzyme active site, so substrate cannot bind (eg the product of a reaction may bind, preventing too much product being formed)
Non-competitive inhibitors (allosteric) usually bind somewhere other than the active site, denaturing the enzyme permanently (eg many poisons, such as heavy metal ions, act this way)
Reactions stops or occurs at a very low rate
Inhibitor concentration
Rat
e of
rea
ctio
n
Rate of reaction decreases rapidly.
EFFECT OF INHIBITOR CONC. ON REACTIONS
Temperature
At low temperatures enzymes and substrates have less energy, so they move less and collide less often.
However, at temperatures a little over the optimal temperature the enzymes become denatured and reaction comes to a stop.
Denaturation = change in protein shape enzyme can’t function
Temperature
Rat
e of
rea
ctio
n
↑ temp ↑ energy ↑ reaction rate
OptimumDenaturation
– irreversible shape change
- can’t function
Low temp. limits the rate of reaction, up to an optimal temp.
EFFECT OF TEMPERATURE ON REACTIONS
3-dimensional enzyme structure
Globular proteins with a very specific shapeChange shape active site changes Cannot bind substrate Enzyme cannot function
pH
Rat
e of
rea
ctio
n
PEPSIN AMYLASE TRYPSIN
Each enzyme has its own optimal pH
At pHs above or below this optimum, enzyme denatures
pH 2 pH7 pH9
EFFECT OF pH ON ENZYME ACTIVITY
MENU
Key Words
Exercises
Quick Quiz
MCQ
Videos
Key words• Limiting• Reaction rate• Optimal• Substrate conc.• Enzyme conc.• Co-factor• Co-enzyme• Activating ion• Competitive inhibitor• Non-competitive
inhibitor
• Temperature• Denaturation• pH
Back to menu
Quick quiz1. ____ factors prevent reaction occurring at optimal
rate
2. As substrate conc. increases, the chance of a ____ between substrate and enzyme increases
3. As enzyme increases, reaction rate increases until another factor becomes ____
4. Enzyme ____ include co-enzymes and ions
5. ____, such as poisons, decrease reaction rates
6. 35-37oc is the ____ range of temp.s for most enzymes
7. Enzymes become ____ at high temperatures
8. Different enzymes have different ____ pH ranges
9. Vitamins often act as ____
10.Low temperatures denature enzymes – True / False?
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Answers
Quick quiz1. Limiting factors prevent reaction occurring at optimal rate
2. As substrate conc. increases, the chance of a collision between substrate and enzyme increases
3. As enzyme concentration increases, reaction rate increases until another factor becomes limiting
4. Enzyme co-factors include co-enzymes and ions
5. Inhibitors, such as poisons, decrease reaction rates
6. 35-37oc is the optimal range of temp.s for most enzymes
7. Enzymes become denatured at high temperatures
8. Different enzymes have different optimal pH ranges
9. Vitamins often act as co-enzymes
10.Low temperatures denature enzymes – True / False?
Back to menu
Exercise
• Workbook pp50-52, qq18-20 and 22-23• Booklet p3
Back to menu
READING• Pathfinder p48 (or p42) Enzymes & reaction
rate• Excellence in Biology pp114-6
Competitive and non-competitive inhibitors
Videos
• http://www.youtube.com/watch?v=E90D4BmaVJM&feature=related (10 min, includes biotechnology)
http://www.youtube.com/watch?v=2DRWqBld7XU&feature=related (feedback inhibition, 1 min)
http://www.youtube.com/watch?v=PILzvT3spCQ&feature=related (1 min)
Question 1
Increasing substrate conc. increases rate of reaction because...
(a) the activation energy decreases
(b) the enzymes act as catalysts
(c) there is more chance of a collision with enzymes
(d) the enzymes are proteins
34
√
Question 2
Increasing enzyme conc. increases the reaction rate because....
(a) enzymes form an enzyme-substrate complex
(b) reactions are exergonic
(d) enzymes are specific
35
√(c) there is more chance of a collision with substrate
Question 3
At very high substrate concentrations, the increase in reaction rate levels off because...
(a) enzymes become limiting
(b) the substrate precipitates out
(c) the enzymes become denatured
(d) a combination of these
37
√
Question 4
Co-factors
(a) lower reaction rates
(b) inhibit the action of enymes
(c) slow down a reaction
(d) include co-enzymes and activating ions
38
√
Question 5
Co-factors
(a) complete or alter the active site of the enzyme
(b) are always organic molecules
(c) are a type of enzyme
(d) can be any chemical substance in a cell
39
√
Question 6
Inhibitors
(a) combine with different substrates
(b) form different kinds of end-product
(c) function at temperatures above 90oC
(d) may be competitive or non-competitive
40
√
Question 7
Inhibitors can include
(a) co-enzymes
(b) poisons
(c) co-factors
(d) activating ions
41
√
Question 8
After being exposed to a high temperature an enzyme cannot function because.....
(a) it has been broken down
(b) its shape has been changed
(c) its composition has been changed
(d) it cannot separate from its substrate
42
√
ANSWER
Correct
√
43
click arrow to return
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ANSWER
IncorrectX
44
click arrow to return
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