Limiting factors

Factors affecting the rate of reactions

Substrate concentration

Pro

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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

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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

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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

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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

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↑ 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

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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

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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?

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Exercise

• Workbook pp50-52, qq18-20 and 22-23• Booklet p3

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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

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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

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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

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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

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ANSWER

Correct

√

43

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ANSWER

IncorrectX

44

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