Factors affecting enzyme action

Question 1

What two things must happen for the enzyme to work?

Answer 1

The enzyme must:

• Come into physical contact with its substrate
• Have an active site which fits the substrate

Question 2

What are the two ways that you can measure the progress of an enzyme-catalysed reaction?

Answer 2

• The formation of products of the reaction e.g the vol. of O2 produced when the enzyme ‘catalase’ acts on hydrogen peroxide
• The disappearance of the substrate e.g the reduction in concentration of starch when it is acted upon by amylase

Question 3

Describe + explain what happens in an enzyme-catalysed reaction when the line is steep (product vs time graph)

Answer 3

• At first there is a lot of substrate but no product
• It is very easy for substrate molecules to come into contact with empty active sites on the enzyme molecules
• All the enzyme active sites are filled at any given moment + the substrate is rapidly broken down into its products

Question 4

Describe what happens in an enzyme-catalysed reaction when the graph gets less steep (product vs time graph)

Answer 4

• The amount of substrate decreases as it is broken down, resulting in an increase in the amount of product
• As the reaction proceeds, there is less + less substrate + more + product
• It becomes more difficult for the substrates to come into contact with the enzymes as there are fewer substrate molecules + also the product molecules may ‘get in the way’ of the substrate molecules + prevent them from reaching an active site

Question 5

Describe what happens in an enzyme-catalysed reaction when the graph starts to flatten out (product vs time graph)

Answer 5

• It therefore takes longer for the substrate molecules to be broken down by the enzyme + so its rate of disappearance slows + consequently the rate of formation of products also slows. Both graphs ‘tail off’
• The rate of reaction continues to slow until there is so little substrate that any further decrease in its concentration cannot be measured
• The graphs flatten out because all the substrate has been used up + so no new product can be produced

Question 6

How do you measure the rate of change at any point on the graph?

Answer 6

• Measure the gradient at chosen point. Grad. equal to grad. of tangent to the curve at that point
• To work out the gradient draw tangent line on the point on the curve and draw two other lines to make a right angle triangle
• The other two sides should be worked out using x-axis (time) for one side and y-axis (mass) for the other
• Do mass/time to find rate of change

Question 7

How many variables should you change in measuring rate of change experiment?

Answer 7

Only 1 single variable in each experiment. All the other variables must be kept constant.

Question 8

What term should you use to describe a substrate and active site that fit together?

Answer 8

They are complementary to one another

Question 9

Explain effect of temperature on enzyme action in terms of kinetic energy

Answer 9

• A rise in temp. increases the kinetic energy of molecules
• As result. molecules move around more rapidly + collide more often
• In enzyme-catalysed reaction, it means there are more effective collisions which result in more enzyme-substrate complexes being formed
• More energy also means activation energy can be overcome more easily
• Therefore, this increases the rate of reaction

Question 10

What happens to the enzyme if the temp. gets higher (than optimum pH)?

Answer 10

• Causes hydrogen + other bonds in enzyme to break
• Results in enzyme + its active site to change shape
• At first, substrate fits less easily which slows the rate of reaction
• For many human enzymes this happens at around 45 degrees
• At around 60 degrees it denatures (completely stops working)
• Denaturation is permanent change + enzyme does not function again

Question 11

What does the temperature vs rate of reaction graph look like?

Answer 11

Slowly goes up + gets more steep then reaches optimum and curves back down at a relatively constant rate

Question 12

Do all enzymes have the same optimum temp.?

Answer 12

No. Optimum temp. differs from enzyme to enzyme. Some work best as low as 10 degrees and some work best at as high as 80.

Question 13

Many enzymes in our body have an optimum temp. of around 40 degrees but our body temp. has evolved to be 37. Why could this be?

Answer 13

• Although higher body temp. would increase metabolic rate slightly, the advantages are balanced out by additional energy (food) that would be needed to maintain the higher temp.
• Other proteins (apart form enzymes) may be denatured at higher temps.
• At higher temps, any further rise in temp. e.g during illness may denature enzymes

Question 14

Do different species of mammals/birds have the same body temps?

Answer 14

No. They have different body temps.

E.g Many birds have around 40 degree body temp. as they have a high metabolic rate for high energy required for flight

Question 15

What is pH of a solution a measure of?

Answer 15

H+ ion concentration

Question 16

What is optimum pH?

Answer 16

pH at which enzyme works its fastest (each enzyme has an optimum pH)

Question 17

How do you calculate the pH of a solution?

Answer 17

Use the formula:
pH = -log(subscript)10
A H+ ion has conc. of 1x10^-9 therefore has a pH 9
10^-7 is pH 7 (neutral)

Question 18

How does pH affect the rate of enzyme reaction?

Answer 18

• An increase or decrease in pH (away from optimum) reduces the rate of enzyme reaction
• If change in pH is more extreme, beyond a certain pH, the enzyme becomes denatured

Question 19

How does pH affect specifically how an enzyme works? (not to do with rate)

Answer 19

• A change in pH alters the charges on the amino acids that make up active site of enzyme - as result, substrate can no longer attach to active site + so enzyme-substrate complex cannot be formed
• Depending how significant change in pH is, it may cause bonds maintaining enzyme’s tertiary structure to break
• Therefore the active site changes shape

Question 20

What is the arrangement of the active site partially determined by?

Answer 20

Hydrogen + ionic bonds between -NH2 and -COOH groups of the polypeptides that make up the enzyme.

Question 21

Are pH fluctuations inside organisms usually large or small?

Answer 21

They are usually small. This means they are far more likely to reduce an enzyme’s activity than to denature it

Question 22

What is the effect on enzyme concentration on rate of reaction?

Answer 22

• Once an active site on enzyme has acted on its substrate, it is free to repeat procedure on another substrate. As enzymes are catalysts they will not be used up in the reaction + so work efficiently at very low concentrations
• As long as there is excess of substrate, an increase in no. of enzymes leads to proportionate increase in rate of reaction
• The graph showing the effect of enzyme conc. will initially show a proportionate increase - as there is more substrate than the than the enzyme’s active site can cope with
• So, if you increase enzyme conc. , some of excess substrate can now be acted upon + rate of reaction will increase
• If substrate is limiting (not sufficient to supply all of enzymes’ active sites at one time), then increase in enzyme conc. will have no effect on rate of reaction. Rate of reaction will stabilise at constant level (graph will level off) - this is because available substrate is already being used as rapidly as it can be by using existing enzyme molecules

Question 23

Describe low enzyme concentration

Answer 23

There are too few enzymes to allow substrates to find an active site at one time

Question 24

Describe intermediate enzyme concentration

Answer 24

There are enough enzymes to allow substrates to find an active site at the same time

Question 25

Describe high enzyme concentration

Answer 25

There are more enzymes (than needed - excess) to allow substrates to find an active site at the same time. Rate of reaction stays the same

Question 26

What is the effect of substrate concentration on rate of enzyme action?

Answer 26

• If conc. of enzyme is fixed + substrate conc. is slowly increased, rate of reaction increases in proportion to conc. of substrate - as at low substrate conc. , enzymes are not working at full capacity
• As more substrate is added, active sites grad. become more filled until point where they’re working as fast as they can (max. rate of reaction)
• After that, addition of more substrate will have no effect on rate of reaction - when there is excess of substrate, rate of reaction levels off

Question 27

Describe low, intermediate and high substrate concentration

Answer 27

Low: Too few substrates to occupy all active sites. Flattens off more quickly on graph
Intermediate: Enough substrates to occupy all active sites at one time
High: Substrates in excess + rate of reaction stays the same