4. Exam Q

Question 1

are the same enzymes needed for the substrate

Answer 1

different dehydrogenase enzymes are needed

as substrate molecules are different

Question 2

name A and B

Answer 2

A - substrate molecule

B - active site

Question 3

describe the relationship between the substrate molecule and the active site (2 marks)

Answer 3

substrate shape is complementary to shape of active site (1)
A and B can bind to form an enzyme substrate complex (1)

Question 4

Explain what C and E tell you about enzymes. [1]

Answer 4

unchanged by the reaction so can be reused (1)

Question 5

Answer 5

(i) tertiary (1)
(ii) globular (1)

Question 6

Catalase acts inside cells. Give the name to this type of enzyme. [1]

Answer 6

intracellular

Question 7

Answer 7

An amino group A
Hydrogen bonds C
A carboxyl group B
The substrate F

Question 8

(b) Under acidic conditions amino groups gain hydrogen atoms and become
positively charged while under alkaline conditions carboxyl groups lose a hydrogen ion and become negatively charged.

	Use this information and the diagram above to explain why a change in pH 		can inactivate an enzyme.							[3]

Answer 8

(b) opposite charges attract / same charges repel (1)

	example of bonds forming / not forming (1)
	e.g.: if charges change then hydrogen bonds may / may not form / 
	ionic bonds may / may not form between -NH3+ and -COO- groups 

shape of active site changes / active site unable to form bonds with substrate / cannot form enzyme-substrate complex (1)

Question 9

Answer 9

(i) the energy of the products of the reaction F
(ii) the activation energy of an enzyme catalysed reaction C
(iii) the energy released during the reaction E
(iv) the formation of an enzyme-substrate complex B

Question 10

(b) (i) An enzyme-substrate complex is formed when a substrate binds to the active site of an enzyme. The lock and key and induced fit hypotheses have been proposed to explain how these complexes are formed. Describe the similarities and differences between these hypotheses. [3]

Answer 10

(b) (i) similarities:
substrate bonds to the enzyme / form an E-S complex (1)
differences:
lock and key model – active site is fixed (1)
induced fit – active site can change shape to improve the bonding to the substrate (1)

Question 11

Citrate and isocitrate are structural isomers of each other. Explain what is meant by the term structural isomer. [1]

Answer 11

Structural isomers are molecules that have the same molecular formula, but have a different arrangement of atoms

Question 12

Explain why the reactions at X and Y must be catalysed by different enzymes. [2]

Answer 12

enzymes are specific to a substrate / each substrate is complementary in shape to the active site (1)

enzymes have the same function but different shaped substrate (1)

Question 13

(iii) Explain why it is not possible to identify the optimum temperature of this enzyme. [2]

Answer 13

mean results for 45 and 60˚C are very similar (1)
overlap in results (1)

Question 14

Answer 14

(b) (i) Adding fruit juice:
lowers / changes pH (1)
enzymes are {below / not at} optimum pH / denatured (1)

	(ii)	Freezing the purée: 		
		molecules have low kinetic energy (1)  
		so low rate of successful collisions (1)

	(iii)	Seal with a vacuum sealer:
		the enzyme is an oxidase / needs oxygen (1)
		no oxygen in a vacuum (1)

Question 15

Explain how each type of inhibition affects the rate of reaction in competitive and non-competitive enzymes. (4 marks)

Answer 15

competitive

inhibitor has a similar shape to substrate (1)
binds to active site + prevents ES complex formation (1)
at high substrate concentration less competition for active site so rate reaches maximum (1)

non-competitive
inhibitor binds to enzyme at allosteric site / not active site (1)
changes shape of active site + substrate cannot fit into active site / ES complexes don’t form (1)
no competition with substrate so high substrate concentration still does not reach maximum rate (1)

Question 16

(iii) When investigating the effect of substrate concentration on the rate of an enzyme-catalysed reaction, explain how and why the following factors must be kept constant. [4]

Temperature
pH

Answer 16

Temperature
change to the kinetic energy affects the rate of successful collisions (1)
high temperature can denature enzyme by changing the shape of the active site (1)
pH
small changes in pH from the optimum have small / reversible changes to shape of active site (1)
so reduce rate of ES complex formation (1)

Question 17

Answer 17

malonate similar shape to succinate (1)
fits into and blocks active site / competes with substrate for active site (1)
prevents formation of ES complexes (1)

Question 18

when data is closer together - what is this called?

Answer 18

standard deviation

Question 19

Answer 19

2. (a) has 100% activity at a wider range of temperatures (35 and 40) compared to free lactase (1)
   remains active at higher temperatures e.g. 25% at 55˚C compared to only 10% for free lactase (1)
   less variation in results / smaller standard deviation (1)(b) can control the rate by adding / removing immobilised enzyme (1)
   can reuse the enzyme so reduces costs (1)(c) 37˚C body temperature of humans (1)

Question 20

(a) Explain what is meant by the term extracellular enzyme

Answer 20

an enzyme that is secreted by a cell and acts outside the cell

Question 21

Explain why the trypsin was left in the buffer for 5 minutes before adding the milk.

trypsin - a protease enzyme

Answer 21

to allow time for the change in pH to affect all enzyme molecules before being mixed with the substrate

Question 22

explain the use of buffers in enzyme reaction investigating pH

Answer 22

to maintain constant pH at each pH being investigated

Question 23

how to increase accuracy of absorbance readings

Answer 23

accuracy
absorbance only measured at 15 second intervals + measure absorbance continually
absorbance only measured until there is little change + measure absorbance until no change

Question 24

A student concluded that the CuSO4 was acting as a non-competitive inhibitor.
To what extent do your results agree with this conclusion? Explain your answer.

Answer 24

agree - non-competitive inhibitor
time taken with copper sulphate did not decrease at higher concentration of peroxide
if competitive you would expect time for disc to sink and rise at higher concentrations of peroxide