Biology for CAPE Original material Cambridge University Press 2011 1

4 Answers to end-of-chapter questions 1 A [1]

2 B [1]

3 C [1]

4 B [1]

5 C [1]

6 B [1]

7 B [1]

8 D [1]

Structured questions

9 a Succinic acid [1] b Malonic acid is a competitive inhibitor

Since it has a similar structure to succinic acid the substrate it competes for the active site and binds to it

Less substrate (succinic acid) attaches to the active site of enzyme Less product (fumaric acid) is formed Explanation [2]

Diagram only [1] Explanation on diagram [3] Max [3] c Ethanol has a similar molecular structure to ethylene glycol /

substrate Therefore, it would also be complementary to the active site of

enzyme Ethanol would act as a competitive inhibitor It would compete for the active site of enzyme It would prevent ethylene glycol (the substrate) from binding with

active site 56 points [3]

substrate succinic acid

malonic acid competitive inhibitor, similar shape to substrate

enzyme succinic dehydrogenase

inhibitor binds to active site hence no enzymesubstrate complex formed; no products formed

Biology for CAPE Original material Cambridge University Press 2011 2

Fewer enzymesubstrate complexes and less product (oxalic acid) 34 points [2] formed 12 points [1]

d Heavy metals form covalent bonds with the SH groups of the

enzyme These bonds may be formed in the active site If bonds are formed at the active site, the active site would be

permanently blocked So no enzymesubstrate complexes could be formed If the covalent bonds with the heavy metal and SH groups

are formed away from the active site (at allosteric site), these bonds would disrupt the tertiary structure of the enzyme

This would change the shape of the active site Hence the active site would no longer be complementary to

substrate Substrate would no longer be able to fit in the active site

(lock and key) Or the active site would be prevented from changing shape

to fit the substrate (induced fit) Well explained with No enzymesubstrate complexes formed hence no either bond formation at

products the active site or elsewhere [2]

Good diagram [2] e

heavy metal permanently bonded at the active site so substrate no longer fits in the active site

substrate no longer fits in the active site shape of the active site changes

Biology for CAPE Original material Cambridge University Press 2011 3

Each curve [2] 10 a Metabolic reactions the chemical reactions occurring within an organisms body [2] b Lock-and-key

The substrate molecule is complementary in shape to that of the active site

The active site on the surface of the enzyme is so contoured and charged that it attracts only one substrate and the shape of the active site is complementary to that of the substrate

It was thought that the substrate exactly fitted into the active site of the enzyme molecule like a key fitting into a lock (lock-and-key theory)

Enzymesubstrate complexes formed This explained why an enzyme would only work on one

substrate (specificity) [2] Induced fit Active site is not perfectly contoured to fit substrate When the substrate attaches to the active site, the shape of the

whole enzyme changes slightly so it can accommodate and hold the substrate [1]

c i From 10 C to 41 C , the rate of activity increases from 0% to 100%

For every 10 C increase in temperature, the activity doubled Rate increased because there is more kinetic energy Enzyme and substrate molecules collide more often, also

because more molecules have sufficient energy to overcome the activation energy

Optimum is 41 C Above the optimum temperature, the rate decreases as more of

the enzyme molecules denature The thermal energy breaks the hydrogen bonds holding the

secondary and tertiary structure of the enzyme together So the enzyme loses its shape and becomes a random coil and

the substrate can no longer fit into the active site This is irreversible 5 points well explained [5]

(b)

(d)

Rat

e of

reac

tion

Substrate concentration

Biology for CAPE Original material Cambridge University Press 2011 4

ii

Diagram [3] inactive active inactive Correct activity [2] 11 a disappearance of substrate [1]

appearance of product [1]

COOH +NH3

COO NH2

Biology for CAPE Original material Cambridge University Press 2011 5

b i

x-axis labelled, with appropriate intervals [1] y-axis, with appropriate intervals [1] Graph showing the effect of pH on enzyme activity Points correctly plotted and joined [1] of catalase Title [1] ii optimum pH = 7

lowest activity is at pH 3 active over a narrow range increasing activity as pH increases to optimum / pH 7 45 points [2] decreasing activity as pH increases above optimum / pH 7 23 points [1]

iii At pH 3

high concentration of H+ ions enzyme acts a buffer COOH groups unionized H and ionic bonds broken tertiary structure of enzyme disrupted shape of active site changes few enzymesubstrate complexes formed [1] At pH 7 optimum activity: active site unchanged, enzymesubstrate complexes formed,

maximum products [1] At pH 8 low concentration of H+ ions enzyme acts a buffer NH2 groups unionised Hydrogen bonds and ionic interactions broken tertiary structure of enzyme disrupted shape of active site changes few enzymesubstrate complexes formed [1]

at 22 C

Biology for CAPE Original material Cambridge University Press 2011 6

c See graph above: same basic shape / lower activity [2] d To maintain the pH of each experimental solution [1] e volume of hydrogen peroxide used

volume of enzyme used time for reaction to take place Any 2 points [1]

Essay questions 12 a i

Activation energy is the minimum free energy that must be possessed by the molecules on collision for the particles to react

It is the amount of energy needed to raise the reactants to an activated state

It is amount of energy given temporarily to a substrate to be converted into a product Any point [2]

ii

Each curve [1] b Enzyme has tertiary structure

Active site of enzyme is made up of few amino acids With a specific shape Shape of active site complementary to substrate 7 points [4] Only one substrate or type of substrate will fit into active site 56 points [3] To form enzymesubstrate complexes 34 points [2] Refer to lock-and-key and induced fit 12 points [1]

c Substrate binds to active site of enzyme

Few amino acids are involved Remainder of amino acids maintains the globular shape Shape of active site complementary to substrate Can interact by exact fit: lock-and-key Then moulds around the substrate: induced fit Substrate held to active site by hydrogen bonds and ionic

interactions bonds as well as hydrophobic and hydrophilic interactions

To form the enzymesubstrate complex

Biology for CAPE Original material Cambridge University Press 2011 7

Enzymesubstrate complex activated into forming products Substrate changes shape slightly To put strain on bonds in the substrate / weakens bonds Activated into forming products Which no longer fit the active site Products move way, leaving the active site free to Each point [1]

form more enzymesubstrate complexes Max [7]

13 a i pH

Well drawn and labelled [1] Description Normally enzyme works in narrow pH range Rate reduces quickly when pH changes from optimum pH [1] Explanation Changes in pH from optimum affect H+ ion concentration in solution At low PH high concentration of H+ ions enzyme acts a buffer COOH groups unionised At high pH low concentration of H+ ions enzyme acts a buffer NH2 groups unionised Hydrogen bonds and ionic interactions In both cases tertiary structure of enzyme disrupted shape of active site changes few enzymesubstrate complexes formed [2]

ii Enzyme concentration

Biology for CAPE Original material Cambridge University Press 2011 8

Well drawn and labelled [1]

Description Rate of reaction increases as enzyme concentration increases Rate directly proportional / linear to enzyme concentration [1] Explanation More active sites are available More collisions between enzyme and substrate molecules More enzymesubstrate complexes formed More product as enzyme concentration increases [2]

iii Substrate concentration

Well drawn and labelled [1] Description Initial substrate concentration limits the rate of reaction / rate directly proportional

to substrates concentration Reaches maximum velocity and plateaus [1] Explanation All active sites initially available More frequent collisions between substrate and active sites Then as substrate concentration increases, all active sites become occupied /

saturated No more enzymesubstrate complexes can be formed until product is formed. [2]

iv Inhibitors

Biology for CAPE Original material Cambridge University Press 2011 9

Well drawn and labelled [1]

Description Inhibitor reduces rate of reaction Can be competitive or non-competitive Can be reversible or irreversible [1] Explanation Competitive similar shape to substrate competes for active site / occupies active site / binds at active site blocks entry of substrate less substrate bind / less enzymesubstrate complex formed does not bind permanently to active site increasing concentration lessens effect of inhibitor Non-competitive not similar in shape to substrate binds permanently to active site and blocks it hence irreversible or binds to a site away from active site / allosteric site this distorts the tertiary structure of the enzyme shape of active site changes this could be reversible or irreversible increasing substrate concentration does not lessen

effect of inhibitor Well explained [2] b Held in place in active site by temporary bonds

e.g. hydrogen, ionic, hydrophobic and hydrophilic interactions That form between the substrate and some of the R groups of the

enzymes amino acids 1 point each [max 3]

Biology for CAPE Original material Cambridge University Press 2011 10

14 a

Graph [1] Annotations [3] b i General class is hydrolases General class [1]

For proteins (e.g. blood and grass): proteases Grease/oil: lipases Starch-based products (e.g. gravy / sauces): amylases Example [1]

ii Temperature between 40 C and 50 C Conditions [2]

Cleaning power depends on enzyme activity Optimum temperature for enzyme Maximum collisions between enzyme and substrate More products Substrate / stain broken down Neutral pH / detergent and water only If pH decreases from optimum, H+ ions would interact with amino acids of

enzymes Hydrogen bonds and ionic interactions broken Tertiary structure of enzyme disrupted Shape of active site changes Few enzymesubstrate complexes formed Well explained [3]

c i Non-competitive inhibition

Tertiary structure of enzyme distorted when aspirin attaches to R group of amino acid of enzyme

Changes shape of active site Substrate for COX no longer fits in active site No enzymesubstrate complexes formed Well explained [2]

ii Competitive inhibition occurring

Since penicillin resembles the substrate It is also irreversible since bonds permanently at the active site Therefore stops activity of transpeptidase Well explained [2]

at low temperatures: slow reaction; less kinetic energy; fewer collisions between enzyme and substrate; fewer enzymesubstrate complexes formed, enzyme is not denatured; for every 10 C increase in temperature, rate

optimum temperature = 40 C: more kinetic energy, more collisions between enzyme and substrate, more enzymesubstrate complexes formed

temperature above optimum: more kinetic energy; molecules vibrate quickly; hydrogen bonds broken in tertiary structure of enzyme; shape of active site changes; fewer/no enzymesubstrate complexes formed; enzyme denatured; irreversible