Topic 2.5
Proteins
Amino acids, proteins and enzymes
Aims
In this exercise your students can use their own knowledge from several areas of the
specification and elsewhere to answer the questions that are related amino acids, glutathione,
proteins and enzymes (mainly the effects of temperature on the rate of reaction). Your
students can use the information provided along with their knowledge to identify trends.
Teaching notes
A variety of question types are provided. Where applicable, encourage your students to write
short sentences, using technical terms, with one point per sentence. Suggested marking
scheme points are separated by semi-colons.
Answers
1
R
2
Peptide
3
4
Carboxyl and amino/amine
H 2 N–A–COHN–B–COOH + H 2 O
5
6
This tripeptide has a free amino group on glycine, whereas glutathione has a free
carboxyl group on glycine/glycine is at the N-terminus in this tripeptide, whereas in
glutathione it is at the C-terminus.
7
8000 (from 20 × 20 × 20)
8
a) When it is composed of just one polypeptide chain/has less than two polypeptides
b) Disulfide bridges/ionic or hydrogen bonds/hydrophobic effects/van der Waals forces
9
c) Through coiling of the chain/the alpha helix; Stabilised by hydrogen bonds;
Between the O of the CO group, with its partial negative charge, and the H of the NH
group, with its partial positive charge
a) To set a certain pH and help to restrict pH changes (during the reaction);
Suitable? At/close to the optimum pH
AQA Biology AS Level Extension Teacher Notes © Nelson Thornes Ltd 2008
1
Topic 2.5
Proteins
b)
% reduction =
reduction
12.0 − 7.4
× 100; =
× 100 = 38.3%
control
12.0
c) Reaction rate decreases with increased time at 55 °C;
Decrease is most rapid in the first 10 minutes (then continues but is less marked);
d) With reference to the structure of enzymes, explain why the reaction rate was slower
after 40 minutes of immersion at 55 °C compared with the control, and was also
slower than after 10 minutes of exposure to 55 °C. THE ANSWER REQUIRES
TWO “PUSHES”, i.e. before and after the underlined “and”.
Denaturation after 40 minutes at 55 °C;
But it is incomplete;
Active site changed;
Poorer fit of substrate molecules;
Higher temperatures increase vibration of polypeptide chains;
Weak bonds (of tertiary structure) are broken;
Greater degree of denaturation (after 40 minutes)/denaturation is progressive (to
answer: “was also slower than after 10 minutes of exposure to 55 °C”).
10 a) Temperature coefficient (Q 10 ) = 6/15 = 0.40; (NOT 2.50 [15/6];
nor 1.25 [50 °C/40 °C])
b) D: Positive, increasing gradient
AQA Biology AS Level Extension Teacher Notes © Nelson Thornes Ltd 2008
2