| Date | November 2010 | Marks available | 2 | Reference code | 10N.3.SL.TZ0.10 |
| Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Discuss | Question number | 10 | Adapted from | N/A |
Question
Superoxide dismutase is an enzyme used by cells to protect themselves against oxidative damage. These enzymes can have different metals as part of their structure.
A study to compare two dismutases from Escherichia coli bacteria and two dismutases from eukaryotic cells was undertaken. The following enzymes were used:
- E. coli dismutase with iron (Fe)
- E. coli dismutase with manganese (Mn)
- eukaryotic mitochondrial dismutase with manganese (Mn)
- eukaryotic cytoplasmic dismutase with copper-zinc (Cu-Zn).
The following shows part of the amino acid sequences of these enzymes. Boxes enclose identical amino acids in the sequence of the two E. coli and mitochondrial dismutases.
State how many amino acids are in the same position in the E. coli (Fe), E. coli (Mn) and the mitochondrial dismutase sequences shown.
State the amino acids which are present in the same position in at least one bacterial dismutase and in both eukaryotic dismutases.
Compare the E. coli (Mn) and the mitochondrial dismutases.
The sequences of the two bacterial dismutases and the mitochondrial dismutase show a high degree of homology. Discuss how this supports the endosymbiotic theory for the origin of mitochondria.
Markscheme
11
Ile and Glu (need both to award the mark)
a. share 17 (out of 29) amino acids in common / more amino acids similar than different;
b. both have Mn in the enzyme (as cofactor);
c. greatest difference between them is from amino acid 18 to 22;
d. mitochondrial has Gly (position 12) while E. coli (Mn) never has Gly;
e. Leu is most common amino acid in both appearing four times / other valid comparison;
a. endosymbiotic theory states bacteria were engulfed by organisms to become mitochondria;
b. sequence comparison between mitochondrial and bacterial dismutase supports this hypothesis;
c. more similarity in the amino acid sequence between mitochondrial and bacterial dismutase than between mitochondrial and cytoplasmic dismutase;
Examiners report
Almost all indicated correctly that 11 amino acids were in the same position in the three dismutase sequences.
Most were able to correctly identify Ile and Glu as present in the same position.
Most were able to get two correct comparisons. Some carelessly indicated that the common element was magnesium rather than manganese despite the fact this was stated in the stem.
Most candidates received 2 marks for this section. Good descriptions of the endosymbiotic theory were often given.
