| Date | May 2013 | Marks available | 2 | Reference code | 13M.3.hl.TZ1.B4 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Describe | Question number | B4 | Adapted from | N/A |
Question
Enzymes are biological catalysts. Catalases are highly efficient enzymes found in cells. Each catalase molecule can decompose millions of hydrogen peroxide molecules per second.
Metal-based inorganic catalysts are also common. In 2009, at Cardiff University in Wales, a new catalyst was developed by Hutchings and co-workers using gold–palladium nanoparticles in the direct synthesis of hydrogen peroxide.
Enzymes are affected by inhibitors which can be either competitive or non-competitive.
Describe the characteristics of an enzyme (such as catalase).
State how inhibitors affect the initial rate of reaction of an enzyme with its substrate.
Explain the action of competitive and non-competitive inhibitors on enzymes in terms of where the inhibitor binds to the enzyme.
Competitive inhibitors:
Non-competitive inhibitors:
State how inhibitors affect the values of \({V_{{\text{max}}}}\) and the Michaelis constant, \({K_{\text{m}}}\), by completing the table below.
Markscheme
proteins;
enzyme activity depends on tertiary and quaternary structure/nature of active site;
lock and key / induced fit (hypothesis/theory);
Allow enzymes are specific (for a particular reaction).
initial rates reduced;
Competitive inhibitors:
(similar shape to substrate so) fits inside active site instead of substrate / OWTTE;
Non-competitive inhibitors:
binds to enzyme not at active site and changes shape of active site so substrate no longer fits / OWTTE;
Allow at allosteric site instead of not at active site.
Award [1 max] for both Vmax correct.
Award [1 max] for both Km correct.
Examiners report
In part (a), many candidates managed to score both marks, and most stated that enzymes are proteins, which scored at least one mark.
In (b), (i) and (ii) were well answered though for the non-competitive inhibitors some did not state explicitly that they change the shape of the active site so the substrate no longer fits, in addition to stating that they bind to the enzyme but not at the active site.
In (b), (i) and (ii) were well answered though for the non-competitive inhibitors some did not state explicitly that they change the shape of the active site so the substrate no longer fits, in addition to stating that they bind to the enzyme but not at the active site.
In (iii), the better candidates scored full marks, though many scored at least one mark, usually for getting the two \({V_{{\text{max}}}}\) correct.
