Enzymes as Biocatalysts: MDCAT Chemistry notes
Enzymes as Biocatalysts for MDCAT: active site, effect of pH and temperature, inhibitors, urease, alkaline phosphatase and medical uses.
What enzymes are
Enzymes are biological catalysts. Almost all are globular proteins. Like other catalysts they speed up a reaction by lowering its activation energy, are not used up, and do not change the position of equilibrium. They are far more efficient and far more specific than inorganic catalysts, and they work under mild conditions of body temperature and pH.
How an enzyme works
The substrate binds to a small region of the enzyme called the active site, forming an enzyme–substrate complex. The reaction occurs there and the products are released, leaving the enzyme free again:
$$\mathrm{E + S \rightleftharpoons ES \rightarrow E + P}$$
- Lock-and-key model: the active site has a fixed shape that fits only one substrate.
- Induced-fit model: the active site changes shape slightly as the substrate binds.
Many enzymes need a non-protein helper: a metal ion cofactor or an organic coenzyme (often derived from a vitamin).
Factors affecting enzyme activity
| Factor | Effect |
|---|---|
| Temperature | Rate rises up to an optimum (about 37$^\circ$C for human enzymes); above it the protein is denatured and activity falls sharply. |
| pH | Each enzyme has an optimum pH: pepsin works in the acidic stomach (about pH 2); salivary amylase near neutral (about pH 6.8); trypsin in the alkaline small intestine (about pH 8). |
| Substrate concentration | Rate rises until all active sites are occupied, then levels off. |
| Inhibitors | Reduce activity (see below). |
Example: salivary amylase begins digesting starch in the mouth. When the food reaches the stomach, the strongly acidic pH inactivates the amylase and its action comes to an end.
Inhibitors
- Competitive inhibitors resemble the substrate and occupy (block) the active site, so the substrate cannot bind. Adding more substrate can overcome them.
- Non-competitive inhibitors bind elsewhere on the enzyme and change the shape of the active site.
Inhibitors act on the enzyme, not by combining with the product or merging with the substrate.
Sources and uses of some enzymes
| Enzyme | Source / role |
|---|---|
| Urease | Present in soya bean (and jack bean); hydrolyses urea to $\mathrm{NH_3}$ and $\mathrm{CO_2}$ |
| Zymase, invertase | Yeast; fermentation of sugars |
| Diastase | Germinating barley (malt); starch to maltose |
| Pepsin, trypsin | Stomach, pancreas; protein digestion |
Enzymes in medicine
- Alkaline phosphatase in blood is raised in rickets (and other bone diseases), because bone-forming cells are overactive.
- L-asparaginase is used to treat leukaemia (blood cancer): it destroys asparagine, which the leukaemic cells need.
- Serum amylase and lipase rise in pancreatitis; cardiac enzymes such as creatine kinase rise after a heart attack.
Common MDCAT traps
- The stomach stops amylase, not lipase or pepsin (pepsin needs acid).
- Rickets: alkaline phosphatase, not lactic dehydrogenase.
- L-asparaginase is for blood cancer, not jaundice or rickets.
- Urease is associated with soya bean, not yeast or grapes.
Quick revision
- Enzymes are protein biocatalysts that lower activation energy.
- Substrate binds at the active site.
- High temperature and extreme pH denature enzymes.
- Competitive inhibitors block the active site.