Enzymes: MDCAT Biology notes
Enzymes MDCAT notes: nature of enzymes, cofactors, activators, prosthetic groups, coenzymes, holoenzyme, zymogens, ribozymes and enzyme classes.
What enzymes are
Enzymes are biological catalysts. They increase the rate of chemical reactions without being used up. After a reaction the enzyme is released unchanged and can be used again. Enzymes do not change the nature of the end product; they only speed up how quickly it forms.
- Almost all enzymes are proteins, built from amino acids. They are globular, and their activity depends on tertiary structure.
- Ribozymes are the exception: catalysts made of RNA.
- Enzymes are specific, work in small amounts, and are sensitive to temperature, pH and inhibitors.
Cofactors: the non-protein helpers
Many enzymes need a non-protein part called a cofactor to work.
| Term | Meaning | Example |
|---|---|---|
| Apoenzyme | The protein part alone, which is inactive | — |
| Holoenzyme | Apoenzyme + cofactor, the active enzyme | — |
| Prosthetic group | An organic cofactor bound covalently (permanently) | Haem in cytochromes |
| Coenzyme | An organic cofactor bound loosely, which detaches after the reaction; often derived from vitamins | NAD, FAD, NADP |
| Activator | An inorganic metal ion that is detachable | $\mathrm{Mg^{2+}}$ for hexokinase; $\mathrm{Zn^{2+}}$ in carboxypeptidase |
So if a question says an enzyme needs $\mathrm{Mg^{2+}}$, call it an activator. If it says "covalently bonded organic part", call it a prosthetic group. NAD is a coenzyme, not an enzyme or a vitamin (although it is made from niacin).
Zymogens
Some enzymes are secreted in an inactive precursor form called a zymogen (proenzyme). This stops them digesting the cells that make them.
- Pepsinogen becomes pepsin when HCl removes a small polypeptide fragment.
- Trypsinogen becomes trypsin in the small intestine.
Classes of enzymes
| Class | Reaction | Example |
|---|---|---|
| Oxidoreductases | Oxidation-reduction (transfer of H or electrons) | Dehydrogenases |
| Transferases | Transfer a group, such as phosphate, between molecules | Kinases; hexokinase moves phosphate from ATP to glucose |
| Hydrolases | Split bonds by adding water | Digestive enzymes |
| Lyases | Remove groups without water | Decarboxylases |
| Isomerases | Rearrange a molecule into its isomer | Phosphoglucose isomerase |
| Ligases | Join molecules using ATP | DNA ligase |
Grouping by substrate
Digestive enzymes are also grouped by what they act on. Pepsin, trypsin and chymotrypsin are proteases, which digest proteins. Lactase is a carbohydrase, which digests lactose. So in a list of those four, lactase is the odd one out.
Worked examples
- An enzyme needs $\mathrm{Fe^{2+}}$ that it loses easily after the reaction. The $\mathrm{Fe^{2+}}$ is an activator, because it is inorganic and detachable.
- An enzyme with its haem group firmly attached is a holoenzyme, and the haem is its prosthetic group.
Common MDCAT traps
- Holoenzyme = apoenzyme + cofactor. The apoenzyme on its own is inactive.
- A prosthetic group is covalently bound; a coenzyme is loosely bound.
- Moving phosphate from ATP is done by kinases, a type of transferase, not ligases or isomerases.
- Enzymes are reused after the reaction; they are not consumed or inactivated.
- Ribozymes are RNA, not protein.
Quick revision
- Enzymes are protein catalysts that increase reaction rate.
- NAD is a coenzyme.
- $\mathrm{Mg^{2+}}$ activates hexokinase.
- Pepsinogen is a zymogen.
- Enzymes depend on tertiary structure.