Definition
An enzyme is a protein that speeds up (catalyses) a biochemical reaction without being used up itself.
Metabolism is the sum of all chemical reactions in an organism: catabolism breaks larger molecules down (releasing energy), anabolism builds larger molecules up (requiring energy input).
Remember
An enzyme is specific to its substrate because its active site's shape matches only that substrate — like a key fitting only one lock, which is why one enzyme can't substitute for another.
Rule
Lock-and-key model: the active site's shape is a fixed, rigid match for its one substrate.
Induced fit model: the active site changes shape slightly as the substrate binds, moulding around it for a closer fit — the currently accepted, more accurate model.
Definition
Temperature: activity rises with temperature up to an optimum, then falls sharply as heat denatures the enzyme (permanently changes its shape, destroying the active site).
pH: each enzyme has its own optimum pH; moving away from it in either direction reduces activity, and extremes denature it — digestive enzymes have very different optima (pepsin works best in the stomach's acidic pH, while others work best near neutral pH in the small intestine).
Substrate concentration: activity increases with more substrate until all active sites are occupied, after which it levels off.
Example 1 — pH away from the optimum
An enzyme works best at pH . If placed in an acidic solution at pH , what happens to its activity?
- pH is far from the optimum .
- The enzyme's active site shape is disturbed, reducing (or, at extremes, permanently destroying) its activity.
Answer: Activity decreases sharply; at very low pH the enzyme may denature
Definition
Competitive inhibitors resemble the substrate and bind directly to the active site, blocking the substrate from entering.
Non-competitive inhibitors bind elsewhere on the enzyme (not the active site), changing its shape so the active site no longer works properly.
Remember
A prosthetic group is a non-protein part permanently attached to some enzymes, often essential for their activity — different from a substrate, which binds only temporarily.
Quick Recap
- ●Enzyme: a protein catalyst, specific to its substrate, unchanged after the reaction
- ●Catabolism breaks down (releases energy); anabolism builds up (needs energy)
- ●Lock-and-key: rigid fit. Induced fit: active site moulds around the substrate (current model)
- ●Activity peaks at an optimum temperature and pH, then falls; rises with substrate concentration until saturated
- ●Competitive inhibitor: blocks the active site. Non-competitive: changes the enzyme's shape from elsewhere
