Enzymes
Biology · GCSE · The School
Biological catalysts
An ENZYME speeds up a reaction without being used up. Each has an ACTIVE SITE shaped to fit one substrate, which is why amylase breaks down starch and does nothing to protein. The lock-and-key model captures the specificity; the more accurate induced-fit model adds that the site adjusts slightly as the substrate arrives.
Temperature: up, then off a cliff
Raising the temperature raises the rate, because molecules collide more often and with more energy. Then, above the optimum, the rate collapses — not gradually, but sharply. The bonds holding the enzyme's shape break, the active site loses its shape, and the substrate no longer fits. That is DENATURING, and it is permanent: cooling a boiled egg does not un-boil it.
pH does the same thing
Every enzyme has an optimum pH, and moving away from it distorts the active site in the same way. Stomach protease works best around pH 2 in stomach acid; the same enzyme would be useless in the small intestine, where pancreatic enzymes work around pH 8. The body does not have one condition — it has several, each matched to its enzymes.
Measure how fast an enzyme works from 10 °C to 70 °C. The rate rises to a peak around 37 °C — more energy means more successful collisions with the substrate — then falls off a cliff above about 45 °C, and does not recover on cooling. That last part is the giveaway: the enzyme has been DENATURED, its active site permanently changed shape. Rising and falling look symmetrical on the graph and are completely different events.