It's usually the enzyme that heat and pH change
- The idea
- Predict
- See it
- Why
- Check
The idea
“Heat slowed the reaction by destroying the substrate, leaving the enzyme nothing to work on.”
Heat damages things, so it's easy to blame the substrate. Its covalent bonds hold up fine at these temperatures, though. The enzyme's shape rests on weak bonds, and those break.
The short version
The mistake: Heat slowed the reaction by destroying the substrate, leaving the enzyme nothing to work on.
What's actually true: Heat above the optimum and extreme pH slow the reaction by changing the enzyme's shape or charges. The substrate is usually fine.
- The active site is where the substrate binds. Its shape and charges come from how the protein folds, held in place by weak bonds like hydrogen and ionic bonds.
- Heat past the optimum breaks those bonds, and the enzyme denatures, or unfolds. Extreme pH changes its charges, which can block binding, stop the reaction, or unfold it.
- Most substrates, like sugars, are held by sturdy covalent bonds and come through these temperatures unchanged.
On the exam: If more substrate can't bring the rate back, the enzyme is what changed.