Same sugar and size don't make an enzyme fit
- The idea
- Predict
- See it
- Why
- Check
The idea
“If two molecules are the same size and the same sugar, one enzyme should work on both.”
Starch and cellulose are both chains of glucose. What sets them apart is how each glucose joins the next.
The short version
The mistake: If two molecules are the same size and the same sugar, one enzyme should work on both.
What's actually true: An active site fits a specific shape, including how the subunits are linked, so starch and cellulose need different enzymes.
- An active site is a pocket with a specific shape. Only a substrate that matches it binds well enough to react.
- Alpha links, in starch and glycogen, keep every glucose facing the same way. Beta links, in cellulose, flip every other one.
- You make amylase but not cellulase. So you digest the starch in bread, and lettuce cellulose passes through as fiber.
On the exam: Ask which link the enzyme is shaped for. A different link needs a different enzyme.