what is the missing word? high temperatures change an enzyme's __________ and stop it working.

The missing word is shape.

Explanation

Enzymes are proteins that must keep a precise three‑dimensional shape to work, especially at the active site where the substrate binds.

High temperatures break the weak bonds holding this shape, so the enzyme is denatured and can no longer function.

So the sentence should read:

“High temperatures change an enzyme's shape and stop it working.”

Information gathered from public forums or data available on the internet and portrayed here.