Question

salivary amylase is an enzyme that aids in the breakdown of starch into sugars. the effect of temperature on amylase activity is shown.
the effect of temperature on amylase activity

1. which statement best explains why the reaction rate decreases when the temperature increases above 38 degrees celsius?

the concentration of the enzyme increases.
the amino acid sequence of the enzyme changes.
the number of enzyme - substrate collisions increases.
the shape of the enzymes active site changes.

Explanation:

Enzymes are proteins, and their function is highly dependent on their three - dimensional shape, especially the active site. The active site is where the substrate binds. When the temperature increases beyond a certain point (in this case, above 38 degrees Celsius), the heat causes the enzyme's structure to change. This is called denaturation. The amino acid sequence of the enzyme (its primary structure) is not immediately changed by moderate temperature increases (it would require extreme conditions like very high heat for a long time to break peptide bonds). An increase in enzyme - substrate collisions would increase the reaction rate, not decrease it. An increase in enzyme concentration would also increase the reaction rate if substrate is available. But when the shape of the enzyme's active site changes (due to temperature - induced denaturation), the substrate can no longer bind effectively, and the reaction rate decreases.

Answer:

The shape of the enzyme's active site changes.