When do enzymes allow chemical reactions to occur?

When the substrates happen to fit the enzyme’s active sites.

Explanation:

Enzymes are protein molecules in which the amino acid chains are twisted and folded to create active sites with a specific shape.

They can hold other molecules (the substrates) that fit the active sites in place and in the correct orientation to react (see image below).

When the reaction is complete, the product molecules leave the enzyme-substrate complex and more substrate molecules can attach themselves  to the enzyme.

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