Think of a enzyme as a glove and its substrate as a baseball. They are made for each other. One captures the other. Now think about a baseball glove with a large softball in it. The glove can no longer effectively capture the baseball because there is another ball blocking the pocket. It much more complicated than this, but the essence is something is blocking something else.
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Please review the linked page for context.
If you can think of something better than this,
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