How does a competitive inhibitor slow enzyme catalysis?

By preventing the binding of substrate to active site

Explanation:

Competitive inhibitors exhibit a type of reversible inhibition. These are the substances that bind to the binding site of the substrate on the enzyme, that is the active site.

One the competitive inhibitor is bound to the active site on the enzyme, the substrate cannot bind to it and there is no enzyme-substrate complex formation. Hence, the competitive inhibitor inhibits/slow down the enzyme catalysis by occupying the active site of the enzyme and thereby not allowing the substrate to bind to the enzyme.  

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