Why do Mars and Mercury have many more craters compared to Venus?

They have a thinner atmosphere.

Explanation:

Venus has a very dense atmosphere compared to Mars and Mercury. This causes many small meteorites and asteroids to break apart before they reach the surface. Thus, Venus has fewer craters than Mars and Mercury.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products