The rate of intracellular reactions is a function of the availability of substrates and enzymes.
Enzyme activity is controlled at two different levels.
Controlling the amount of enzyme that is present is considered coarse control, while controlling the activity level of the enzyme that is present is termed fine control. The underlying principle in regulation is to maintain a favorable intracellular environment in the most economical manner. Quotes are from chapters 1 and 3 of Biochemistry by G.L. Zubay, Fourth edition, Watson Brown, 1998.