Most of the enzymes, particularly the rate-limiting ones, are present in very low concentration. Many rate-limiting enzymes have short, half lives. This helps in efficient regulation of enzyme levels.

There are two types of enzymes:

1. Constitutive enzymes: Housekeeping enzymes. The levels of these enzymes are not controlled and remain fairly constant.
2. Adaptive enzymes: Their concentration increases or decreases as per the body needs and are well regulated.

Induction: The term induction is used to represent increased synthesis of enzymes which are under gene control through mediation of hormones or other substances.

Example: Hormone insulin increases the synthesis of synthetase, glucokinase, phosphofructokinase, and pyruvate kinase. All these are involved in the utilisation of glucose repress the synthesis of many key enzymes involved in glucose synthesis (gluconeogenesis).

Repression: The term repression is used to represent decreased synthesis.

Example: In many instances, substrate can repress the synthesis of enzyme. Pyruvate carboxylase is the key enzyme in the synthesis of glucose from non-carbohydrate sources like pyruvate and amino acids. If there is sufficient glucose available, there is no necessity for its synthesis. This is achieved through repression of pyruvate carboxylase by glucose.