78. Constitutive enzymes are
(A) Induced by effector molecules
(B) Repressed by repressors
(C) Encoded by sequences that occur as part of an operon
(D) Always produced in the cell

Constitutive enzymes are always present and active in cells at constant levels, regardless of substrate availability. The correct answer to the query is (D) Always produced in the cell.​​

Correct Answer

Option (D) Always produced in the cell accurately defines constitutive enzymes. These enzymes, often called housekeeping enzymes, are synthesized continuously at a constant rate to support essential cellular functions like glycolysis, independent of environmental cues or substrate presence.​

Option Explanations

• (A) Induced by effector molecules: Incorrect, as this describes inducible enzymes, which require specific inducers like lactose in the lac operon to trigger production.​

• (B) Repressed by repressors: Incorrect, since repressible enzymes are inhibited by end-product repressors, such as tryptophan in its operon; constitutive enzymes lack such regulation.​

• (C) Encoded by sequences that occur as part of an operon: Incorrect, because while some operons exist, constitutive enzymes are not defined by operon membership; their genes are expressed constitutively, not coordinately regulated like inducible operons.​

• (D) Always produced in the cell: Correct, as these enzymes maintain steady production for vital processes, unaffected by substrate concentration.​