Enzyme regulation — An enzyme that is synthesized continuously, regardless of substrate presence or metabolic end-product levels, is termed:

Introduction / Context:
Cells regulate enzyme levels to conserve resources. Some enzymes are produced only when needed (inducible) or downregulated by end products (repressible), while others are made continuously. Recognizing these categories is essential in microbial genetics and metabolic engineering.

Given Data / Assumptions:

• Focus is on expression pattern, not catalytic mechanism.
• We distinguish among constitutive, inducible/repressible regulation, allostery, and isoenzymes.
• The question asks for the enzyme produced regardless of environmental cues.

Concept / Approach:
Constitutive enzymes are expressed at relatively constant levels. Repressible enzymes are part of feedback control loops (e.g., biosynthetic pathways) and decrease in response to end-product accumulation. Allosteric enzymes are regulated by effectors binding at non-active sites; this is post-translational and different from expression control. Isoenzymes are different enzyme forms catalyzing the same reaction but encoded by distinct genes and often differently regulated.

Step-by-Step Solution:

Verification / Alternative check:
Housekeeping enzymes such as those for central metabolism can be constitutive in many organisms, though levels may still vary subtly with growth rate.

Why Other Options Are Wrong:

• Isoenzyme: refers to multiple forms, not constant expression.
• Repressible enzyme: expression decreases with end-product accumulation.
• Allosteric enzyme: regulated at the activity level, not necessarily continuously produced.

Common Pitfalls:
Conflating allosteric regulation with gene regulation; they operate at different layers (activity vs. expression).

Final Answer:
a constitutive enzyme