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I found that inducible enzymes can be regulated by many ways, but I cannot find how constitutive enzymes are regulated...

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A constitutive or "housekeeping" enzyme is usually defined as an enzyme whose expression level (amount of protein) is always constant, independent of any environmental factors. Hence, regulation of constitutive enzymes must be post-translational, for example allosteric regulation of the enzymatic activity by substrates or products.

Note that constitutive enzymes is mostly a theoretical concept. There are probably no enzymes whose expression is completely unaffected by any environmental factor.

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  • $\begingroup$ why do you say constitutive enzymes is mostly a theoretical concept? $\endgroup$ – george Jun 28 '16 at 10:12
  • $\begingroup$ Because I have yet to see an enzyme whose expression is not regulated at the transcriptional/translational level in some experimental condition. An enzyme's protein level might be constant in some particular experiment, but who's to say what will happen in another setting. It's the same problem as with "housekeeping genes" in RT-PCR. $\endgroup$ – Roland Jun 28 '16 at 17:35

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