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Thread: What is a constitutive enzyme

  1. #1 What is a constitutive enzyme 
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    I have two definitions of what a constitutive enzyme is

    "constitutive enzyme An enzyme that is always produced whether or not a suitable substrate is present"

    And

    "Constitutive enzymes are produced in constant amounts without regard to the physiological demand or the concentration of the substrate. They are continuously synthesized because their role in maintaining cell processes or structure is indispensable."


    Now my enzyme is always expressed even if no substrate is present (its an organic nitrogen metabolising enzyme expressed even when only inorganic nitrogen is present)

    According to the first definition this is therefore a constitutive enzyme. However other definitions (such as the second one) indicate that expression levels are constant irrespective of substrate levels. Now expression of my enzyme is further induced (relative activity normalised to total protein) by the presence of the substrate. Which would suggest it is not a constitutive enzyme.


    Does anyone have a better definition ? Or which way I should be leaning towards?


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    Forum Freshman Samuel P's Avatar
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    I would suggest that the first definition is just a simplified version of the second.

    Now my enzyme is always expressed even if no substrate is present (its an organic nitrogen metabolising enzyme expressed even when only inorganic nitrogen is present)
    Are you saying that you are studying an enzyme and you're struggling to define whether it is a constitutive enzyme or not?

    I'm not really too sure on what you're trying to say in this entire area, but if an enzyme is produced relative to protein levels, it is not constitutive enzyme.


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    Sorry for not making it clear,

    Yes I am struggling to define if it is constitutive enzyme or not. I too was leaning for towards the second definition.

    However I wasn't sure whether it was an ambiguous term that could be applied in this case. The reason I was asking if it is constitutive enzyme it implies it is essential for normal function which would help me explain the whole story.

    Its a bit too complex to explain in here but basically I have two enzymes X and Y which both work on the same substrate forming the same product.


    X is expressed even if no substrate is present, but levels of expression can double with increased substrate.

    Y on the other hand is expressed only when there is excess substrate or X is inhibited (therefore is clearly an induced enzyme)


    When discussing the two enzymes it would help if there was a proper term to distinguish the two types of activity seen.
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  5. #4  
    Forum Freshman Samuel P's Avatar
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    From all the evidence, I would say, as you have, that X is a constitutive enzyme and Y is an induced enzyme. Although induced enzymes are only usually stimulated to be produced by an excess of a substance, or the introduction of a new substance.

    That's about the extent of my knowledge. But you look like you're on the right lines ^_^.
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