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Can we consider insulin receptor an enzyme? In other words, does the insulin receptor have enzymatic characteristics?

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Yes, the insulin receptor can be viewed as an enzyme, since it catalyzes a reaction -- phosphorylation of tyrosine residues on its substrates. But since the protein has multiple functions, it's probably better to say that the insulin receptor "has enzymatic activity", rather than "is an enzyme".

The notion "enzyme" is not restricted to catalysis of reactions involving small molecules, like those in central metabolism. The IUPAC definition of an enzyme reads: "Macromolecules, mostly of protein nature, that function as (bio)catalysts by increasing the reaction rates". So "enzyme" is a broad term, including catalysis of all chemical reactions. DNA replication, mRNA transcription, protein synthesis and posttranslational modifications all involve enzymes. Those enzymes that act on small metabolites can be called "metabolic enzymes" to clarify their function.

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  • $\begingroup$ Insulin receptor tyrosine kinase, is for all intends and purposes an enzyme. It catalyzes the transfer of a Gamma phosphate from ATP to its substrates (a tyrosine), require divalent metal ions (Mg2+) and have maximal activity when bound to an activator such as Insulin. As such it is perfectly acceptable to call it an enzyme. $\endgroup$ – Jeppe Nielsen Oct 20 '17 at 6:48
  • $\begingroup$ Also what other activities are you talking about? The insulin receptor to my knowledge bind extracellular insulin or closely related hormones, become activated and start phohorylation of intracellular substrates. Which substrates and adaptor proteins that are present and their concentrations are what determine downstream signalling. $\endgroup$ – Jeppe Nielsen Oct 20 '17 at 6:56
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    $\begingroup$ @JeppeNielsen I only meant that the protein has multiple functions since it is both receptor and an enzyme. I agree it's fine to call it an enzyme, but it can be useful to indicate that it's not only an enzyme. $\endgroup$ – Roland Oct 20 '17 at 15:01

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