[Protein-analysis] Re: Question about enzymes

r norman via proteins%40net.bio.net (by r_s_norman from _comcast.net)
Mon Oct 16 12:37:04 EST 2006

On 16 Oct 2006 10:19:14 -0700, yubbers9 from yahoo.com wrote:

>Hi all,
>I'm taking an introductory biology course, and I've been told
>that enzymes always have only one active site.
>I'm not sure about this however, because it seems to me
>that there must be some enzyme that has 2 or more,
>after all it's just a lumpy 3-dimensional tangle of protein,
>so why wouldn't there be more than 1 active site?
>For every shape on an enzyme there must be
>some chemical that fits in there, right?
>Can anyone please explain why an enzyme is only
>expected to have one active site?

It is not easy for evolution to figure out a way of making a chain of
specific amino acids in a specified sequence produce a complex
three-dimensional object that actually does something.  And then
evolution has to figure out how to make it do that thing very well,
indeed.  It really is asking too much to have it do two things at the
same time!

Ordinarily, enzymes catalyze one specific reaction (or category of
reactions) using one specific precursor (or category of precursors).
This bind at one specific active site to make things happen.  That
doesn't mean that the enzyme doesn't also have all sorts of other
binding sites where other things can bind and influence the reaction
that occurs at the "real" active site.  In fact it is quite common for
binding of ligands at a second site, or the phosphorylation of an
amino acid at a second site, to have very large consequences on the
function of the enzyme.  It is just that the enzyme really only
catalyzes one reaction.

Note:  just because a ligand happens to "fit into" and even "bind" to
a second site doesn't necessarily mean that anything will happen as a
result.  Hence it isn't really a site at all, just an accidental and
incidental binding.

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