[Protein-analysis] Re: Question about enzymes
(by twic from urchin.earth.li)
Tue Oct 17 13:51:56 EST 2006
On Mon, 16 Oct 2006 yubbers9 from yahoo.com wrote:
> I'm taking an introductory biology course, and I've been told that
> enzymes always have only one active site.
You have been lied to.
> 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?
You're quite right, as others have pointed out. On top of their examples,
- the mammalian fatty acid synthase, a homodimer which catalyses no less
than four reactions 
- the F1 ATPase, in which there are not only three active sites, but
three active sites whose reaction cycles are rigidly interlinked 
- erythrocruorin; i don't know if you count haemoglobin as an enzyme (you
certainly shouldn't, but never mind), but earthworms have a polymeric
version with no less than 144 oxygen-binding sites 
> For every shape on an enzyme there must be some chemical that fits in
> there, right?
An infinite number, in fact. That doesn't make it a catalyst!
> Can anyone please explain why an enzyme is only expected to have one
> active site?
Because, having looked at more enzymes than you can shake a
spectrophotometer at, biochemists have observed that most do only have one
active site. And why is that? Good question; i'm not aware that there's a
definitive answer, and not sure that there even could be one.
Taking care of business
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