[Protein-analysis] Re: Question about enzymes

[Tom Anderson]

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, 
consider:

- the mammalian fatty acid synthase, a homodimer which catalyses no less 
than four reactions [1]

- the F1 ATPase, in which there are not only three active sites, but 
three active sites whose reaction cycles are rigidly interlinked [2]

- 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 [3]

> 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.

tom

[1] http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16513973

[2] http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=15139813

[3] http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=10860978

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