[Protein-analysis] Re: Question about enzymes

Dr Engelbert Buxbaum via proteins%40net.bio.net (by engelbert_buxbaum from hotmail.com)
Thu Oct 19 05:28:39 EST 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.

They have at least one, otherwise they could not work. However, there
may be more than one. 

First of all there are enzymes with more than one substrate, which have
sites for each of them. Of course, depending on definition, one could
claim that all substrate binding sites make up one active site. For
adenylate kinase (AMP + ATP <-> 2 ADP) there is a film available on the
net that shows how that works (don't recall the URL).

There are also enzymes with two different activities (Rubisco is a
famous example), which use partly different sites.

Also, there are enzymes which have duplicated sites, like the multiple
drug resistance transporter Mdr1 (Pgp, ABCB1p) for ATP. Usually such
duplication results in co-operativity (the iron transporter transferrin
is the only example I know with 2 binding sites and no co-operativity).
Mdr1 can transport various +/- hydrophobic substrates from the internal
leaflet of the plasma membrane to the extracellular fluid. It appears
that there are different, but overlapping, sites for different transport
substrates.

Follow up set to bionet.molbio.proteins



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