[Protein-analysis] Re: Question about enzymes
lucasea from sbcglobal.net
(by lucasea from sbcglobal.net)
Tue Oct 17 21:34:32 EST 2006
"Tom Anderson" <twic from urchin.earth.li> wrote in message
news:Pine.LNX.4.62.0610171928500.24308 from urchin.earth.li...
> - the mammalian fatty acid synthase, a homodimer which catalyses no less
> than four reactions 
Just curious (this is an honest question), are these four reactions
catalyzed by four different active sites? I'm not a biochemist, but I
understand it's not uncommon for a particular active site to catalyze
multiple different reactions that have similar mechanistic requirements,
particularly if the binding pocket isn't too tight/selective.
> 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.
Again, not the opinion of a biochemist, but I suspect it's basically a
probability/entropic argument. I suspect that the chances of a new function
arising in a pre-existing enzyme the has some other function without
destroying that original function is less than the function arising in a new
protein or than the new function arising in the original enzyme but at the
same time destroying the original function of that enzyme. Certainly
there's a finite probability that a mutation that helps form the second
active site in a pre-existing enzyme will destroy the function of the
original active site by changing the tertiary structure in the wrong way.
Intuition also tells me that it would be a low entropy configuration to have
two active sites in a single enzyme, as compared to the same two active
sites in two separate enzymes. Maintaining this lower-entropy configuration
costs the organism more free energy, and that's usually not in the best
interests of the organism, evolutionarily speaking.
These are just the intuitions of a physical organic chemist with a
smattering of understanding of biochemistry, but I do understand that
evolution can behave in counter-intuitive ways.
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