tsig
Posts: 339
Joined: Aug. 2006
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| Quote (Daniel Smith @ April 27 2009,19:10) | | Quote (JAM @ April 26 2009,21:54) | | Quote (Daniel Smith @ April 26 2009,17:59) |
Please do. |
1. acetohydroxy acid synthase (x 3 isozymes)
2. acetohydroxy acid isomeroreductase
3. dihydroxy acid dehydratase
4. multiple transaminases that use both isoleucine and valine as substrates
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Thank you JAM for that. I've only just begun to look at these enzymes but I am immediately struck by the fact that we have not defined terms here.
How do you define "specific" in a biological sense?
One of the many dictionary definitions of "specific" is "Something particularly fitted to a use or purpose." This is more in line with the meaning I have in mind. It doesn't necessarily mean "exclusive", although those are related terms. An enzyme becomes more specific as more substrates, reactions and products are excluded. It also doesn't mean "unique". Specificity is relative not absolute. One of the abstracts I looked at has even defined a "specificity ratio" for the three isozymes you mentioned.
What "specific" means to me is the level to which an enzyme is ideally suited for the job it does. If it is limited to one substrate and one product, then I'd call it "highly specific". If it will accept a limited amount of substrates or produces a limited amount of products, it is just "specific". If it will accept a wide range of substrates or will produce a wide range of products, it would be "non-specific".
I don't see where the enzymes you cited are non-specific in that sense. For instance the three isozymes of acetohydroxy acid synthase--as near as I can tell--are limited to two substrates and two products each, thus meeting my definition of "specific". What's more, they complement each other, with one being more effective at producing acetolactate when pyruvate levels are low, thus enabling a bacterium to cope with poor carbon sources. So they vary in their specificity for both product and substrate.
But all of this is moot if we don't agree on the definition of "specific". So again JAM, how do you define "specific"? |
You say you were struck with a fact. I don't think you've ever got that close to one and if you did you could dodge it with ease.
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