Description of limitations, often extreme, on what molecules can serve as substrates for particular metabolic reactions.
Enzyme specificity limits what reactions can occur within or associated with an organism by limiting what molecules can be acted upon. The result is substantial control over metabolism in terms of what can occur catalytically as well as when and where it can occur.
Note that enzyme specificity tends to not be 100%, that is, at low rates substrate molecules other than primary substrates may be acted upon. This flexibility probably represents a key means by which new metabolic functions can evolve, that is, with "new" functions representing quantitative changes in terms of what already occurs rather than qualitative ones. Also in terms of evolution, their likely are limits to the benefits associated with increasing enzyme specificities to 100%.
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