David Chipman claimed that "it is very clear and very obvious what is meant
by "a key
metabolite in [the nonmevalonate] pathway".
I intended to comment on this contribution when it appeared, especially the
last paragraph ("I don't think it is wise for one school of theoreticians
to lay absolute claim to the legitimate use of terms [e.g., 'key'] that
they didn't invent..."), but as Pedro did that very effectively already
there didn't seem much point.
However, I was reminded of Douglas's original point when the latest issue
of Eur. J. Biochem. appeared (vol. 266, number 1, Nov. 1999), as it
contains a review (pp. 1-16) whose authors like the phrase "key
intermediate" so much that they use it not only in the title but also in
the first sentence of the abstract and in the first caption of the
introduction. However, try as I might I cannot see that they are using in
it the sense that David Chipman finds so obvious.
They seem just to mean that phosphatidic acid is an essential substrate for
various enzymes of lipid metabolism, not that "this is one of the
metabolites which DEFINES
in the conceptual sense" lipid metabolism, which would surely be absurd.
So does the "very clear and very obvious" sense apply only to the
nonmevalonate pathway, or is phosphatidic acid not truly a key intermediate?
Among other things, Pedro wrote "'Key' used in this way is really a useless
word, since then all enzymes are key... Remove any enzyme out of a pathway
and it will no
longer function."
True enough, of course, but it may be worth recalling the time 30 years ago
(too long ago for Pedro to remember...) when what many people regarded as
an important enzyme (I don't think anyone called it key) was chymotrypsin.
Despite being one of the few possible exceptions to Pedro's rule, being one
of the few enzymes that one can readily imagine life getting on quite well
without (about as unkey as you could ask for), it had a considerable
importance by virtue of being an enzyme that was available in large amounts
and was reasonably stable. It thus allowed a lot of the groundwork of
enzymology to be laid that would have been impossible to do with enzymes
that might have seemed more interesting biologically.
Athel
================================================================
Athel Cornish-Bowden
Laboratoire de Chimie Bacterienne, Centre National de la Recherche
Scientifique, 31 chemin Joseph-Aiguier, B.P. 71, 13402 Marseille Cedex 20,
France
athel at ibsm.cnrs-mrs.fr
Phone: + 33 491 16 41 38; fax: + 33 491 71 89 14
http://ir2lcb.cnrs-mrs.fr/~athel/homepage.htm
Now available: Basic Mathematics for Biochemists (2nd edn.)
http://ir2lcb.cnrs-mrs.fr/~athel/basmaths.htm
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