At 01:39 25/04/97 -0700, Athel wrote:
>Yang-Seok Kim asked: "Vitamin B1 synthetic genes are scattered in E.coli
>chromosome. However, I found, two of them are joined and bacame
>bifunctional protein in plant. Does anybody know the benifit of
>bifuntionallity in evolution or the reference papers about
>bifuntionallity?"
>
>If the two enzymes catalyse non-consecutive reactions it isn't easy to
>think of advantages,
One advantage that has been suggested is coordinate regulation of the two
enzymes as suggested by Michael Savageau (Biochemical Systems Analysis,
Addison Wesley 1974 - though you might find my recent account in
Understanding the control of metabolism, Portland Press, 1996, both more
available and more approachable). The argument is particularly applicable
to branched pathways subject to feedback inhibition. This can occur in
various forms, and one form, nested inhibition, requires that the
sensitivities of the enzymes affected by the feedback metabolite maintain
an appropriate ratio, otherwise the system performs badly and can even lose
stability. If both activities of the bifunctional enzyme respond to a
common regulatory site, there will be a closely defined relationship
between their activities suitable for implementing nested feedback
inhibition. A possible example of this is aspartate kinase : homoserine
dehydrogenase in the nested feedback inhibition control of the threonine
biosynthesis pathway in E. coli.
Patterns of feedback control are very variable between different organisms,
presumably as the result of different selective pressures on different
modes of metabolism in different environments, and this could account for
why particular bifunctional enzymes of this type are not universally
distributed.
David Fell
=======================================================
David Fell, daf at bms.brookes.ac.uk (usual & current)
GESBI, UFR de Biochimie, Universite de Bordeaux II, 146 rue Leo Saignat,
33076 Bordeaux CEDEX, FRANCE.
David.Fell at u-bordeaux2.fr (April-June '97)
