Download Checks and Balances in the Lipid Pathways/Network

A1 8 Biochemical Society Transactions (1999) 27
F12 Checks and Balances in the Lipid PathwaydNetwork
F14 Forging the Link, International Networking
Ian D G Battle, ACTIN Director
Pira House, Randalls Road, Leatherhead KT22 7RU
Slabas. T.
University of Durham, Department of Biology, South Road,
Durham, DH13LE
G1
F13 Turnover and Sequestration of Plant Secondary Products
Robert Edwards, Department of Biological Sciences, University
of Durham, Durham DHl 3LE.
Although there has been considerable progress in understanding
how plant secondary products are synthesised,surprisingly little is
known concerning how they are turned over and degraded. As the
final concentrations of secondary products are determined by a
combination of synthesis and turnover, it is important to
understand how these metabolites are degraded if we are to
successfully engineer secondary metabolism in plants. Many plant
secondary products are conjugated with sugars and stored in the
vacuole. But this is not an end-point of metabolism and these
conjugates undergo rapid cycling and can be re-released or further
degradaded. Using our studies on inducible isoflavonoid
phytoalexin metabolism in legumes as an example, the pathways
responsible for the conjugation, storage and turnover of phenolic
secondary metabolites will be discussed. In alfalfa (Medicago
sufivu)the isoflavonoid phytoalexin medicarpin is degraded by a
combination of oxidative enzymes and sugar conjugation.
Evidence will be presented that the enzymes responsible for the
conjugation of isoflavonoids are highly specific and their
expression regulated by exposure to their aglycone substrates.
After conjugation,the sugar moiety of isoflavonoid conjugates
becomes 6"-O-malonylatedand this modification appears to target
secondary products for vacuolar deposition. The mechanisms
whereby conjugates of secondary metabolites are taken up into the
vacuole is not well understood, but drawing parallels with the
metabolism of xenobiotics, probably involves active transport and
ATP-binding cassette transporter proteins, which may act in
conjunction with glutathione transferases. Finally these
conjugates are readily turned over by re-release into the cytoplasm
by specific hydrolases and can be recycled back into bioactive
isoflavonoids for use in defence against fungal infection.
Live Control of the Living Cell
Wally C. van Heeswijk, Barbara M. Bakker, Bas Teusink,
Boris N. Kholodenko, Oscar J.G. Somsen, Jacky L. Snoep and
Hans V. Westerhoff
Molecular Cell Physiology & Mathematical Biochemistry,
BioCentrum Amsterdam, Biology, De Boelelaan 1087
NL- 1081 HV Amsterdam, European Union
Many studies of metabolic control address the question which
molecular processes determine a flux. Metabolic Control
Analysis has rephrased this question as the extent to which
activation of any such process activates that flux. We shall
discuss how the control of the glycolytic flux in T. brucei, a
mammalian parasite involved in sleeping disease, is distributed
over glucose transport and a number of glycolytic enzymes.
The distribution of control is itself not constant but a strong
hnction of extracellular glucose concentration and of the
activity of the glucose carrier.
The phenomenon of a strong dependence of metabolic control
on the activities of certain cellular components suggests that
living cells may be able to adjust their metabolic control to
their needs: Metabolic control itself may be a live property of
living cells, through differential activation of the cellular
control hierarchies. The variety of glucose transporters in S.
cerevisiae might serve this purpose. We shall discuss an
example of such live control, i.e., that of ammonia assimilation
in E. coli by the pivotal protein PI1 and its homologue GlnK.
Depending on the growth history of the cells, GlnK attenuates
the control exerted by PII. Molecular intelligence may be the
result.