In reply to the question by Athel Cornish-Bowden on PPi metabolism in plants:
PPi-dependent PFK and PPDK (pyruvate orthophosphate dikinase) seem to be
present in the cytosol of many plants (co-existent with ATP-PFK and
pyruvate kinase)
It has been hypothesized that in plants and phototrophic protists a
(Fru-2,6-P2 sensitive) PPi-PFK plays a role in (the forward direction) when
oxygen levels are low and in other situations where the ATP/ADP ratio may
be lowered (for a short review, see Mertens, E., FEBS Lett., 1991, 285,
1-5.
PPDK is responsible for PEP formation in leaves of C4 plants. In these
plants, PEP serves as the site of CO2 fixation during photosynthesis.
Therefore, PPDK is under strict light/dark control. A reference: Edwards et
al, Annu Rev. Plant Physiol. 1985, 36, 255-
Paul Michels
>My question is stimulated by David's comment, though it may not be
>directly relevant to it. Does anyone know to what extent it is true (as
>I was told a long time ago) that in plants inorganic phosphate fulfils
>the role associated with ATP in animals, i.e. that phosphorylation of
>many molecules is coupled to conversion of PPi to Pi rather than
>conversion of ATP to ADP? If this is true,
>>1. Are there parallel (alternative) ATP-using and PPi-using enzymes?
>>2. Or are many of the usual ATP-dependent enzymes missing from plants?
>>3. If alternative enzymes exist, how do they affect energy regulation,
>and do they permit the sort of response to loss of ATP production that
>David refers to above?
>>4. References to reviews?
>>Athel
>>Home page: http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/home_page.html>MCA FAQ: http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/mcafaq.html>MCA chapter from my book:
>http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/fek10/mcai.html
Paul Michels
Research Unit for Tropical Diseases (TROP).
International Institute of Cellular and Molecular Pathology (ICP).
Avenue Hippocrate 74, B-1200 Brussels, Belgium.
tel: +32-2-764.74.73 / 764.74.39 (secr); fax: +32-2-762.68.53
e-mail: michels at trop.ucl.ac.be
