On 11 Feb 1997 00:59:38 -0800, athel at IR2CBM.CNRS-MRS.FR (Athel) wrote:
>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?
To start with, a textbooky answer (from Molecular Activities of Plant
Cells by Anderson, J.W. and Beardall, J., Blackwell, 1991).
The only plant-specific PPi-dependent reaction they refer to is an
alternate route catalysed by FP-PTase
fructose-6-P + PPi ---> fructose-1,6-bisP + Pi
Here it seems that there are two parallel ATP and PPi-using enzymes.
However, the PPi one is thought to work principally in the reverse
direction to supply PPi for sucrose breakdown via UDP-glucose
pyrophosphorylase. FP-PTase has been found only in higher plants, some
algae, ryophytes and Rhodospirillum.
So much for received wisdom. Maybe there is something else in the
primary literature.
Jannie
Prof. Jannie Hofmeyr Tel: +27 21 808 3039
Dept. of Biochemistry Fax: +27 21 808 3022
University of Stellenbosch e-mail: jhsh at maties.sun.ac.za
Stellenbosch 7600
South Africa
