Page 110 - The Biochemistry of Inorganic Polyphosphates

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Functions of polyphosphate and polyphosphate-dependent enzymes
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Vacuoles also contain an important phosphorus reserve in yeast and fungi (Indge,
1968a,b,c; Urech et al., 1978; Cramer and Daves, 1984). Under phosphate overplus, the
content of PolyP in vacuoles of Saccharomyces carlsbergensis grew dramatically (Lichko
et al., 1982). Some mutants of S. cerevisiae having no vacuoles possess low levels of PolyP
and are unable to grow on a medium without P i (Shirahama et al., 1996).
Thecellsofprotozoa(DocampoandMoreno,2001)andalgaChlamydomonasreinhardtii
(Ruizetal.,2001b)possessspecificPolyPandCa 2+ storageorganelles,i.e.acidocalcisomes,
which are similar to vacuoles in some properties, especially in the presence of proton-
pumping pyrophosphatase. These organelles act as phosphate storage systems for the above
lower eukaryotes.
In eukaryotes, the function of PolyP as a phosphate reserve is probably related to the
action of different forms of exopolyphosphatases and endopolyphosphatase.
It is possible, however, that in some cases utilization of PolyP does not involve hydrolysis
to P i , but rather phosphate transfer without loss of the energies of the phosphoric anhydride
bonds to other compounds. It seems unlikely that the energy stored in PolyPs would be
dissipated without being utilized for energy-requiring processes.

7.2 Energy Source

The phosphoanhydride bonds of PolyPs have free energies of hydrolysis similar to that
of ATP and occupy an intermediate position in the free energy scale of phosphorylated
compounds. Thermodynamically, the standard free energy of hydrolysis of the anhydride
linkage yields about 38 kJ per phosphate bond at pH 5. It can therefore act as both a
donor and an acceptor of phosphate groups. Belozersky was the first to suggest that PolyP
in very primitive organisms could perform the functions of energy-rich compounds as an
evolutionary percursor of ATP (Belozersky, 1958).

7.2.1 Polyphosphates in Bioenergetics of Prokaryotes
In many prokaryotes, PolyP is a direct phosphorus donor for biochemical reactions due to
the action of enzymes such as polyphosphate–glucose phosphotransferase and NAD kinase.
Polyphosphate kinases and PolyP:AMP phosphotransferase link nucleoside–polyphosphate
and inorganic PolyP. Polyphosphate kinases 1 and 2 can use PolyPs for the synthesis of
different nucleoside triphosphates.
A specific way of using PolyP as an energy source was found in the PolyP-accumulating
bacterium Acinetobacter johnsonii. When high-P i -grown cells of strictly aerobic A. john-
sonii 210A are incubated anaerobically, their PolyP is degraded and P i is excreted (Van
Veen et al., 1994; Kortstee et al., 2000). The energy of PolyP is mobilized by two systems.
The polyphosphate:AMP phosphotransferase/adenylate kinase system is responsible for
the direct formation of ATP from PolyP, while a constitutive, bidirectional, low-affinity P i
transport system mediates the uptake and efflux of MeHPO 4 . The uptake is driven by the
proton motive force, while the electrogenic excretion of MeHPO 4 in conjunction with a
proton generates this force (Van Veen et al., 1994). Exopolyphosphatase may enhance the