Page 113 - The Biochemistry of Inorganic Polyphosphates

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Cations sequestration and storage 97
in these organelles under glucose repression. Therefore, their role as an alternative energy
reserve seems to be similar to that of pyrophosphate (Mansurova et al., 1973a,b; Mansurova,
1989).
It should be noted that the relation of PolyP and transmembrane gradients in yeast has
been confirmed more conclusively than that of PolyP and ATP pools.

7.3 Cations Sequestration and Storage

7.3.1 In Prokaryotes

Complexes of PolyP with common cations (Mg ,Ca 2+ and K ) have been found in many
+
2+
prokaryotes. One more important function of the PolyP is involvement in the detoxication
of heavy metal cations. PolyP sequesters Ni 2+ in Staphylococcus aureus (Gonzales and
Jensen, 1998). The cells of Anacystis nidulans with high intracellular PolyP levels showed
a greater tolerance to Cd 2+ than those with small PolyP reserves (Keyhani et al., 1996). The
Cd 2+ tolerance of E. coli also depends on PolyP metabolism (Keasling and Hupf, 1996). The
PolyP produced in a recombinant E. coli strain with mer operon encoding mercury transport
systemswascapableofchalatingandreducingthecytotoxityofHg 2+ (Pan-Houetal.,2002).
However, degradation of PolyP was observed during growth in the presence of heavy metals
(Keyhani et al., 1996; Keasling and Hupf, 1996; Keasling, 1997; Keasling et al., 2000).
The PolyP metabolic pathways in E. coli were genetically manipulated to test the effect of
PolyP on tolerance to cadmium (Keasling and Hupf, 1996; Keasling et al., 2000). A strain
mutant in the genes for polyphosphate kinase (ppk1) and polyphosphatase (ppx) produced
no PolyP, whereas the same strain carrying multiple copies of ppk on a high-copy plasmid
produced significant amounts of PolyP. The cell-doubling time of both strains increased
with increasing Cd 2+ concentration. In contrast, the mutant strain carrying multiple copies
of ppk and ppx produced one tenth of the PolyP found in the strain carrying multiple copies
of ppk only and showed no significant increase in cell-doubling time over the same Cd 2+
concentration range. Therefore, not only the large amount of intracellular PolyP but also the
ability to synthesize and degrade PolyP is important for tolerance to heavy metals (Keasling
and Hupf, 1996; Keasling et al., 2000). The following mechanism of PolyP participation
in the detoxication of heavy metals has been proposed. PolyP sequesters heavy metals, on
the one hand, and the entry of metal cations into the cells stimulates exopolyphosphatase
−
activity, which releases P i from PolyP, on the other hand. The MeHPO 4 ions are then
transported out of the cells (Keasling, 1997; Keasling et al., 2000).

7.3.2 In Eukaryotes

In the lower eukaryotes, cation sequestration and storage are observed in vacuoles. Vacuoles
of yeast accumulate amino acids (Wiemken and D¨urr, 1974), K, Mg 2+ and Mn 2+ (Okorokov
et al., 1980; Lichko et al., 1982) (Table 7.1), and Ca 2+ (Ohsumi and Anraku, 1983; D¨unn
et al., 1994). PolyP, which is able to confine different cations in an osmotic inert form,
was also found in these storage organelles (Indge, 1968a,b,c; Westenberg et al., 1989).