Page 62 - The Biochemistry of Inorganic Polyphosphates

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Forms of polyphosphates cells
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2+
PolyPs can form complexes with arginine, spermidine, lysine, Mg ,Ca 2+ and Mn 2+ in
vacuoles of yeast (Wiemken and D¨urr, 1974; D¨urr et al., 1979; Okorokov et al., 1980; Lichko
et al., 1982; Westenberg et al., 1989; D¨unn et al., 1994) and Neurospora crassa (Cramer
and Davis, 1984). The cells of protozoa (Docampo and Moreno, 2001; Rodrigues et al.,
2002a,b) and some algae (Ruiz et al., 2001b) possess an acidic organelle, acidocalcisome,
2+
2+
which contains pyrophosphate and PolyPs bound with Ca ,Mg ,Zn 2+ and other cations.
It should be noted that in these organelles, low-molecular-weight PolyPs, including PolyP 3 ,
are present in the bound state.

2+
4.2 Polyphosphate–Ca –Polyhydroxybutyrate
Complexes

Specific complexes containing polyhydroxybutyrate (PHB) and PolyPs have been found in
membranes of many organisms (Reusch and Sadoff, 1988; Reusch, 1992, 1999a, 2000).
When such components of E. coli membranes were isolated and analysed, Ca 2+ was found
to be the predominant neutralizing cation (Reusch and Sadoff, 1988). The polymer length
was 130–150 residues for PHB (∼ 12 kD), as measured by non-aqueous size-exclusion
chromatography (Seebach et al., 1994a), and 55–70 residues for PolyP (∼ 5 kD), as deter-
mined by acrylamide gel electrophoresis (Castuma et al., 1995). The molecular weight of
the complex was estimated as 17 (± 4) kD by non-aqueous size-exclusion chromatography
(Reusch et al., 1995). These measurements indicate a 1:1 ratio between the two polymer
strands and a 2:1 ratio of monomer residues for PHB:PolyP.
The detailed structure of PolyP–PHB complexes is still unknown. However, some as-
sumptions were made on the basis of physical properties and sizes of the polymers, and
the low dielectric environment they inhabit. It is clear that the highly polar polyanionic
PolyPs must be shielded from the hydrophobic region of the bilayer by amphiphilic PHB.
The models for membrane channel complexes were proposed by Reusch and co-workers
(Reusch and Sadoff, 1988; Reusch et al., 1995) and by Seebach and co-workers (Seebach
et al., 1994b, 1996). The first model proposes that PHB has a coiled conformation such as
it displays in solution (Figure 4.1), while the second one assumes that PHB maintains the
folded helix form of its solid-state structure (Figure 4.2). A consequence of both arrange-
ments is the formation of multiple parallel ‘lanes’ between the two polymers, with multiple
cation-binding sites lining each of these lanes.
In the ‘Reusch model’, the complexes have the liquid properties of polymer electrolytes
and this suggests a family of conformations rather than a single defined structure. In the
‘Seebach model’, several PHB molecules surround the PolyP unit. Individual PHB chains
are free to adopt various positions in the phospholipid lattice; hence, a well-defined structure
is again unlikely. Further studies may help us in choosing one of these two proposed models.

4.3 Complexes of Polyphosphates
with Nucleic Acids

PolyP–ribonucleic acid complexes have been isolated from a variety of organisms
(Belozersky, 1955, 1958, 1959a; Chayen et al., 1955; Chaloupka and Babicky, 1957, 1958;