Page 210 - The Biochemistry of Inorganic Polyphosphates

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Polyphosphates in chemical and biological evolution
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10.1 Abiogenic Synthesis of Polyphosphates
and Pyrophosphate
Phosphorus, being one of constituent elements of living cells, must without doubt have
played an important role at the earliest stages of emergence and evolution of life. It was
considered that even when the Earth possessed a reducing atmosphere, phosphorus was
present as phosphate rather than in a reduced form such as phosphite (Miller and Parris,
1964). However, model experiments showed that electrical discharges in water-saturated
N 2 containing 1–10 % CH 4 reduce phosphate to phosphite. This mechanism was suggested
as a possible source of water-soluble phosphorus-containing compounds in volcanic en-
vironments on the prebiotic Earth. By introducing small amounts of H 2 and CO into gas
mixtures, in which CO 2 and N 2 are the main components, surprisingly high conversions
to phosphite were obtained and several percent reduction of apatite occurred (De Graaf
and Schwartz, 2000). Phosphites are known to be highly unstable compounds (Miller and
Parris, 1964; Schwartz, 1971), but their occurrence from insoluble apatite might provide a
possibility for engaging the insoluble forms of phosphate in different chemical, and later
biochemical, processes. It appears that, even at the very earliest stages of life on Earth,
phosphorus was taken up by primitive living organisms from the environment in the form
of phosphate or its derivatives.
Condensed inorganic phosphates could arise on the primitive Earth through a wide vari-
ety of abiogenic processes. They could be formed by condensation of inorganic phosphates
at high temperatures (Schramm et al., 1962, 1967), in the reaction between calcium phos-
phate and cyanide (Miller and Parris, 1964), and under the action of heat on mixtures of
ammonium phosphate and urea (Ostenberg and Orgel, 1972). They could therefore easily
be present at the time when life first appeared on the Earth. It was shown both in the ex-
periments that simulate magmatic conditions and in the analysis of volatile condensates
in volcanic gas, that volcanic activity can produce water-soluble PolyPs (Yamagata et al.,
1991). Some authors, however, doubt that PolyP synthesis by heating phosphate minerals
under geological conditions on the primitive Earth may be an effective process, but they
do not exclude an undiscovered robust prebiotic synthesis of PolyP or mechanisms for
concentrating it (Keefe and Miller, 1996). PolyP production as a result of heating the min-
eral apatite in the presence of other minerals has been reported (De Graaf and Schwartz,
2000).
On the other hand, pyrophosphate (the lowest member of this homologous series of
compounds) could also be formed on the primitive Earth. It could arise from orthophosphate,
either by inorganic redox reactions or following preliminary activation of the phosphate by
cyanogen, cyanate or dicyandiamide. Miller and Parris (1964), Degani and Halmann (1971)
and Steinman et al. (1964) have demonstrated this in model experiments. These activating
agents could apparently have existed on the primeval Earth. Furthermore, from the work of
Orgel and co-workers (Beck and Orgel, 1965; Lohmann and Orgel, 1968), the Miller–Parris
reaction (i.e. the conversion of hydroxyapatite into calcium pyrophosphate in the presence
of cyanates) can take place under aqueous conditions.
The pyrophosphate formed in this or other ways could be, according to Lipmann (Lip-
mann, 1965, 1971), ‘the simplest compound present on the primeval Earth to be involved
in the accumulation and transfer of energy-rich bonds’. Pyrophosphate is an energy-rich