Page 221 - The Biochemistry of Inorganic Polyphosphates

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Changes in the role of polyphosphates 205

Figure 10.5 Hypothetical scheme for the formation and reactions of high-molecular-weight PolyPs,
pyrophosphate and ATP at different stages of appearance of life on Earth (Kulaev and Skryabin, 1974).

on the Earth, was their main, although by no means, their only function. In general, it
is assumed that the polyfunctionality of compounds was an important criterion in their
selection as components of living cells.
It is possible, for instance, that in protobionts PolyP was not merely a coupling com-
pound, but also provided a comparatively long-term depot for phosphorus and energy, which
enabled organisms to become independent of their environment to some extent. PolyP was
able to detoxify free orthophosphoric acid and its salts, which could accumulate in large
amounts in cells. Being an excellent ion exchanger, PolyP could also be a regulator of cation
metabolism in the earliest organisms, since it could regulate the functions of many enzymes
through binding one cation and liberating another. At some stage of evolution, however,
the high-molecular-weight PolyPs ceased to fully satisfy the requirements of an organism.
More specific compounds were needed, which would have had structures capable of an even
greater variety of functions, as well as more precise and specific interactions with other cell
components.
In general, it is very likely that the ability to interact with other cell components has also
been an important factor in the evolution of metabolism of living organisms. Thus, PolyP,
which possesses a monotonic macromolecular, essentially linear, structure without any
special features, could have become a somewhat unsatisfactory compound at a certain stage
of cell development. The limited capacity of PolyP for precise and very specific interactions
with other cellular metabolites resulted in an inconsistency with its function of coupling exo-
and endoenergetic processes. Hence, ATP was selected for the above functions, because it
has a much more specific, and therefore more readily recognized, structure. Moreover, ATP
was capable of many other functions, which could not be performed by PolyP.
Numerous investigations (Ponnamperuma et al., 1963; Rabinowitz et al., 1968; West and
Ponnamperuma, 1970; Oro, 1983) have demonstrated a relative easiness of formation of