Page 223 - The Biochemistry of Inorganic Polyphosphates

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Changes in the role of polyphosphates 207
only via ATP. In this respect, the phosphagens differ essentially from inorganic PolyPs,
which can also be utilized and synthesized, as we have seen, by other routes.
It is therefore reasonable to suppose that the phosphagens, as distinct from inorganic
PolyPs, arose in the course of evolution subsequent to ATP. The function of detoxification
of orthophosphate has in most cases, however, evidently ceased to be necessary as a result of
evolutionary development in the higher organisms of a very delicately balanced metabolism
andofmechanismsforitsprecisecontrol.Inmanycases,theneedofeliminationofP i surplus
in a form, which is non-toxic to the cell, has again appeared in certain tissues of the higher
organisms.
Such a case is apparently the accumulation in the seeds of some plants of significant
amounts of phytin, calcium and magnesium salts of inositol hexaphosphoric acid. A number
of investigators, in particular, Sobolev (1962), have in fact observed that phytin is formed
in large amounts during the ripening of the seeds of higher plants, in parallel with the accu-
mulation of reserve substances such as starch and fats. The accumulation of phytin under
these conditions, in the light of our hypothesis, can be regarded as a means of detoxifying
orthophosphate, which is liberated during the synthesis of starch and fats and cannot be
removed from the ripening seeds by any other means. This method of P i detoxification, i.e.
deposition as phytin, may be regarded as much more advanced from the evolutionary point of
view.ThisfollowsfromtheworkcarriedoutbyKulaevandco-workers(Kulaevetal.,1964c)
on themetabolismof phosphorus during germination of cotton seeds. It wasshown,in partic-
ular, that one of the products of phytin cleavage during germination was 3-phosphoglycerie
acid. From the evolutionary point of view, this is obviously a greatly improved process, as a
result of which large amounts of 3-phosphoglyceric acid accumulate during seed germina-
tion. This compound is known to occupy the central position in the Calvin photosynthetic cy-
cle and other energetic and plastic processes in the higher plants. However, high-molecular-
weight PolyPs are also present at the sites where large amounts of phytin accumulate in the
higher plants (e.g. the seeds) (Asamov and Valikhanov, 1972), which is highly significant.
For example, high-molecular-weight polyphosphates occur in the brains of mammals, i.e.
at a site where usually a typical phosphagen, creatine phosphate, is found as well.
In the course of evolution from prokaryotes to eukaryotes, the energetic role of
PolyP decreased. However, other functions came to the fore, such as phosphate storage,
cation chelation, regulation of enzyme activities, gene expression and membrane transport
(Figure 10.6). The significance of the regulatory functions of PolyP increased in eukaryotes.

RNA world Prokaryotes Lower eukaryotes Higher eukaryotes
Gene activity control Gene activity control Gene activity control
RNA–PolyP complexes
(regulation of ribozyme activities)
Energy reserve Energy reserve Membrane transport
PolyPs
Membrane transport Membrane transport
Condensing factor Phosphate reserve Cationes chelation
Compounds with macroenergetic bonds Phosphate reserve
Cationes chelation Cationes chelation Enzyme activities
regulation
Enzyme activities Enzyme activities
regulation regulation
Figure 10.6 Changes in the functions of PolyPs during evolution.