Page 102 - The Biochemistry of Inorganic Polyphosphates

WU095/Kulaev
WU095-06
Enzymes of polyphosphate biosynthesis and degradation
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Many enzymes, however, were demonstrated to catalyse the reaction:
(6.15)
PolyP + H 2 O −−→ Pyrophosphate + P i
3
First, there are inorganic pyrophosphatases (EC 3.6.1.1) which can hydrolyse tri- and
tetraphosphate (Baykov et al., 1999). The ability of some pyrophosphatases to split these
substrates depends on pH and divalent cations (Baykov et al., 1999). The most effective
hydrolysis of low-molecular-weight PolyPs was observed for inorganic pyrophosphatase
isolated from the archaeon Metanotrix soehgenii. This hydrolyses PolyP 3 and PolyP 4 for
44 and 8 %, respectively, of the PP i hydrolysis rate (Jetten et al., 1992) and may therefore
be involved in their metabolism.
Secondly, adenosylmethionine synthetase, in addition to synthetase reaction, catalyses
tripolyphosphatase reactions stimulated by adenosylmethionine. Both of the enzymatic
activities of the enzyme, which has been purified to homogeneity from E. coli, require a
divalent metal ion and are markedly stimulated by certain monovalent cations (Markham
et al., 1980). Tripolyphosphatase activity is also associated with S-adenosylmethionine
synthetase isozymes from rat liver (Shimizu et al., 1986).
Thirdly, some RNA triphosphatases possess a weak tripolyphosphatase activity, and
PolyP 3 is a potential competitive inhibitor (Yu et al., 1997; Gong and Shuman, 2002).
Specific tripolyphosphatase was purified from Thermobacterium thermoautotrophicum
(Van Alebeek et al., 1994). The enzyme of 22 kDa hydrolyses tripolyphosphates five times
more actively than PolyP 15 .
Tripolyphosphatase was purified from Neurospora crassa. This had a molecular mass of
50 kDa, and its activity strongly depended on divalent metal cations (Kulaev et al., 1972a,c;
Umnov et al., 1974; Egorov and Kulaev, 1976).
In S. cerevisiae, tripolyphosphatase activity is an inherent property of exopolyphos-
phatases of the cell envelope, cytosol and mitochondrial matrix (Andreeva and Oko-
rokov, 1993; Andreeva et al., 1996; Lichko et al., 2000), which are encoded by the PPX1
gene. In Leishmania major, exopolyphosphatase is also responsible for PolyP 3 degradation
(Rodrigues et al., 2002a).
Theputativeabilityofsometripolyphosphatasesfrommicroorganismstosplitlong-chain
PolyPs under suitable conditions needs further investigation.

6.2.6 Endopolyphosphatase (Polyphosphate Depolymerase,
EC 3.6.1.10)

Endopolyphosphatase splits long-chain PolyP molecules into shorter ones. The product
contains four to five phosphate residues.
PolyP + H 2 O −−→ Oligopolyphosphates (6.16)
n
Malmgren (1952) and Mattenheimer (1956) were the first to observe this reaction in yeast
and fungi. Endopolyphosphatase activity was investigated in the fungi Aspergillus niger
(Malmgren, 1952) and Neurospora crassa (Kritsky et al., 1972; Kulaev et al., 1972a–c).
This enzyme was purified from the yeast (Kumble and Kornberg, 1996). It is a dimer
of 35 kDa sub-units, and its activity requires divalent metal cations. Mn 2+ is more active