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J Biol Chem, Vol. 275, Issue 6, 3977-3983, February 11, 2000
From the Department of Biochemistry, Stanford University,
Stanford, California 94305-5307
Polyphosphate kinase (PPK), the principal enzyme
required for the synthesis of inorganic polyphosphate (polyP) from ATP,
also exhibits other enzymatic activities, which differ significantly in
their biochemical optima and responses to chemical agents. These
several activities include: polyP synthesis (forward reaction), nATP
polyPn + nADP (Equation 1); ATP synthesis from
polyP (reverse reaction), ADP + polyPn
ATP + polyPn
1 (Equation 2); general nucleoside-diphosphate
kinase, GDP + polyPn
GTP + polyPn
1
(Equation 3); linear guanosine 5'-tetraphosphate (ppppG) synthesis, GDP + polyPn
ppppG + polyPn
2 (Equation 4); and autophosphorylation, PPK + ATP
PPK-P + ADP (Equation 5).
The Mg2+ optima are 5, 2, 1, and 0.2 mM,
respectively, for the activities in Equations 1, 2, 3, and 4. Inorganic
pyrophosphate inhibits the activities in Equations 1 and 3 but
stimulates that in Equation 4. The kinetics of the activities in
Equations 1, 2, and 3 are highly processive, whereas the transfer of a
pyrophosphoryl group from polyP to GDP (Equation 4) is distributive and
demonstrates a rapid equilibrium, random Bi-Bi catalytic mechanism.
Radiation target analysis revealed that the principal functional unit
of the homotetrameric PPK is a dimer. Exceptions are a trimer for the
synthesis of ppppG (Equation 4) and a tetrameric state for the
autophosphorylation of PPK (Equation 5) at low ATP concentrations. Thus, the diverse functions of this enzyme involve different subunit organizations and conformations. The highly conserved homology of PPK
among 18 microorganisms was used to determine important residues and
conserved regions by alanine substitution, by site-directed mutagenesis, and by deletion mutagenesis. Of 46 single-site mutants, seven exhibit none of the five enzymatic activities; in one mutant, ATP
synthesis from polyP is reduced relative to GTP synthesis. Among
deletion mutants, some lost all five PPK activities, but others
retained partial activity for some reactions but not for others.
To whom correspondence should be addressed. Tel.: 650-723-6167;
Fax: 650-723-6783; E-mail: akornber@cmgm.stanford.edu.
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