Page 103 - The Biochemistry of Inorganic Polyphosphates

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Enzymes of polyphosphate degradation 87
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than Mg , with an optimum concentration of about 2.5 mM. The enzyme hydrolyses
PolyPs to shorter chain lengths and even to tripolyphosphate. These authors suggest the en-
dopolyphosphatase to be localized in vacuoles (Kumble and Kornberg, 1996). The activity
has been partially purified from rat and bovine brain, where its abundance is about 10 times
higher than in other tissues but much less than in yeast (Kumble and Kornberg, 1996). En-
dopolyphosphatase has escaped detection in procaryotes (Kumble and Kornberg, 1996). The
presence of this enzyme in eucaryotes is supposed to be associated with the redistribution
of PolyP pools in different compartments and PolyP transport between the compartments.

6.2.7 PolyP:AMP Phosphotransferase

The most well-known databases (http://www.expasy.org, and http://www.chem.qmul.ac.uk/
iubmb/enzyme) have no mention of this enzyme.
The reaction was found first in Corynebacterium xerosis (Dirheimer and Ebel, 1965):
PolyP + AMP PolyP + ADP (6.17)
n n − 1
PolyP:AMP phosphotransferase was partly purified from A. jonsonii strain 210A
(Bonting et al., 1991). This had a molecular mass of 55 kDa. The kinetic studies showed
apparent K m values of 0.26 mM for AMP and 0.8 µM for PolyP 35 . The highest activity was
found with PolyPs of 18 to 44 phosphate residues. The PolyPs were degraded completely by
a processive mechanism. No activity was revealed with pyrophosphate, PolyP 3 and PolyP 4.
Some authors believed that this enzyme, in concert with adenylate kinase, is responsible
for utilization of the greater part of PolyP in A. johnsonii (Kortstee and van Aeen, 1999;
Kortstee et al., 2000):

PolyP + AMP PolyP + ADP (6.18)
n n − 1
2ADP ATP + AMP (6.19)
The resulting reaction is as follows:

PolyP + ADP PolyP n − 1 + ATP (6.20)
n
This pathway retains the phosphoanhydride-bound energy of PolyP. The enzymes from
A. johnsonii were used to create an ATP-regenerating system (Resnick and Zehnder, 2000).
The significance of PolyP:AMP phosphotransferase in different bacteria is still in ques-
tion. Therefore, in E. coli the ADP formation from PolyP by chain shortening was explained
by a joint action of polyphosphate kinase and adenylate kinase, which formed a complex
with the PolyP (Ishige and Noguchi, 2000):

PolyP + ADP PolyP + ATP (6.21)
n n − 1
AMP + ATP ADP (6.22)

This conclusion was grounded on the observation that over-expression of polyphosphate
kinase in E. coli caused a sharp increase of the PolyP:AMP phosphotransferase activity
(Ishige and Noguchi, 2000). Moreover, in vitro PolyP:AMP phosphotransferase activity