Page 122 - The Biochemistry of Inorganic Polyphosphates

WU095/Kulaev
WU095-07
Functions of polyphosphate and polyphosphate-dependent enzymes
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cytoplasmic side of the endoplasmic reticulum interacts with the phosphate residue of Dol–
P. The Dol–P–P–mannose is transported across the membrane so that the phosphomannose
residue enters the lumen, where mannosyl transferase and Dol–P–P:PolyP phosphotrans-
ferase reactions occur. As a result, Dol–P is formed, which again crosses the membrane
and could interact on its cytoplasmic side with a new molecule of GDP–mannose. The
mannoproteins and PolyP are transported to the cell envelope by special vesicles.
One of the specific processes of cell–cell interactions in the lower eukaryotes is the
symbiosis between fungi and plants. It was observed that mycorrhiza possesses a lot of
phosphorus and PolyPs. For example, microsclerotia of the root-inhabiting fungus Phialo-
cephala fortinii at an early stage of interaction with the roots of Asparagus officinalis was
shown to accumulate PolyPs (Yu et al., 2001). PolyPs were found in vacuoles of fungal
cells in Eucalyptus pilularis/Pisolithus tinctorius ectomycorrhizas (Ashford et al., 1999).
The mycorrhization of corn plants by the fungi Glumus mosseae and Glumus fasciculatum
was shown to stimulate phosphorus uptake and accumulation (Shnyreva and Kulaev, 1994).
It cannot be excluded that PolyPs, located on cell surfaces of the lower eukaryotes, may
play a certain role in cell–cell interactions and especially in the interactions of fungi and
plant cells during mycorrhiza development.

7.6 Regulation of Enzyme Activities

Being a polyanion, PolyP can interact with many proteins and enzymes, especially those
rich in cationic amino acid residues. For example, in the presence of PolyP, cytochrome C
forms stable protein aggregates as a result of binding of the polymer at a single site close
to lysines 13, 86 and 87 on the protein surface (Concar et al., 1991).
It should be noted that the effect of PolyP on enzymatic activities might involve different
mechanisms. First, there is a competition with the substrate for the binding site. It is probable
that inhibition by PolyP of polygalacturonase activity, which is important for pathogenicity
of the fungus Botrytis cinerea (Mellerharel et al., 1997) and restriction endonucleases of
the fungus Colleotrichum (Rodriguez, 1993), is realized in such a way.
Secondly, there is an interaction of PolyP with polycationic activators. As for yeast
trehalase, the inhibitory effect is probably due to the interactions with polyamines, which are
activators of the enzyme (App and Holzer, 1985). PolyP inhibited trehalase from vegetative
yeast cells and, to a lesser extent, that from the spores (Wolska-Mitaszko, 1997).
As for deaminase, the kinetic analysis suggests a partial mixed-type inhibition mecha-
nism. Both the K i value of the inhibitor and the breakdown rate of the enzyme–substrate–
inhibitor complex are dependent on the chain length of the PolyP, thus suggesting that the
breakdown rate of the enzyme–substrate–inhibitor complex is regulated by the binding of
Polyphosphate to a specific inhibitory site (Yoshino and Murakami, 1988). More compli-
cated interactions were observed between PolyP and two oxidases, i.e. spermidine oxidase
of soybeen seedling and bovine serum amine oxidase. PolyP competitively inhibits the ac-
tivities of both enzymes, but may serve as an regulator because the amino oxydases are also
active with the polyamine–PolyP complexes (Di Paolo et al., 1995).
The complexing of cations important for enzyme activities may be the third way of
PolyP action on enzyme activity. An example of such action is the mechanism leading to
growth inhibition, morphological changes and lysis of Bacillus cereus when challenged