http://en.wikipedia.org/wiki/Polyphosphate
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Triphosphates are
salts or
esters of polymeric
oxyanions formed from
tetrahedral PO
4 (
phosphate)
structural units linked together by sharing oxygen atoms. When two
corners are shared the polyphosphate may have a linear chain structure
or a cyclic ring structure. In biology the polyphosphate esters
AMP,
ADP and
ATP
are involved in energy transfer. A variety of polyphosphates find
application in mineral sequestration in municipal waters, generally
being present at 1 to 5 pm.
[1] GTP,
CTP, and
UTP are also nucleotides important in the protein synthesis, lipid synthesis and carbohydrate metabolism, respectively.
Structure & Formation
The structure of tripolyphosphoric acid illustrates the principles
which define the structures of polyphosphates. It consists of three
tetrahedral PO
4 units linked together by sharing oxygen
atoms. Structurally, the outer tetrahedra share one vertex with the
central tetrahedron; the central tetrahedron shares two corners with the
other tetrahedra. The corresponding phosphates are related to the acids
by loss of the
acidic protons. In the case of the cyclic trimer each tetrahedron shares two vertices with adjacent tetrahedra.
Sharing of three corners is possible as in the sheet-structure
Phyllosilicates, but such structures occur only under extreme conditions. Three-corner sharing also occurs in
phosphorus pentoxide, P
4O
10, which has a 3-dimensional structure.
Chemically, the polymerization reaction can be seen as a condensation
reaction. The process begins with two phosphate units coming together.
- 2 PO43− + 2 H+
P2O74− + H2O
It is shown as an
equilibrium reaction because it can go in the reverse direction, when it is known as an
hydrolysis reaction because a water molecule is split (
Lysed). The process may continue in steps; at each step another PO
3 unit is added to the chain, as indicated by the part in brackets in the illustration of polyphosphoric acid. P
4O
10
can be seen as the end product of condensation reactions, where each
tetrahedron shares three corners with the others. Conversely, a complex
mix of polymers is produced when a small amount of water is added to
phosphorus pentoxide.
Acid-base and complexation properties
Polyphosphates are
weak bases. A lone pair of electrons on an oxygen atom can be donated to a
hydrogen ion (proton) or a metal ion in a typical
Lewis acid-
Lewis base
interaction. This has profound significance in biology. For instance,
adenosine triphosphate (ATP) is about 25% protonated in aqueous solution at
pH 7.
[2]
- ATP4- + H+ ATPH3-, pKa
6.6
Further protonation occurs at lower pH values.
ATP forms
chelate complexes with metal ions. The
stability constant for the equilibrium
- ATP4- + Mg2+ MgATP2-, log β 4
is particularly large.
[3]
The formation of the magnesium complex is a critical element in the
process of ATP hydrolysis, as it weakens the link between the terminal
phosphate group and the rest of the molecule.
[2][4]
The "high energy" phosphate bond
The energy released in ATP hydrolysis,
- ATP4- + H2O → ADP3- + Pi-
at ΔG
-36.8 kJ mol
−1 is large by biological standards. P
i
stands for inorganic phosphate, which is protonated at biological pH.
However, it is not large by inorganic standards. The term "high energy"
refers to the fact that it is high relative to the amount of energy
released in the
organic chemical reactions that can occur in living systems.
High-polymeric inorganic polyphosphates
High-polymeric inorganic polyphosphates were found in living
organisms by L. Liberman in 1890. These compounds are
linear polymers
containing a few to several hundred residues of
orthophosphate linked by energy-rich
phosphoanhydride bonds.
Previously, it was considered either as “molecular fossil” or as only
a phosphorus and energy source providing the survival of microorganisms
under extreme conditions. These compounds are now known to also have
regulatory roles, and to occur in representatives of all kingdoms of
living organisms, participating in metabolic correction and control on
both genetic and enzymatic levels. Polyphosphate is directly involved in
the
switching-over of the genetic program characteristic of the
exponential growth stage of bacteria to the program of cell survival
under stationary conditions, “a life in the slow line”. They participate
in many regulatory mechanisms occurring in bacteria:
- They participate in the induction of rpoS, an RNA-polymerase subunit
which is responsible for the expression of a large group of genes
involved in adjustments to the stationary growth phase and many
stressful agents.
- They are important for cell motility, biofilms formation and virulence.
- Polyphosphates and exopolyphosphatases
participate in the regulation of the levels of the stringent response
factor, guanosine 5'-diphosphate 3'-diphosphate (ppGpp), a second
messenger in bacterial cells.
- Polyphosphates participate in the formation of channels across the
living cell membranes. The above channels formed by polyphosphate and
poly-b-hydroxybutyrate with Ca2+ are involved in the transport processes in a variety of organisms.
- An important function of polyphosphate in microorganisms—prokaryotes
and the lower eukaryotes—is to handle changing environmental conditions
by providing phosphate and energy reserves. Polyphosphates are present
in animal cells, and there are many data on its participation in the
regulatory processes during development and cellular proliferation and
differentiation—especially in bone tissues and brain.
In humans polyphosphates are shown to play a key role
in blood coagulation. Produced and released by platelets[5]they activate
Factor XII
which is essential for blood clot formation. Furthermore
platelets-derived polyphosphates activate blood coagulation factor XII
(Hageman factor) that initiates fibrin formation and the generation of a
proinflammatory mediator, bradykinin that contributes to
leakage from the blood vessels and thrombosis.
[6][7]
See also
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