Polyphosphate
A polyphosphate is a
Structure
-
Structure of triphosphoric acid
-
Polyphosphoric acid
-
Cyclic trimetaphosphate
-
Adenosine diphosphate (ADP)
The structure of tripolyphosphoric acid illustrates the principles which define the structures of polyphosphates. It consists of three tetrahedral PO4 units linked together by sharing oxygen centres. For the linear chains, the end phosphorus groups share one oxide and the others phosphorus centres share two oxide centres. The corresponding phosphates are related to the acids by loss of the
Sharing of three corners is possible. This motif represents
Formation and synthesis
Polyphosphates arise by polymerization of phosphoric acid derivatives. The process begins with two phosphate units coming together in a condensation reaction.
- 2 H(PO4)2− ⇌ (P2O7)4− + H2O
The condensation is shown as an equilibrium because the reverse reaction, hydrolysis, is also possible. The process may continue in steps; at each step another (PO3)− unit is added to the chain, as indicated by the part in brackets in the illustration of polyphosphoric acid. P4O10 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
- ATP4− + H+ ⇌ ATPH3−, pKa 6.6
Further protonation occurs at lower pH values.
The "high energy" phosphate bond
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 energy released in ATP hydrolysis,
- ATP4− + H2O → ADP3− + Pi−
at ΔG -36.8 kJ mol−1 is large by biological standards. Pi 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 molecular weight polyphosphates are well known.
In nature
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
Previously, it was considered either as “
- 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.[clarification needed]
- 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[7] they activate blood coagulation factor XII which is essential for blood clot formation. Factor XII, also called Hageman factor, initiates fibrin formation and the generation of a proinflammatory mediator, bradykinin, that contributes to leakage from the blood vessels and thrombosis.[8][9] Bacterial-derived polyphosphates impair the host immune response during infection and targeting polyphosphates with recombinant exopolyphosphatase improves sepsis survival in mice.[10] Inorganic polyphosphates play a crucial role in tolerance of yeast cells to toxic heavy metal cations.[11]
Use as food additives
Sodium polyphosphate (E452(i)), potassium polyphosphate (E452(ii)), sodium calcium polyphosphate (E452(iii)) and calcium polyphosphate (E452(iv)) are used as food additives (emulsifiers, humectants, sequestrants, stabilisers, and thickeners).[12] They are not known to pose any potential health risk other than those generally attributed to other phosphate sources (including those naturally occurring in food). While concerns have been raised regarding detrimental effects on the bones and cardiovascular diseases, as well as hyperphosphatemia, these seem to be relevant only for exaggerated consumption of phosphate sources. In all, reasonable consumption (up to 40 mg phosphate per kg of body weight per day) seem to pose no health risk.[13][14]
See also
- Phosphoric acids
- Sodium trimetaphosphate
- Sodium hexametaphosphate
References
- S2CID 238989161.
- ^ PMID 11772.
- PMID 1645933.
- PMID 3537318.
- ISBN 978-0-08-037941-8.
- PMID 15308650.
- PMID 20005807.
- ^ "Newly discovered mechanism by which blood clots form". physorg.com. December 10, 2009. Retrieved 13 December 2009.
- PMID 32788578.
- PMID 23663411.
- ^ "E452 Polyphosphates". openfoodfacts.org. Retrieved 2022-03-18.
- ^ EFSA Panel on Food Additives and Flavourings (FAF), Younes, M., Aquilina, G., Castle, L., Engel, K. H., Fowler, P., ... & Mennes, W. (2019). Re‐evaluation of phosphoric acid–phosphates–di‐, tri‐and polyphosphates (E 338–341, E 343, E 450–452) as food additives and the safety of proposed extension of use. EFSA Journal, 17(6), e05674.
- ^ Ritz, E., Hahn, K., Ketteler, M., Kuhlmann, M. K., & Mann, J. (2012). Phosphate additives in food—a health risk. Deutsches Ärzteblatt International, 109(4), 49.
External links
- Pavlov E, Grimbly C, Diao CT, French RJ (September 2005). "A high-conductance mode of a poly-3-hydroxybutyrate/calcium/polyphosphate channel isolated from competent Escherichia coli cells". FEBS Lett. 579 (23): 5187–92. S2CID 35616647.
- Kulaev I, Vagabov V, Kulakovskaya T (1999). "New aspects of inorganic polyphosphate metabolism and function". J. Biosci. Bioeng. 88 (2): 111–29. PMID 16232585.
- Kulaev I, Kulakovskaya T (2000). "Polyphosphate and phosphate pump". Annu. Rev. Microbiol. 54: 709–34. PMID 11018142.