The thermodynamics and kinetics of phosphoester bond formation, use, and dissociation in biology, with the example of polyphosphate in platelet activation, trasience, and mineralization.

Abstract:

Mitochondria condense orthophosphates (Pi), forming phosphoester bonds for ATP production that is important to life. This represents an exchange of energy from dissociated carbohydrate bonds to phosophoester bonds. These bonds are available to phosphorylate organic compounds or hydrolyze to Pi, driving many biochemical processes.

The benthic bacteria T. namibiensis ¹ and Beggiatoa ² condense Pi into phosphate polymers in oxygenated environments. These polyphosphates (polyPs) are stored until the environment becomes anoxic, when these bacteria retrieve the energy from polyP dissociation into Pi³. Dissociated Pi is released outside of the bacteria, where it precipitates as apatite.

The Gibbs free energy of polyP phosphoester bond hydrolysis is negative, however, the kinetics are slow⁴. Diatoms contain a polyP pool that is stable until after death, after which the polyPs hydrolyze and form apatite⁵. The roles of polyP in eukaryotic organism biochemistry continue to be discovered.

PolyPs have a range of biochemical roles, such as bioavailable P-storage, stress adaptation, and blood clotting⁶. PolyP-containing granules are released from anuclear platelets to activate factor V⁷ and factor XII in the blood clotting process due to their polyanionic charge⁸. Platelets have a lifespan of approximately 8 days, after which they undergo apoptosis⁹. Data will be presented that demonstrate the bioactive, thermodynamically unstable polyP pool within older platelets in vitro can spontaneously hydrolyze and form phosphate minerals. This process is likely avoided by platelet digestion in the spleen and liver, possibly recycling platelet polyPs with their phosphoester bond energy for other biochemical roles.

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