Bacterial Phosphate Granules Contain Cyclic Polyphosphates: Evidence from ³¹P Solid-State NMR

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• Polyphosphates (polyPs) are essential polymers in organisms, traditionally viewed as linear chains.
• Recent studies suggest cyclic metaphosphates may interact with enzymes, questioning their natural occurrence.
• Previous reports of metaphosphates were often dismissed as artifacts of extraction methods.

Purpose of the Study:

• To investigate the chemical structure of polyphosphates in whole bacterial cells using nondestructive methods.
• To determine if cyclic metaphosphates occur naturally in the bacterium *Xanthobacter autotrophicus*.
• To explore the potential biological significance and cellular localization of metaphosphates.

Main Methods:

• Employed nondestructive ³¹P solid-state Nuclear Magnetic Resonance (NMR) spectroscopy on whole cells and insoluble fractions.
• Analyzed isotropic and anisotropic ³¹P chemical shifts to identify phosphate structures.
• Compared cellular spectra with synthetic metaphosphates of known structures for identification.

Main Results:

• ³¹P NMR data revealed the coexistence of linear and cyclic phosphates in hydrated whole cells of *X. autotrophicus*.
• Substantial amounts of cyclic phosphates, with an average size of 3-8 phosphate units, were detected under specific growth and sample conditions.
• Metaphosphates were found to be enriched in insoluble, electron-dense granules, and exogenous hexametaphosphate was metabolized to trimetaphosphates.

Conclusions:

• This study provides the first definitive evidence for the presence of biologically relevant metaphosphates in whole bacterial cells.
• The findings suggest that metaphosphate concentrations can be significant in fresh cells and indicate a potential biological role.
• Opens new avenues for investigating the function of metaphosphates across various organisms.