Phosphate binding proteins

Similarly to dyes, some fluorescent proteins can be incorporated into polymeric beads to be used as an alternative for ion sensing. For example, a reporter protein (composed of a phosphate-binding protein, a FRET donor (cyan fluorescent protein) and a FRET acceptor (yellow fluorescent protein)) was incorporated into polyacrylamide nanobeads by Sun et al. [46]. FRET was inhibited upon binding of phosphate. Kopelman and co-workers [47] used a similar approach to design a nanosensor for copper ions. They have found that fluorescence of red fluorescent protein DsRed (commonly used as a label) is reversibly quenched by Cu2+ and Cu+. Both DsRed and Alexa Fluor 488 (used as a reference) were entrapped into polyacrylamide nanobeads. Typically, up to 2 ppb of copper ions can be reliably measured. It should be mentioned, that in contrast to much more robust dyes, mild conditions upon polymerization and purification are very important for immobilization of the biomolecule to avoid degradation. 

There has been some development of optical biosensors. Nitrate reductase was immobilised within a sol-gel matrix, with binding of nitrate ion (down to a limit of 10"6 M) causing a characteristic change in the optical absorption [132]. It is notable that this sensor was reversible, allowed selective nitrate detection over other physiologically significant anions and did not lose activity even over six months. Phosphate-binding protein was immobilised on a fibre-optic detector and could be used to measure phosphate with a detection limit of about 10 6 M [133]. 

Brune, M. et al. 1998. Mechanism of inorganic phosphate interaction with phosphate binding protein from Escherichia coli. Biochemistry 37, 10370-10380. 

Katayev EA, Sessler JL, Khrustalev VN et al (2007) Synthetic model of the phosphate binding protein solid-state structure and solution-phase anion binding properties of a large oligopyr-rolic macrocycle. J Org Chem 72 7244—7252... 

Human phosphate binding protein (HPBP), an apolipoprotein that binds inorganic phosphate in blood, was serendipitously discovered. Its three-dimensional structure and complete amino acid sequence were solved (Morales et al, 2006 Diemer et al, 2008). The conditions found to separate HPBP and PONl in vitro indicated that HPBP is strongly associated with PONl (Renault et al, 2006). Moreover, the stabilization of the active form(s) of human PONl by HPBP suggests that HPBP could be a functional chaperone for PONl (Rochu et al, 2007b, c). 

Renault, F., Chabriere, E., Andrieu, J.P., Dublet, B., Masson, P., Rochu, D. (2006). Tandem purification of two HDL-associated partner proteins in human plasma, paraoxonase (PONl) and phosphate binding protein (HPBP) using hydroxyapatite chromatography. J. Chromatogr. B 836 15-21. 

Rochu, D., Renault, F., Clery-Barraud, C., Chabriere, E., Masson P. (2007b). Stability of highly purified human paraoxonase (PONl) association with human phosphate binding protein (HPBP) is essential for preserving its active conformation (s). Biochim. Biophys. Acta 1774 874-83. 

There is indirect evidence for the existence of an SRP-like entity in E. coli. Pagds et al. (1985) have presented preliminary findings that indicate that a translation block may occur during the synthesis of pre-PhoS, a periplasmic phosphate-binding protein. It is known that the translation rate in E. coli is nonuniform. Pause sites (Pag s et al., 1985) occur at codons complementary to uncommon tRNAs. However, a pause site in PhoS elongation corresponding to a peptide of 8 kDa is not accounted for by the presence of such codons. This peptide was subsequently con-... 

Furuichi T, Yoshikawa S, Miyawaki A, Wada K, Maeda N, Mikoshiba K (1989) Primary structure and functional expression of the inositol 1,4,5-tris-phosphate-binding protein P400. Nature. 342, 32-38. 

Besides the role of 1,25-(OH)2D3 in the transfer of calcium across the intestinal membrane it is known that this substance also activates the transfer of inorganic phosphate across intestinal illeum and jejunum 307-3io) process has been shown to be independent of calcium transport and thus represents an entirely different function of the vitamin. Evidence has been presented that 1,25-(OH)2D3 rather than 25-OH-D3 functions in this process Little is known concerning the mechanism of phosphate transfer except that it is a sodium dependent and active process ° So far a specific phosphate binding protein has not yet been found. 

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