PMC:3248852 / 19448-21951 JSONTXT

{"project":"2_test","denotations":[{"id":"22054214-17314313-9450432","span":{"begin":267,"end":269},"obj":"17314313"},{"id":"22054214-2225770-9450433","span":{"begin":373,"end":375},"obj":"2225770"},{"id":"22054214-14435647-9450434","span":{"begin":584,"end":586},"obj":"14435647"},{"id":"22054214-5950403-9450435","span":{"begin":1080,"end":1082},"obj":"5950403"},{"id":"22054214-14220729-9450436","span":{"begin":1535,"end":1537},"obj":"14220729"},{"id":"22054214-5940939-9450437","span":{"begin":1560,"end":1562},"obj":"5940939"},{"id":"22054214-5461218-9450438","span":{"begin":1777,"end":1779},"obj":"5461218"},{"id":"22054214-5461218-9450439","span":{"begin":2349,"end":2351},"obj":"5461218"}],"text":"Phosphorylation of yolk platelet phosvitin\nAs seen in the 31P-NMR spectrum in Figure 2, phosvitin is the major phosphoprotein in the amphibian oocyte. Phosvitin is a glycosylated, serine-rich peptide with reported masses of 16-19 kDa, 25 kDa, or 31 kDa (reviewed in [19]). A single resonance (2.59 ppm) dominates the proton-decoupled 31P-NMR spectrum of Xenopus phosvitin [20]. Comparison of the phosvitin spectra with and without proton decoupling suggests a triplet splitting pattern for the major resonance, presumably due to coupling to methylene protons.\nRabinowitz and Lipmann [21] were the first to demonstrate reversible phosphate transfer between yolk phosphoprotein and ATP. Attempts were made to determine the equilibrium constant of the reaction between ATP and phosphoprotein. Figures varying from 20 to 50 were obtained for the forward reaction. However, their experiments indicated a non-homogenous phosphate population. The authors suggested that the \"thermodynamic potential of phosphoryl (groups) in phosvitin to be not far below that of ATP\". Mano and Lipmann [22] subsequently found that only more highly phosphorylated forms of phosvitin were good acceptors of phosphate from protein kinase and ATP. This suggests that a large fraction of the phosvitin serine phosphates do not turn over in situ. Our data indicate (Table 1) that only a small fraction of the serine phosphates in yolk phosvitin may be available for reversible phosphoryl exchange with ADP/ATP.\nPhosvitin also contains firmly bound, non-heme iron [23]. Grant and Taborsky [24] suggested that at alkaline pH, autoxidation of iron converts phosvitin-bound serine phosphate to the corresponding enol phosphate, an energy-rich structure. However, subsequent studies by Rosenstein and Taborsky [25] failed to find evidence for the production of a stable phosphoenol product and a demonstration of the stability of the C-H bond at the α-carbon of the oxidized residue further ruled it out. Their finding that phosphate release occurs by P-O bond cleavage is consistent with a mechanism by which an oxidatively generated carbonium ion derivative of phosphoserine is converted into a stable product by the direct formation of the free aldehyde and a monomeric metaphosphate ion, the latter reacting with water to yield inorganic orthophosphate. Rosenstein and Taborsky [25] proposed that yolk phosvitin would provide the developing embryo with a potential phosphorylating agent (HPO42-) which becomes activated by oxidation."}