Inorganic-pyrophosphate-dependent phosphorylation of spinach thylakoid proteins

Inorganic tripolyphosphate (PPP(i)) as a phosphate donor for human deoxyribonucleoside kinases

Inorganic tripolyphosphate (PPP(i)) and pyrophosphate (PP(i)) were examined as potential phosphate donors for human deoxynucleoside kinase (dCK), deoxyguanosine kinase (dGK), cytosolic thymidine kinase (TK1), mitochondrial TK2, and the deoxynucleoside kinase (dNK) from Drosophila melanogaster. PPP(i) proved to be a good phosphate donor for dGK, as well as for dCK with dCyd, but not dAdo, as acceptor substrate, illustrating also the dependence of donor properties on acceptor. Products of phosphorylation were shown to be 5(')-phosphates. In striking contrast to ATP, the phosphorylation reaction follows strict Michaelis-Menten kinetics, with K(m) values of 74 and 92 microM for dCK and dGK, respectively, and V(max) values 40-50% that for ATP. With the other three enzymes, as well as for dCK with dAdo as acceptor, no, or only low levels (</=1% of that for ATP) of activity were observed. PP(i) was inactive (<0.1%) as a phosphate donor with all enzymes, but was a competitive inhibitor vs ATP, as was PPP(i) in systems with no or low donor activity. This is the first report on inorganic tripolyphosphate as a phosphate donor for nucleoside kinases, in particular human deoxyribonucleoside kinases.

Inorganic pyrophosphate generation and disposition in pathophysiology

Inorganic pyrophosphate (PP(i)) regulates certain intracellular functions and extracellular crystal deposition. PP(i) is produced, degraded, and transported by specialized mechanisms. Moreover, dysregulated cellular PP(i) production, degradation, and transport all have been associated with disease, and PP(i) appears to directly mediate specific disease manifestations. In addition, natural and synthetic analogs of PP(i) are in use or currently under evaluation as prophylactic agents or therapies for disease. This review summarizes recent developments in the understanding of how PP(i) is made and disposed of by cells and assesses the body of evidence for potentially significant physiological functions of intracellular PP(i) in higher organisms. Major topics addressed are recent lines of molecular evidence that directly link decreased and increased extracellular PP(i) levels with diseases in which connective tissue matrix calcification is disordered. To illustrate in depth the effects of disordered PP(i) metabolism, this review weighs the roles in matrix calcification of the transmembrane protein ANK, which regulates intracellular to extracellular movement of PP(i), and the PP(i)-generating phosphodiesterase nucleotide pyrophosphatase family isoenzyme plasma cell membrane glycoprotein-1 (PC-1).

Purification and characterization of thylakoid membrane-bound inorganic pyrophosphatase from Spinacia oleracia L

An inorganic pyrophosphatase (PPase) was purified from thylakoid membrane of spinach leaves to electrophoretic purity by methods including detergent solubilization, ammonium sulfate fractionation, and successive chromatographic techniques. Current protocol yielded about 10% recovery of total activity with a 30-fold purification. The specific activity of the purified enzyme was approximately 400 micromol PPi consumed/mg protein x h. This enzyme is a monomer with a molecular mass of 55 kDa. Several properties, including subunit composition, substrate specificity, ion requirements, inhibitor sensitivities, and amino acid composition, have been studied. Mg2+ is an essential cofactor for the thylakoid PPase. The preferred substrate for the hydrolytic reaction of PPase appears to be dimagnesium pyrophosphate. K+ could not stimulate the enzymatic activity of thylakoid PPase, while F- was a potent inhibitor. Group-specific modification of the thylakoid PPase demonstrates possible involvement of carboxylate residues in the enzymatic activity. Furthermore, antibodies raised against thylakoid PPase in a rabbit could inactivate the PPi hydrolysis of thylakoid and the purified enzyme, but not that of vacuolar H+-PPase, indicating both PPi hydrolases are structurally distinct.

Stabilization of photosystem two dimers by phosphorylation: implication for the regulation of the turnover of D1 protein

A general feature of many membrane protein complexes is that they have oligomeric organisation in vivo. Photosystem II (PSII) is one such example and the possible functional significance of this is explored in this work. Monomeric and dimeric forms of the core complex of PSII have been isolated from non-phosphorylated and phosphorylated thylakoid membranes prepared from spinach. These complexes had the same complement of proteins including, D1 (PsbA), D2 (PsbD), alpha-(PsbE) and beta-(PsbF) subunits of cytochrome b559, CP47 (PsbB), CP43 (PsbC), 33 kDa (PsbO) extrinsic protein and some other smaller subunits, such as PsbH, but did not contain Cab proteins. D1, D2, CP43 and PsbH were the phosphorylated components. Whether phosphorylated or not, the dimeric form of the PSII complex was more stable than the monomeric form. However, when treated with photoinhibitory light the isolated dimers converted to monomers in their non-phosphorylated state but not when phosphorylated. Phosphorylation, however, did not prevent photoinhibition as judged by the loss of oxygen evolving activity. A model is suggested for the role of PSII phosphorylation in controlling the conversion of dimeric PSII to its monomeric form and in this way regulate the rate of degradation of D1 protein during the photoinhibitory repair cycle.

Characterization of a mitochondrial inorganic pyrophosphatase in Saccharomyces cerevisiae

We have studied a mitochondrial inorganic pyrophosphatase (PPase) in the yeast Saccharomyces cerevisiae. The uncoupler FCCP (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and the ionophores valinomycin and nigericin stimulate the PPase activity of repeatedly washed yeast mitochondria 2-3-fold. We have previously cloned a yeast gene, PPA2, encoding the catalytic subunit of a mitochondrial PPase. Uncouplers stimulate the PPase activity several-fold in mitochondria from both cells that overexpress PPA2 from a high copy number plasmid and cells with normal expression. These results indicate that the PPA2 polypeptide functions as an energy linked and membrane associated PPase. The stimulation of mitochondrial PPase activity by FCCP, but not by valinomycin and nigericin, was greatly enhanced by the presence of DTT. The antibiotics Dio-9, equisetin and the F0F1-ATPase inhibitor oligomycin also increase mitochondrial PPase activity several fold. This stimulation is much higher, whereas basal PPase activity is lower, in isotonic than in hypotonic solution, which indicates that intact membranes are a prerequisite for maximal effects.

Protein phosphorylation by inorganic pyrophosphate in yeast mitochondria

Inorganic pyrophosphate can function as phosphate donor in protein phosphorylation reactions in yeast mitochondria. It was shown that, when PPi substitutes for ATP as inhibitor of the pyruvate dehydrogenase reaction, maximal activity is reached after a lag-period of 30-60 minutes. 32P-labeling of peptides shows that [32P]PPi gives about 25% of the labeling obtained by [gamma-32P]ATP in the protein kinase reaction. The PPi dependent phosphorylation is increased several fold by the presence of cold ATP.