Inorganic polyphosphate functions at various stages of cell evolution

Enzymes of inorganic polyphosphate metabolism

Inorganic polyphosphate (PolyP) is a linear polymer containing a few to several hundred orthophosphate residues linked by energy-rich phosphoanhydride bonds. Investigation of PolyP-metabolizing enzymes is important for medicine, because PolyPs perform numerous functions in the cells. In human organism, PolyPs are involved in the regulation of Ca(2+) uptake in mitochondria, bone tissue development, and blood coagulation. The essentiality of polyphosphate kinases in the virulence of pathogenic bacteria is a basis for the discovery of new antibiotics. The properties of the major enzymes of PolyP metabolism, first of all polyphosphate kinases and exopolyphosphatases, are described in the review. The main differences between the enzymes of PolyP biosynthesis and utilization of prokaryotic and eukaryotic cells, as well as the multiple functions of some enzymes of PolyP metabolism, are considered.

New aspects of inorganic polyphosphate metabolism and function

The review analyzes the results of recent studies on the biochemistry of high-molecular inorganic poly-phosphates (PolyPs). The data obtained lead to the following main conclusions. PolyPs are polyfunctional compounds. The main role of PolyPs is their participation in the regulation of metabolism both at the genetic and metabolic levels. Among the functions of PolyPs known at present, the most important are the following: phosphate and energy storage; regulation of the levels of ATP and other nucleotide and nucleoside-containing coenzymes; participation in the regulation of homeostasis and storage of inorganic cations and other positively charged solutes in an osmotically inert form; participation in membrane transport processes mediated by poly-beta-Ca2+-hydroxybutyrate complexes; participation in the formation and functions of cell surface structures; control of gene activity; and regulation of activities of the enzymes and enzyme assemblies involved in the metabolism of nucleic acids and other acid biopolymers. However, the functions of PolyPs vary among organisms of different evolutionary levels. The metabolism and functions of PolyPs in each cellular compartment of procaryotes (cell wall, plasma membrane, cytosol) and eucaryotes (nuclei, vacuoles, mitochondria, plasma membrane, cell wall, mitochondria, cytosol) are unique. The synthesis and degradation of PolyPs in the organelles of eucaryotic cells are possibly mediated by different sets of enzymes. This is consistent with of the endosymbiotic hypothesis of eucaryotic cell origin. Some aspects of the biochemistry of high-molecular PolyPs are considered to be of great significance to the approach to biotechnological, ecological and medical problems.

Membrane-bound and soluble polyphosphatases of mitochondria of Saccharomyces cerevisiae: identification and comparative characterization

Isolated mitochondria of Saccharomyces cerevisiae possess polyphosphatases insensitive to a number of inhibitors of ATPase and pyrophosphatase of the same organelles and differing from the last two by neutral pH optima and molecular masses. After subfractionation of mitochondria, the polyphosphatase activity is distributed among the membrane and soluble preparations. The membrane-bound and soluble polyphosphatase activities are represented by different enzymes distinguished by molecular masses, substrate specificity, Km values, and relation to mono- and divalent cations. The membrane-bound polyphosphatases have molecular masses of 120 and 76 kDa, and the soluble one of about 36 kDa. All three enzymes appear to have a monomeric structure. The soluble polyphosphatase activity is stimulated by divalent cations in contrast to the membrane-bound one which is inhibited by the same cations, including Mg2+. Monovalent cations do not actually change the activity of the soluble enzyme, but stimulate it in the membrane preparation. Specific activities for the hydrolysis of polyphosphates with average chain lengths of 9-188 phosphate residues increase under increasing degree of substrate polymerization in the membrane preparation and are actually unchanged in the soluble one. The affinity of the soluble enzyme to polyphosphates is 5-10 times higher than that of the membrane-bound polyphosphatases.

Purification and properties of exopolyphosphatase isolated from Saccharomyces cerevisiae vacuoles

An exopolyphosphatase (polyPase) with a specific activity of 60 U/mg protein has been purified from the vacuolar sap of Saccharomyces cerevisiae. The molecular mass of the intact enzyme was found to be 245 kDa. It is highly specific towards high-molecular polyphosphates (polyP). The activity with polyP9 is 24% of that with polyP208. The apparent Km for polyP15 and polyP208 hydrolysis is 93 and 2.4 microM, respectively. The enzyme is slightly active with polyP3 and adenosine-5'-tetraphosphate, but does not hydrolyze pyrophosphate, ATP, GTP and p-nitrophenylphosphate. It is stimulated by divalent metal cations. Co2+, the best activator, stimulates it 6-fold. Antibodies that inhibit the cell envelope and cytosol polyPases of S. cerevisiae have no effect on the vacuolar polyPase. The vacuolar polyPase differs from other yeast polyPases in molecular mass, substrate specificity and effects of activators.

Purification and properties of polyphosphatase from Saccharomyces cerevisiae cytosol

A homogenous polyphosphatase preparation was obtained from Saccharomyces cerevisiae cytosol with a 3.8% yield and 3540-fold purification. The enzyme hydrolysed polyphosphate (polyP) with various chain lengths, including polyP3, and split Pi off the end of the chain. It was inactive with respect to ATP, PPi, and p-nitrophenylphosphate. Its specific activity with polyP15 was 283 U/mg protein. The polyphosphatase was a monomeric protein with a molecular mass of 40 kDa. This enzyme was inactive without divalent cations and with Cu2+ and Ca2+. The ability of other divalent cations to activate the enzyme decreased in the following order; Co2+ > Mn2+ > Mg2+ > Zn2+. A kinetic model of the hydrolysis of polyP3 and action of Mg2+ is proposed.

Comparison of exopolyphosphatases of different yeast cell compartments

Purified cell-envelope polyphosphatase as well as polyphoshatase activities of cytosol and isolated vacuoles, of nuclei and mitochondria of the yeast Saccharomyces cerevisiae were compared. The polyphosphatases of cell envelope and cytosol are similar, the polyphosphatases of nuclei, vacuoles and mitochondria differ in their kinetic properties, substrate specificity, requirements in divalent cations and in some effector actions both from these and from each other.