In enzymology, a polyphosphate kinase (), or polyphosphate polymerase, is an enzyme that catalyzes the formation of polyphosphate from ATP, with chain lengths of up to a thousand or more orthophosphate moieties.
ATP + (phosphate)n ADP + (phosphate)n+1
Thus, the two substrates of this enzyme are ATP and polyphosphate [(phosphate)n], whereas its two products are ADP and polyphosphate extended by one phosphate moiety [(phosphate)n+1].
This enzyme is a membrane protein and goes through an intermediate stage during the reaction where it is autophosphorylated with a phosphate group covalently linked to a basic amino acyl residue through an N-P bond.
Several enzymes catalyze polyphosphate polymerization. Some of these enzymes couple phosphotransfer to transmembrane transport. These enzyme/transporters are categorized in the Transporter Classification Database (TCDB) under the Polyphosphate Polymerase/YidH Superfamily (TC# 4.E.1) and are transferases that transfer phosphoryl groups (phosphotransferases) with polyphosphate as the acceptor. The systematic name of this enzyme class is ATP:polyphosphate phosphotransferase. This enzyme is also called polyphosphoric acid kinase.
Families
The Polyphosphate Polymerase Superfamily (TC# 4.E.1) includes the following families: 4.E.1 - The Vacuolar (Acidocalcisome) Polyphosphate Polymerase (V-PPP) Family 9.B.51 - The Uncharacterized DUF202/YidH (YidH) Family
The Vacuolar (Acidocalcisome) Polyphosphate Polymerase (V-PPP) Family
Eukaryotes contain inorganic polyphosphate (polyP) and acidocalcisomes, which sequester polyP and store amino acids and divalent cations. Gerasimaitė et al. showed that polyP produced in the cytosol of yeast is toxic. Reconstitution of polyP translocation with purified vacuoles, the acidocalcisomes of yeast, showed that cytosolic polyP cannot be imported whereas polyP produced by the vacuolar transporter chaperone (VTC) complex, an endogenous vacuolar polyP polymerase, is efficiently imported and does not interfere with growth. PolyP synthesis and import require an electrochemical gradient, probably as a (partial) driving force for polyP translocation. VTC exposes its catalytic domain to the cytosol and has nine vacuolar transmembrane segments (TMSs). Mutations in the VTC transmembrane regions, which may constitute the translocation channel, block not only polyP translocation but also synthesis. Since these mutations are far from the cytosolic catalytic domain of VTC, this suggests that the VTC complex obligatorily couples synthesis of polyP to its vesicular import in order to avoid toxic intermediates in the cytosol. The process therefore conforms to the classical definition of Group Translocation, where the substrate is modified during transport. Sequestration of otherwise toxic polyP may be one reason for the existence of this mechanism in acidocalcisomes. The vacuolar polyphosphate kinase (polymerase) is described in TCDB with family TC# 4.E.1.
Function
CYTH-like superfamily enzymes, which include polyphosphate polymerases, hydrolyze triphosphate-containing substrates and require metal cations as cofactors. They have a unique active site located at the center of an eight-stranded antiparallel beta barrel tunnel (the triphosphate tunnel). The name CYTH originated from the gene designation for bacterial class IV adenylyl cyclases (CyaB), and from thiamine triphosphatase (THTPA). Class IV adenylate cyclases catalyze the conversion of ATP to 3',5'-cyclic AMP (cAMP) and PPi. Thiamine triphosphatase is a soluble cytosolic enzyme which converts thiamine triphosphate to thiamine diphosphate. This domain superfamily also contains RNA triphosphatases, membrane-associated polyphosphate polymerases, tripolyphosphatases, nucleoside triphosphatases, nucleoside tetraphosphatases and other proteins with unknown functions.
The generalized reaction catalyzed by the vectorial polyphosphate polymerases is: