inositol hexakisphosphate & diphosphoinositol-pentakisphosphate kinase 1 (insP6 & PP-IP5 kinase 1, PP-IP5 kinase 1, inositol pyrophosphate synthase 1, hsVIP1, IP6 kinase, histidine acid phosphatase domain-containing protein 2A, HISPPD2A, IP6K, IPS1, KIAA0377, PPIP5K1, VIP1)

Function

• bifunctional inositol kinase
• catalyzes formation of diphosphoinositol pentakisphosphate (insP7 or PP-insP5) & bi-diphosphoinositol tetrakisphosphate (insP8 or PP2-insP4)
• converts inositolitol hexakisphosphate (insP6) to insP7
• also able to convert insP7 to insP8
• probably specifically mediates formation of 4PP-insP5 & 6PP-insP5 insP7 isomers but not of 5PP-IP5 insP7 isomer
• activated when cells are exposed to hyperosmotic stress

ATP + 1D-myo-inositol hexakisphosphate
               <-->
ADP + 5-diphospho-1D-myo-inositol (1,2,3,4,6)pentakisphosphate

ATP + 1D-myo-inositol 1,3,4,5,6-pentakisphosphate
               <-->
ADP + diphospho-1D-myo-inositol tetrakisphosphate
     (isomeric configuration unknown)

ATP + 1D-myo-inositol 5-diphosphate pentakisphosphate
                <-->
ADP + 1D-myo-inositol bisdiphosphate tetrakisphosphate
     (isomeric configuration unknown)

Kinetic parameters:

• KM=0.12 uM for insP6
• KM=0.10 uM for insP7
• Vmax=0.03 nmol/min/mg enzyme with insP6 as substrate
• Vmax=0.13 nmol/min/mg enzyme with insP7 as substrate

Structure

• belongs to the histidine acid phosphatase family, VIP1 subfamily
• although related to histidine acid phosphatases, it lacks the conserved active sites, suggesting that it has no phosphatase activity

Compartment

cytoplasm, cytosol

Alternative splicing

named isoforms=7

Expression

• widely expressed
• high expression in skeletal muscle, heart & brain

More general terms

• kinase or ATP-phosphotransferase
• phosphoprotein

Database

• UniProt: http://www.uniprot.org/uniprot/Q6PFW1.html