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Example sentences for: ntpdases
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E174 in ACR3 and S218 in ACR4 are required for catalytic function [ 12 ] . Substitution of H59 in ACR1 converted CD39 into an ADPase in a quaternary structure dependent manner [ 13 ] . Mutation of W187A in ACR3 affected CD39 folding and translocation, while mutation of W459A in ACR5 increased ATPase activity but diminished ADPase activity [ 14 ] . Mutations of D62 and G64 of ACR1 and D219 and G221 of ACR4 demonstrated that the nucleotide phosphate binding domains of NTPDases are similar to those present in the actin/heat shock protein/sugar kinase superfamily [ 15 ] . These results suggest that the conserved residues of the ACR1 to 5 regions are involved in the catalytic mechanism of CD39.
The catalytic mechanism of NTPDases is not known even though some basic facts of the catalysis have been established.
NTPDases are important in many physiological processes like cell motility, adhesion, nonsynaptic information transfer, secretion, regulation of hemostasis and ectokinases [ 1 ] . Understanding the enzymatic mechanisms of the NTPDases will help description of their physiological functions, and development of strategies to regulate the functions of the enzymes.
These results will provide guides for further studies of the catalytic mechanism of NTPDases.
NTPDases do not form phosphorylated intermediates during catalysis, a conclusion also supported by lack of vanadate sensitivity and Pi product inhibition [ 3 4 5 6 ] . The catalytic reaction appears to be irreversible and no partial reactions have been observed [ 7 8 ] . Divalent cations like Ca 2+or Mg 2+are required for activity, and maximal activity is reached when the concentrations of substrates and divalent cations are equal [ 1 ] . The specific activities of NTPDases vary over a broad range from ten thousand units for potato apyrase to less than one hundred units for chicken gizzard ecto-ATPase [ 9 10 ] . Sequence comparisons indicate that most of NTPDases contain five highly conserved regions, apyrase conserved region, ACR1 - ACR5 [ 9 11 ] . However, the catalytic sites have not been identified, although ACR1 and ACR4 have been implicated in β- and γ-phosphate binding, respectively [ 9 ] .
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