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Example sentences for: apppa
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Catalytic activity of the Fhit ApppA hydrolase depends on His96 [ 14 16 ] . To test whether the active site of Hnt2 is necessary to control dinucleoside polyphosphate accumulation in vivo , wild-type and mutant alleles of HNT2 that differ at His109, corresponding to human Fhit His96, were expressed from the HNT2 promoter on plasmids (Table 3) in hnt2ΔADE2 strain BY71-6c.
The middle histidine of the histidine triad (His96 in human Fhit), which is critical for hydrolysis of ApppA by Fhit [ 14 16 ] , is not necessary for tumor suppression [ 17 18 ] . Nonetheless, wild-type and His96Asn forms of Fhit are saturated by ApppA in the low micromolar range and form stable complexes with non-hydrolyzable ApppA in which two ApppA analogs are bound per Fhit dimer and all phosphates cluster on one surface of the protein [ 16 ] . These observations suggested that Fhit-substrate complexes may be the active, signaling form of Fhit and that the function of the catalytically essential histidine may be to terminate the lifetime of signaling complexes [ 16 ] .
Fhit proteins from humans [ 19 ] and worms [ 15 ] bind ApppA and AppppA with K m values of 2 to 3 μM.
His114 corresponds to His98 of Fhit, which was localized in low barrier hydrogen-bonding distance to the α-β bridging oxygen of a bound ApppA analog in co-crystal structure with Fhit His96Asn [ 35].
Human Fhit [ 14 ] and the S. cerevisiae Fhit homolog [ 13 ] , which was called Aph1 but is here termed Hnt2 under nomenclature aproved by the Saccharomyces Genome Database, cleave ApppA more readily while Aph1, the S. pombe homolog, cleaves AppppA more readily [ 20 ] . Consistent with the ApppA hydrolase activity of purified Fhit protein, most cancer cell lines that are Fhit negative at the protein level have higher levels of ApppA than cell lines that are Fhit positive [ 21 ] . Nonetheless, the actual concentrations of dinucleoside polyphosphates were submicromolar in every cell culture sample [ 21 ] and thus, under the reported culture conditions, the measured dinucleoside polyphosphates would not be expected to occupy the Fhit active site substantially [ 19 ] . Dinucleoside polyphosphate levels were measured in adenine-requiring S. cerevisiae strains before or after disruption of the Fhit-homologous HNT2 gene [ 13 ] and in adenine-requiring S. pombe strains as a function of disruption and overexpression of the Fhit-homologous aph1 gene [ 22 ] . Recently, it was observed that diadenosine polyphosphates undergo a divalent cation-dependent conformational change that might mediate their biosynthesis, catabolism or signaling properties [ 23 ] .
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