The crystal structure of human angiogenin (reported in the preceding paper in this issue) reveals that the site that corresponds to the pyrimidine binding site of RNase A is obstructed by Gln-117. Mutation of this residue to Ala and Gly is here found to increase activity 11- to 18-fold and 21- to 30- fold, respectively, toward dinucleotide, polynucleotide, and cyclic nucleotide substrates, but without changing specificity. The enhanced activity of Q117G toward CpA is due to a 5-fold decrease in K(m) and a 6- fold increase in k(cat). Its K(i) value for 2'-CMP is 5-fold lower than that of native angiogenin, whereas its K(i) value for 5'-AMP is unchanged. It has been reported previously that mutating Asp-116 to Ala increases activity 15- fold. The double mutant D116A/Q117A is shown to be only slightly more active than each individual mutant. The present results demonstrate that Gln-117 impedes the ribonucleolytic activity of angiogenin, as predicted by x-ray crystallography. Moreover, they suggest that prior to or during catalysis angiogenin must undergo a conformational change to reorient the C-terminal segment that contains this residue, and that a similar reorganization is required for the mutants as well. This view is supported by molecular modeling of an angiogenin-uridine vanadate complex. These in vitro findings have implications for the angiogenic activity of angiogenin in vivo.
|Number of pages||5|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - 12 Apr 1994|
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