THE C-TERMINAL REGION OF HUMAN ANGIOGENIN HAS A DUAL ROLE IN ENZYMATIC-ACTIVITY

The ribonucleolytic activity of angiogenin (Ang) is essential to Ang's capacity to induce blood vessel formation, Previous x-ray diffraction and mutagenesis results have shown that the active site of the human protein is obstructed by Gln-117 and imply that the C-terminal region of Ang must undergo a conformational rearrangement to allow substrate binding and catalysis, As a first step toward structural characterizat ion of this conformational change, additional site-directed mutagenesi s and kinetic analysis have been used to examine the intramolecular in teractions that stabilize the inactive conformation of the protein, Tw o residues of this region, Ile-119 and Phe 120, are found to make hydr ophobic interactions with the remainder of the protein and thereby hel p to keep Gln-117 in its obstructive position, Furthermore, the suppre ssion of activity by the intramolecular interactions of Ile-119 and Ph e-120 is counterbalanced by an effect of the adjacent residues, Arg-12 1, Arg-122, and Pro-123, which do not appear to form contacts with the rest of the protein structure, They contribute to enzymatic activity, probably by constituting a peripheral subsite for binding polymeric s ubstrates, The results reveal the nature of the conformational change in human Ang and assign a key role to the C-terminal region both in th is process and, presumably, in the regulation of human Ang function.