The Specific Guanine Binding Site of the Ribonuclease Tj Family Enzymes and of G-Proteins Is Modeled in the Cocrystal Formed by 7-Methylguanosine-5'-Phosphate and Phenylalanine1

Abstract
In the cocrystal formed by 7-methylguanosine-5'-phosphate-phenylalanine-6H2O, the interactions between guanine and phenylalanine are similar to those observed in the complex of ribonuclease T1; with 2'-guanylic acids, and those of the two G-proteins, Elongation Factor-Tu and ras oncogene p21, with GDP. They are similar in the following three points: (a) guanine N(1)H and N(2)H donate cyclic N-H-O hydrogen bonds to the carboxylate group of phenylalanine in the former cocrystal and to the side chain carbox-ylate group of Asp or Glu in the latter proteins, (b) O(6) of guanine accepts hydrogen bond(s) from main-chain NH group(s), and (c) the purine moiety is sandwiched between aromatic (or hydrophobic) amino acid side chains.