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 carboxylate 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 .
C0061472
glu
C0002520
amino acid