A reinvestigation of residues 64-68 and 175 in papain. Evidence that residues 64 and 175 are asparagine.
The tryptophan-containing peptides were isolated from the chymotryptic digest of S-carboxymethylated papain. Residue 175, which is strongly hydrogen-bonded to the active-site histidine residue in the tertiary structure of papain, is asparagine, confirming the work of Kimmel, Rogers & Smith (1965). Its function is probably to maintain the orientation and tautomeric state of the imidazole ring of histidine-159. The amino acid sequence predicted from the electron-density map of papain for residues 64-68 was confirmed, but residue 64 is asparagine, not aspartic acid. This residue, which is about 10 A from the thiol group of the active-site cysteine-25, cannot therefore be a site of electrostatic attraction for substrates of basic amino acids.
D000595:Amino Acid Sequence; D001216:Asparagine / Q000032:analysis*; D001665:Binding Sites; D002850:Chromatography, Gel; D002918:Chymotrypsin; D003545:Cysteine; D006639:Histidine; D007093:Imidazoles; D010206:Papain / Q000032:analysis*; D011485:Protein Binding
D007093:Imidazoles; D006639:Histidine; D001216:Asparagine; D002918:Chymotrypsin; D010206:Papain; D003545:Cysteine