Low-barrier hydrogen bonds.
Frey
P A
PA
D001224:Aspartic Acid / Q000737:chemistry; D002918:Chymotrypsin / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D006860:Hydrogen Bonding
D001224:Aspartic Acid; D006639:Histidine; D002918:Chymotrypsin