TRANSFERASE, HYDROGEN BOND NETWORK, GLUTAMINYL CYCLASE, PYROGLUTAMATE, SITE-DIRECTED MUTAGENESIS, PROTON TRANSFER, ACYLTRANSFERASE, GLYCOPROTEIN, METAL-BINDING, POLYMORPHISM, ZINC CRYSTAL STRUCTURE OF THE HUMAN GLUTAMINYL CYCLASE MUTANT E201L AT 2.08 ANGSTROM RESOLUTION 2ZEG