Human lysozyme is an important enzyme which is part of the human body's defense against some bacteria. It is found in abundance in tears, saliva and mucous. The main action of lysozyme is to perform beta 1-4 glycosidic bond cleavage on the N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) polysaccharide coating of a bacteria cell wall. With the weakened cell wall, the bacteria then burst due to internal pressure..
The structure of human lysozyme is fairly simple with only seven helices and one three stranded beta sheet all made from a single polypeptide chain. Also, there are no subunits and no prosthetic groups present in the enzyme. The cleft where the active site is located is on the surface of the enzyme and is relatively easy to spot when viewing the three-dimensional model. The actual residues involved in the catalytic action of lysozyme are Asp 53 and Glu 35. The mechanism of bond breaking involves hydrogen transfer from Glu 35 to the oxygen of the glycosidic bond. The result is a broken bond and the formation of a carbonium ion on the C1 of the NAG residue The carbonium ion is then removed via a hydroxyl group from the solvent. The actual residues involved in holding the ligand in the active site are still being determined but from preliminary experiments it appears Lys 33, Trp 34, Glu 35, Ser 36, Asn 46, Asn 60, Asp 53, Trp 64, Asp 102, Ala 108, and Val 110 all form hydrogen bonds to the ligand. Human lysozyme is unique in its sequence but not in its reaction mechanism as lysozmyes from other species all seem to function in a similar manner.
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PDB code 1lz1, URL http://www.biochem.ucl.ac.uk/bsm/pdbsum/1lz1/main.html
Protein Data Bank, URL http://pdb.bnl.gov/pdb-bin/pdbids
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