lewis structure and lysozymes

nucleophile

• supplies electrons

electrophiles

• accepts electrons

arrow rules

• curved arrows show the direction of electron movement- pushing electrons

• the base of the arrow begins at the original location of the pair of electrons

• the head of the arrow points to the destination of the electrons

when does simultaneous making and breaking of sigma bonds occur

• when a nucleophile approaches a carbon atom having a leaving group in a direction anti and rearward to the leaving group (substitution nucleophilic reaction) (the nucleophile does not have to attack the carbon)

sigma bond making and pi bond breaking

• condensation of a molecule and an ion into a single ion

• 1 curly arrow points from 1 of the lone pairs of electrons on the hydroxyl to the carbon and the other arrow from the C=O bond to the oxygen

• C changes sp² to sp³ hybrid state

sigma bond breaking and pi bond making

• one curly arrow points from 1 of the lone pairs of electrons on the oxygen to the C-O bond and the other arrow from the C-OH bond to the oxygen

lysozyme

• an enzyme that is involved in the first line of defence against bacterial attacks that cleave peptidoglycan, the polysaccharide complex in the cell walls of gram-positive bacteria

• the enzyme has little effect on gram negative bacteria

where are lysozymes found in humans

• in most bodily secretions including tears and nasal mucus

history of lysozymes

• the term lysozyme was by Fleming in 1922

• the structure of hen egg-white lysozyme, determined in 1965 by x-ray crystallography, was the first to be determined for any enzyme

lysozyme structure

• polypeptide chain: 129 amino acids, 4 s-s bridges

• structure: 2 domains separated by a deep cleft

• (left domain: a small beta sheet of mainly hydrophilic residues)

• (right domain: hydrobic core surrounded by short alpha helices)

• substrate: triNAG

• the active site cleft binds 6 sugars (ABCDEF)

• the glycosidic bond between the 4th and 5th sugar (D-E) is the bond broken in the reaction

lysozyme active site

• lysozyme binds the peptidoglycan carbohydrate polyer so that a NAM ring is at side D and a NAG ring at site E in the binding site on the enzyme surface

• the D-E glycosidic bond is close to the side chain of Glu35 and Asp52. Both have carboxylic acid side chains

Asp52

• is in the carboxylate form as expected at pH 6 (optimum pH for lysozyme reaction)

Glu35

• has an unusual raised pKa because it is in a hydrophobic microenvironment and is in the carboxylic acid form at pH 6

lysozyme mechanism

• nucleophilic attack by Asp52 forms covalent acyl enzyme intermediate

• Glu35 donayes a proton and sugars E-F diffuses away (first product)

• attack by water: OH added to C1 of D and a proton to Glu35

• sugars A-B-C-D are the 2nd product