structure n n n Monomer The primary structure

structure n n n Monomer The primary structure of lysozyme is a single polypeptide containing 129 amino acids. Lysozyme has five helical regions and a three -stranded antiparallel β sheet and a large amount of random coil and β turns. The structure revealed 4 stabilizing disulfide bonds. In physiological conditions, lysozyme is folded into a compact, globular structure with a long cleft in the protein surface.

*四個雙硫鍵 *129個amino acids

Polysaccharide Lysozyme-polysaccharide Intermediate

substrate n chitin, which is a polymer of NAG linked by ß(1 -4) glycosidic bonds. Chitin is found in crustacean(甲殼綱) shell tissue n the polysaccharide component of the cell walls of certain bacteria; the principal function of lysozyme is as an antibacterial agent. This polysaccharide is composed of alternating residues of NAG and NAM. Only the glycosidic bonds between C-1 of NAM and C-4 of NAG are hydrolyzed.

function Abundant in a number of secretions, such as tears, saliva, and mucus. It also can be found in egg whites. n 藉由與細菌的細胞壁結合，減少其上的負電荷， 而促進白血球的吞噬作用(類似無專一性的調理 素)。 n 切割peptidoglycan glycosidase or glycohydrolase(catalyze the transfer of a glycosyl group to water: C-O bond broken) n

function catalyzing hydrolysis of 1, 4 -beta-linkages between N-acetylmuramic acid (NAM)and Nacetyl-D-glucosamine (NAG)residues in a peptidoglycan. n Cleave the cell wall of gram positive bacteria. **(Peptidoglycan forms around 90% of the dry weight of Gram-positive bacteria but only 10% of Gram-negative strains. ) gram-positive bacteria have one lipid layer just inside their cell wall while gram-negative bacteria have two lipid layers, sandwiching their cell wall n

Mechanism n SN 1 and SN 2 arguments n active site作用的位置 為Glu 35 and Asp 52 n Cleave the glycosidic C－O bond between NAM and NAG in site D and E 將Glu 35置換為Gln 35， 藉此觀察二 mechanism之 intermediates n 將F原子取代C－ 1 and C－2 上的H， 改變SN 1之reaction pathway n n SN 2 major

Interaction between lysozyme and substrate Lysozyme's active site is a long, deep cleft in the protein surface. The polysaccharid substrate(e. g. NAM-NAG-NAM-NAG) is properly positioned by hydrogen bonding and hydrophobic interactions. **the hydrolysis of the polysaccharid only happen on the C-O bond between 4 th and 5 th sugar units

Q&A n 為什麼lysozyme只作用在第四和第五個 units上? 1. Site D of Peptidoglycan 結構上的改變 促進接上lysozyme(distortion) 2. Lysozyme對trimer, tetramer, pentamer 的match 3. NAM-NAG可切, NAG-NAM不可以切

disease n hereditary amyloidosis n the cause is a mutation in the lysozyme gene, which leads to accumulations of lysozyme in several tissues

Compare with penicillin Lysozyme 切斷處 Penicillin 切斷處