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Lysozyme
醫學二
生化第五組
structure
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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
chitin, which is a polymer of NAG linked
by ß(1-4) glycosidic bonds. Chitin is
found in crustacean(甲殼綱) shell tissue
 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.
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function
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Abundant in a number of secretions, such as
tears, saliva, and mucus. It also can be found
in egg whites.
 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)。
 切割peptidoglycan
glycosidase or glycohydrolase(catalyze the
transfer of a glycosyl group to water:C-O bond
broken)
function

catalyzing hydrolysis of 1,4-beta-linkages
between N-acetylmuramic acid (NAM)and Nacetyl-D-glucosamine (NAG)residues in a
peptidoglycan.
 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
Mechanism
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SN1 and SN2
arguments
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active site作用的位置
為Glu35 and Asp52
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Cleave the glycosidic
C－O bond between
NAM and NAG in site
D and E
將Glu35置換為
Gln35，藉此觀察二
mechanism之
intermediates
 將F原子取代C－
1and C－2 上的H，
改變SN1之reaction
pathway


SN2 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-NAM-NAG) is properly
positioned by hydrogen bonding and hydrophobic
interactions.
**the hydrolysis of the polysaccharid only
happen on the C-O bond between 4th and
5th sugar units
Q&A
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
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disease
hereditary amyloidosis
 the cause is a mutation in the lysozyme
gene, which leads to accumulations of
lysozyme in several tissues
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Compare with penicillin
Lysozyme 切斷處
Penicillin 切斷處