Everything about Lysozyme totally explained
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| Name = Lysozyme (renal amyloidosis)
| HGNCid = 6740
| Symbol = LYZ
| AltSymbols =;
| OMIM = 153450
| ECnumber = 3.2.1.17
| Homologene = 37278
| MGIid = 96897
| GeneAtlas_image1 = PBB_GE_LYZ_213975_s_at_tn.png
| Function =
| Component =
| Process =
| Orthologs =
}}
Lysozyme is a family of
enzymes which damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between N-acetyl-D-glucosamine residues in chitodextrins. It is abundant in a number of
secretions, such as
tears,
saliva, and
mucus. Lysozyme is also present in
cytoplasmic granules of the
polymorphonuclear neutrophils (PMN). Large amounts of lysozyme can be found in egg whites. C-type lysozymes are closely related to
alpha-lactalbumin in sequence and structure making them part of the same family.
Physiology
Most of the bacteria affected by lysozyme are not pathogenic. In some cases, lysozyme is a primary reason these organisms don't become pathogenic. Lysozyme can act to some extent as an innate
opsonin, or as an actively lytic enzyme.
Lysozyme serves as a non-specific innate
opsonin by binding to the bacterial surface, reducing the negative charge and facilitating phagocytosis of the bacterium before opsonins from the acquired immune system arrive at the scene. In other words, lysozyme makes it easier for phagocytic
white blood cells to engulf bacteria.
The
enzyme functions by attacking
peptidoglycans (found in the cells walls of bacteria, especially
Gram-positive bacteria) and
hydrolyzing the glycosidic bond that connects
N-acetylmuramic acid with the fourth carbon atom of
N-acetylglucosamine. It does this by binding to the
peptidoglycan molecule in the binding site within the prominent cleft between its two domains. This causes the substrate molecule to adopt a strained conformation similar to that of the transition state.
According to Phillips-Mechanism the lysozyme binds to a hexasaccharide. The lysozyme then distorts the 4th sugar in hexasaccharide (the D ring) into a half-chair conformation. In this stressed state the glycosidic bond is easily broken.
The amino acid side chains glutamic acid 35 (Glu35) and aspartate 52 (Asp52) have been found to be critical to the activity of this enzyme. Glu35 acts as a proton donor to the glycosidic bond, cleaving the C-O bond in the substrate, whilst Asp52 acts as a nucleophile to generate a glycosyl enzyme intermediate. The glycosyl enzyme intermediate then reacts with a water molecule, to give the product of hydrolysis and leaving the enzyme unchanged.
Role in disease
In some forms of
hereditary amyloidosis, the cause is a
mutation in the lysozyme
gene, which leads to accumulations of lysozyme in several
tissues.
Whereas the skin is a protective barrier due to its dryness and acidity, the
conjunctiva (membrane covering the eye) is instead protected by secreted enzymes, mainly lysozyme and
defensin. However when these protective barriers fail,
conjunctivitis results.
History
Alexander Fleming (1881-1955), the discoverer of
penicillin, described lysozyme in
1922.
Its structure was described by
David Chilton Phillips (1924-1999) in 1965 when he got the first 2
angstrom (200
pm) resolution image. This work led Phillips to provide an explanation for how
enzymes speed up a chemical reaction in terms of its physical structures. The original mechanism proposed by Phillips was more recently revised.
Howard Florey (1898-1968) and
Ernst B. Chain (1906-1979) also investigated lysozymes. Although they never made much progress in this field, they developed penicillin, which Fleming had failed to do.
Further Information
Get more info on 'Lysozyme'.
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