Muramidase

Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, is an antimicrobial enzyme produced by animals that forms part of the innate immune system. Lysozyme is a glycoside hydrolase that catalyzes the hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in peptidoglycan, which is the major component of gram-positive bacterial cell wall. This hydrolysis in turn compromises the integrity of bacterial cell walls causing lysis of the bacteria. Lysozyme is abundant in secretions including tears, saliva, human milk, and mucus. It is also present in cytoplasmic granules of the macrophages and the polymorphonuclear neutrophils (PMNs). Large amounts of lysozyme can be found in egg white. C-type lysozymes are closely related to alpha-lactalbumin in sequence and structure, making them part of the same glycoside hydrolase family 22. In humans, the C-type lysozyme enzyme is encoded by the LYZ gene. Lysozyme is thermally stable, with a melting point reaching up to 72 ℃ at pH 5.0. However, in human milk it loses activity very quickly at that temperature. Its isoelectric point is 11.35. Lysozyme can survive in a large range of pH (6-9). The enzyme functions by attacking, hydrolyzing, and breaking glycosidic bonds in peptidoglycans. The enzyme can also break glycosidic bonds in chitin, although not as effectively as true chitinases. Lysozymes active site binds the peptidoglycan molecule in the prominent cleft between its two domains. It attacks peptidoglycans (found in the cell walls of bacteria, especially Gram-positive bacteria), its natural substrate, between N-acetylmuramic acid (NAM) and the fourth carbon atom of N-acetylglucosamine (NAG). Shorter saccharides like tetrasaccharide have also shown to be viable substrates but via an intermediate with a longer chain. Chitin has also been shown to be a viable lysozyme substrate. Artificial substrates have also been developed and used in lysozyme. The Phillips Mechanism proposed that the enzyme's catalytic power came from both steric strain on the bound substrate and electrostatic stabilization of an oxo-carbenium intermediate. From X-ray crystallographic data, Phillips proposed the active site of the enzyme, where a hexasaccharide binds. The lysozyme distorts the fourth sugar (in the D or -1 subsite) in the hexasaccharide into a half-chair conformation. In this stressed state, the glycosidic bond is more easily broken. An ionic intermediate containing an oxo-carbenium is created as a result of the glycosidic bond breaking. Thus distortion causing the substrate molecule to adopt a strained conformation similar to that of the transition state will lower the energy barrier of the reaction. The proposed oxo-carbonium intermediate was speculated to be electrostatically stabilized by aspartate and glutamate residues in the active site by Arieh Warshel in 1978. The electrostatic stabilization argument was based on comparison to bulk water, the reorientation of water dipoles can cancel out the stabilizing energy of charge interaction. In Warshel's model, the enzyme acts as a super-sovlent, which fixes the orientation of ion pairs and provides super-solvation (very good stabilization of ion pairs), and especially lower the energy when to ions are close to each other.