Structural Biochemistry/Protein function/Lysozyme

Introduction


Lysozymes, also known as muramidase or N-acetylmuramide glycanhydrolase, are small globular protein enzymes composed of 129 amino acid residues. As one of the first enzymes to be studied, Alexander Fleming had shown them to be produced by phagocytes and epithelial cells (Neufeld).They are part of the glycoside hydrolase family, which are known for damaging the cell walls of bacterial cells by catalyzing hydrolysis of 1,4-beta-linkages. In a peptidoglycan, the 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine are hydrolyzed; in chitodextrins, the 1,4-beta-linkages between N-acetyl-D-glucosamine residues are hydrolyzed. Lysozymes can be found in tears, human milk, saliva, and mucus. As such, lysozymes act as part of the body's defense system against bacteria. High concentrations of lysozymes can also be found in egg white. Their ability to break down bacterial cell walls in order to improve protein and nucleic extraction efficiency make lysozymes important proteins in living organisms. In human beings, the LYZ gene is responsible for encoding the lysozyme enzyme.

History
Laschtschenko first discovered lysozymes in 1909, when he first observed the antibacterial property of hen egg whites. However, it wasn't until about a decade later until scientists used the term. In 1922, Alexander Fleming, who also discovered penicillin, observed the antibacterial effect of treating bacterial cultures with nasal mucus from a patient suffering a cold. As discussed above, he noticed that lysozymes were secreted from such places in the body. It wasn't, however, until 1965 that the three-dimensional structure of lysozyme was determined by David Chilton Phillips. Utilizing X-ray crystallography with a 2 angstrom resolution, a hen egg-white lysozyme model was determined, being the first enzyme structure to be observed using X-ray analysis. The lysozyme was the first enzyme structure to be solved with X-ray diffraction tools. It was also the first enzyme to be fully sequenced and determined to contain all 20 common amino acids. And mechanistically, it was the first enzyme to be well studied and understood.

Overview
The best studied lysozymes are from hen egg whites, from which lysozymes are abundant, and bacteriophage T4. This enzyme was the first enzyme to have its structure determined, although crystal structures of other proteins had been determined previously. Lysozymes are often easy to study through X-ray crystallography due in part to their easy ability to be isolated from egg whites and to be crystallized, features used widely in the purification of lysozymes. Lysozymes are a part of the immune system and can protect against E. coli, Salmonella, and also Pseudomonas.

Lysozyme Active Site

Since Fleming's discovery of lysozyme, undoubtedly the most significant contribution to our knowledge concerning this enzyme was the X-ray crystallographic analysis. The X-Ray Crystallography structure of lysozyme has been determined in the presence of a non-hydrolyzable substrate analog. This analog binds tightly in the enzyme's active site to form the ES complex, but ES cannot be efficiently converted to EP. It would not be possible to determine the X-ray structure in the presence of the true substrate, because it would be cleaved during crystal growth and structure determination.

The active site consists of a crevice or depression that runs across the surface of the enzyme. Look at the many enzymes contacts between the substrate and enzyme active site that enables the ES complex to form. There are 6 subsites within the crevice, each of which is where hydrogen bonding contacts with the enzymes are made. In site D, the conformation of the sugar is distorted in order to make the necessary hydrogen bonding contacts. This distortion raises the energy of the ground state, bringing the substrate closer to the transition state for hydrolysis.

Practical Uses
Chicken-type and goose-type lysozyme have a very high anti-bacterial potential mainly against Gram-positive bacteria, this has practical uses in food, pharmaceutical, and medical industries. The lysozyme's antibacterial properties are most highly effective against those Gram-positive bacteria with a cell wall made of a peptidoglycan layer. The properties of the lysozyme have made it possible to be integrated into food packaging materials which can greatly extend the shelf-life of scarcely processed food, protecting it from microbial contamination. It has also been shown to preserve food items such as vegetables, milk, meat, etc. It has been seen to help control the production of lactic acid in the wine-making process. The pharmaceutical industry has seen success with using hen-lysozymes in inflammatory diseases, as well as bacterial and viral diseases.

Reference
Neufeld, Elizabeth. "From Serendipity to Therapy",Annu. Rev. Biochem, 2011.

Lesnierowski G., Cegielska-Radziejewska R., 2012. Potential possibilities of production, modification and practical application of lysozyme. Acta Sci. Pol, Technol. Aliment. 11(3), 223-230.