From Proteopedia

Hen Egg-White Lysozyme

Background

Hen Egg-White Lysozyme is an enzyme that was first described by the Russian scientist P. Laschtchenko in 1909 and its structure was solved for in 1965 by David Phillips via X-ray crystallography (Garrett, & Grisham, 2005). It is a single polymer of 129 residues (14.3kDa in weight) that catalyzes the hydrolysis of the polysaccharide wall of bacterial cells, breaking the β (1-4) linkage between N-acetylmuramic acid (NAM) and N-acetylglucosamine between the D and E sugars. The D and E sugars are the fourth and fifth sugars, respectively, in reference to the six sugars (oligosaccharide), identified by the letters A-F, that function as the ligand and bind to lysozyme. In hens (Gallus gallus), lysozyme is heavily concentrated in the egg white, serving as an anti-biotic as well as a nutrient to the developing eggs ^[1]. Lysozyme is not only found in hen egg-whites but has many homologs that occur in a wide variety of organisms including humans. The gene for lysozyme in hens is expressed in the oviduct and in macrophages, which directly reflects its purposes (i.e. nutrition and defense). The one gene is controlled at the level of transcription by different means for the different locations and resulting functions (Worthington, 2010).

Structure

The Hydrophobic Effect

There are many levels of organization that contribute to protein stability, the strongest of which being the hydrophobic effect. The reason the hydrophobic effect is so powerful in determination of the overall protein structure is because proteins exist and function in solvents. Since the body is mostly water, regions that are polar like water (hydrophilic regions) will congregate near it, while regions that not polar will repel from it (hydrophobic), will try to distance themselves from water by being isolated towards the inside or orienting towards other hydrophobic groups so that they might stabilize. This is all a result of entropy because it takes more energy for water to surround and stabilize hydrophobic regions (Garrett, & Grisham, 2005).

van der Waals Forces

The few hydrophobic residues that do exist on the surface are in the active site and a stabilized by van der Waals forces. Van der Waals forces also exist within the enzyme and stabilize the enzyme of its own need while also stabilizing the bound ligand.

Primary, Secondary, and Tertiary Structure

The primary structure 129 residues of 1hew result in a secondary structure of 5 alpha helices and 5 beta sheets. They are mostly stabilized by hydrogen bonding between secondary elements, nitrogen, oxygen, hydrogen’s on polar particles and water. The tertiary structure of Lysozyme is made up of several bind motifs. An antiparallel β-sheets occurs between a pseudo β -sheet from bases Lys1-Phe3 and Phe38-Thr40. A helix loop helix occurs from Cys80-Leu84, with a loop occurring from Ser85-Ile88 followed by another helix Thr89-Val99. Also, a β-ladder exists from the antiparallel arrangement of 3 β-sheets from Gln41-Thr47, Gly49-Ile55, and Leu56-Arg61. Each other these structures help reduce the strain on the enzyme ^[2].

Disulfide Linkages

Next, we will observe the disulfide bonds, shown as yellow bars with the bases the join noted. They occur between the following Cystine residues: 6 and 127, 30 and 115, 76 and 94, 64 and 80. Give strong support in the enzyme between pieces of the same chain.

Activity

Functional Preferences

The function of lysozyme is optimal under physiological conditions of a pH 6-9 with maximal function observed at pH= 6.2 and temperatures around 37oC. Furthermore, while lysozyme can lyse short saccharides, it is more efficient when cutting 3 repeating NAG-NAM units (Worthington, 2010).

Mechanism

The active site of Lysozyme has a few key components that are integral parts of its catalytic ability. Glu35 acts as an acid, donating an H+ to the O in the glycosidic bond. Asp52 will covalently catalyze the reaction by binding its carboxyl group to the unstable positive ion. Water then enters the system and a hydroxyl group will add to the sugar of the NAM. Both Glu35 and Asp52 will return to their natural states and will continue as catalysts.

References

^[3]

^[4]

1. ↑ http://www.ncbi.nlm.nih.gov/pubmed/1569548?dopt
2. ↑ http://www.ncbi.nlm.nih.gov/pubmed/1569548?dopt
3. ↑ Worthington, Von. (2010). Lysozyme. Retrieved from http://www.worthington-biochem.com/ly/default.html
4. ↑ Garrett, RH, & Grisham, CM. (2005). Biochemistry:The Third Edition. Belmont, CA: Thomson Brooks/Cole.