Gallus gallus (chicken)
Bacteria and human beings are distinctly different life forms. Nevertheless the human body contains many different kinds of bacteria in large numbers. Bacteria in the stomach can help digest food and yoghurts for instance are carefully prepared cultures of bacteria. Sometimes however bacteria can be deadly, like the O-153 mutant of the common bacterium Escherichia coli. Chemicals are needed to fight bad bacteria and such chemicals are called bactericides. When Alexander Fleming was searching for such compounds, he accidentally discovered one such reagent occurring naturally in the mucus of the nose. This compound was a protein, called lysozyme.
Lysozyme is a fairly small protein shaped liked a disk with a diameter of about 40 A and a thickness of about 20 A. It is an enzyme, that means, it catalyzes, speeds ups chemical reactions. It was only the third protein whose structure was determined and the first structure ever of an enzyme. The first two proteins whose structures were determined, contained only alpha-helices and irregular structure. The structure of lysozyme for the first time showed a new kind of arrangement of atoms. While most of its atoms are arranged in helices and loops, it also contains another type of arrangement, called a beta-sheet, in this case a 3-strand anti-parallel beta sheet. Lysozyme is an enzyme of a type called a hydrolase, that means it cuts chemical bonds between atoms. Bacteria protect themselves by a wall made of pieces of peptides and chains of sugar. Lysozyme is able to break this wall by cutting chemical bonds between sugar atoms. After these bonds are cut, the wall breaks, the interior of the bacterium spills out and the bacterium is dead. Lysozyme for the first x-ray studies was obtained from hen egg white, but it can be found in many other places and many organisms have their own lysozymes. It can be easily obtained pure in large amounts from natural sources, making it a very popular target for x-ray studies. It has probably been studied more exhaustively by protein crystallographers than any other protein. A recent survey of the PDB shows more than 700 entries for lysozyme. In possibly the most exhaustive structural study ever performed, the laboratory of Brian Matthews at the University of Oregon determined the structures of several hundred mutants of T4 bacteriophage lysozyme. The goal of this study was to analyze how individual amino acid changes affect the structure and stability of a protein.
Protein Data Bank (PDB)
author: Arno Paehler