ABSTRACT
In the absence of a map of electron density, the molecular structure of a protein is studied by a diverse collection of techniques. These approaches can be conveniently divided into three classes: the use of chemical probes, the use of immunochemical probes, and the use of physical measurements. A strategic distinction, however, exists between covalent modification with chemical reagents and covalent modification by site-directed mutation. A common goal of both chemical modification and site-directed mutation is to correlate the structure of the protein with its function. The advantage of identifying a site of modification chemically is that several different methods of analysis can be applied to the isolated product; the advantages of mass spectrometry are its rapidity and its sensitivity. Site-directed mutation produces the covalent modification of a protein by converting one particular amino acid in its sequence into another of the 20 amino acids.
