Conformational preferences of amino acids in globular proteins

In a previous paper [Levitt, M., and Greer, J. (1977), J. Mol. Biol. 114, 181--239], an objective compilation of the secondary-structure regions in more than 50 different globular proteins was produced automatically. In the present paper, these assignments of secondary structure are analyzed to give the frequency of occurrence of the 20 naturally occurring amino acids in alpha helix, beta sheet, and reverse-turn secondary structure. Nineteen of these amino acids have a weak but statistically signficant preference for only on type of secondary structure. These preferences correlate well with the chemical structure of the particular amino acids giving a more objective classification of the conformational properties of amino acids than available before.