bridges in bovine ribonuclease A. (b) The bridge between Cys-26 and Cys-84 is shown in stereo. Ref: http://cwx.prenhall.com/horton/medialib/media_portfolio/04.html   2. Methionine (Met, M) is an essential nonpolar amino acid . Methionine and cysteine are the only sulfur containing proteinogenic amino acids. The methionine derivative S-adenosyl methionine (SAM) serves as a methyl