The X-ray structure of hen egg white (HEW) lysozyme. (a) The polypeptide chain is shown with a bound (NAG)₆ substrate (green). The positions of the backbone C_α atoms are indicated together with the side chains of the catalytic residues Asp 52 and Glu 35 (red) and Cys residues that form disulfide bonds (yellow). The substrate's sugar rings are designated A, at its nonreducing end (right), through F, at its reducing end (left). Lysozyme catalyzes the hydrolysis of the glycosidic bond between residues D and E. Rings A, B, and C (darker green) are observed in the X-ray structure of the complex of (NAG)₃ with lysozyme; the positions of rings D, E, and F (lighter green) were inferred from model building studies. [Drawing copyrighted © by Irving Geis.] (b) A ribbon diagram of lysozyme highlighting the protein's secondary structure and indicating the positions of its catalytically important side chains. (c) A computer-generated model showing the protein's molecular envelope (purple dot surface) and C_α backbone (blue). The side chains of the catalytic residues, Asp 52 (above) and Glu 35 (below), are colored yellow. Note the enzyme's prominent substrate-binding cleft. [X-ray structure by David Phillips and Louise Johnson, Oxford University, U.K.]