FIGURE 4 (a) Pairs of Cys residues (numbered for their position in the primary sequence) were engineered into T4 lysozyme. This mutant has three disulfide bonds. (b) Additional disulfide bonds stabilize protein structure relative to wild-type lysozyme. The numbers below the plot indicate the positions of the Cys residues involved in the disulfide bonds. The rightmost column shows the results for the three-disulfide mutant protein in (a). The stability of these proteins is indicated by the temperature at which the protein loses its activity, compared with wild type.