Fig. 2.

Panel A. Distribution of charged amino acids in the monomer glutamate dehydrogenase from Piroccocus furiosus (Pf) [PDB code 1GTM, chain B], PfGDH. Negatively charged amino acids (Asp,Glu) are colored in red while positively charged amino acids (Lys,Arg,His) are colored in blue. In the monomer of the hyperthermophilic variant PfGHD, 40 salt-bridges are detected, while only 20 are present in the mesophilic variant from Clostridium symbiosum. The inset illustrates the distribution of charged amino acids in the active site. A network of three ion pairs is detected involving lysine 104, which is implicated in the activity of the protein (see Ref.³⁶ for details). Panel B. Designed mutant of the human protein acylphosphatase (AcPh-des). The mutations have been generated via charge optimization⁴¹. Amino acid numbering is in accord with the [PDB code 2K7J]. The mutant is obtained by the introduction of 5 punctual mutations. Three mutations (H61E, E64K, K73E) reverse the wild-type amino acid charge, while other two introduce extra charge (Q51K, N82K). The designed mutant maintains the activity of the wild-type (B.1, activity vs substrate concentration) but is more stable (B.2, heat capacity vs temperature. The peak indicates the melting temperature of the system.). Data in B.1 and B.2 have been extracted from the original work⁴¹ using the software PlotDigitizer.