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Cysteine‐191 in aspartate aminotransferases appears to be conserved due to the lack of a neutral mutation pathway to the functional equivalent, alanine‐191

It was previously suggested that the conserved Cys‐191 of aspartate aminotransferases (AATases) is conserved, not because it is essential, but because it is frozen in the sequence, with no neutral corridor to traverse to the similar phenotype of Ala‐191 (Gloss et al., Biochemistry 31:32–39, 1992). This hypothesis has now been tested by additional mutations. All possible one‐base mutations from Cys were made at position 191. All of these variants display kinetic parameters (kcat and kcat/KM values) that differ from the wild‐type enzyme by 30% or more. The non‐conserved cysteines that are predominantly Ala in other AATase sequences (Cys‐82, Cys‐192, and Cys‐401) were mutated to Ser to test the corollary that a neutral Cys → Ala corridor does exist for these positions. These Cys → Ser mutations yielded enzymes with wild‐type‐like kinetic parameters. The pKa values of the internal aldimines of the mutants, Cys‐191 → Ser, Phe, Tyr, and Trp are higher than that of wild type by 0.6–0.8 pH units. The stabilities to urea denaturation of the Cys‐191 mutants are similar to that of wild type, while those of the non‐conserved cysteines show greater variation. Examination of the three‐dimensional environment of the five cysteines showed that the van der Waals contacts of Cys‐191 are more conserved than are those of the non‐conserved cysteines. These data provide further support for the above hypothesis.