The specific activity of HheG A221C was also reduced (75% of wild-type activity), whereas product enantiomeric excess and Tm were similar to that of HheG wild type. Since HheG A221C was the only variant with a cysteine pair for cross-linking in the tangential crystal interface, this variant was included in further studies, despite its slightly reduced specific activity. Both variants, D114C and A221C, as well as wild-type HheG, were further compared regarding their crystallization properties.