Homogenization and crystallization of histidine ammonia-lyase by exchange of a surface cysteine residue.

Histidase (histidine ammonia-lyase, EC 4.3.1.3) from Pseudomonas putida was expressed in Escherichia coli and purified. In the absence of thiols the tetrameric enzyme gave rise to undefined aggregates and suitable crystals could not be obtained. The solvent accessibility along the chain was predicted from the amino acid sequence. Among the seven cysteines, only one was labeled as 'solvent-exposed'. The exchange of this cysteine to alanine abolished all undefined aggregations and yielded readily crystals diffracting to 1.8 A resolution.