The Structure of Rhodanese at 4Å Resolution. The Conformation of the Polypeptide Chain

Rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1), a dimeric enzyme of MW 37,000 was crystallized from an ammonium sulfate solution. The crystals belong to space group C2 with a = 156.1 ± 0.2Å, b = 49.0 ± 0.05Å, c = 42.2 ± 0.1Å and β = 98.50 ± 0.05°. Three dimensional X‐ray data to a resolution of 3.9Å were collected from crystals of the native protein, a mercury‐, and a platinum‐containing heavy atom derivative. The phases of the protein reflexions were determined by the isomorphous replacement method, including anomalous scattering data. In the resulting electron‐density map the course of the polypeptide chain could be followed in both monomers. Six short helices and what seems to be a three strand, parallel twisted pleated sheet are found in each. The monomers are related by a pseudo‐dyad or near‐dyad, which, however, is not obeyed by the chain ends. The carboxyl terminus of one monomer and the amino terminus of the other one are in close contact and obviously form a salt bridge. A correlation with the presently available data of the amino acid sequence cannot yet be made.