Three-dimensional structure of actinoxanthin. III. A 4-Å resolution

The protein actinoxanthin (molecular weight 10,300) crystallizes in space group P212121, with cell dimensions a = 30.9 Å, b = 48.8 Å, c = 64.1 Å, and z = 4. The three‐dimensional structure of actinoxanthin at 4‐Å resolution was determined by x‐ray methods on the basis of experimental data from the native protein and five isomorphous derivatives. At the stage of solving the phase problem, the heavy atoms in the derivatives were located using direct methods. The actinoxanthin molecule can be described as an oblate ellipsoid with approximate dimensions 20 × 30 × 40 Å and consists of two different sizes of folded units separated by a well‐defined cleft. The larger unit, including the N‐ and C‐terminals of the protein chain, is characterized by a significant content of β‐sheet structure. The smaller unit, containing two deca‐ and hexapeptide cycles closed by disulfide bonds, has a mainly irregular structure.