ISOLATION, CRYSTALLIZATION, CRYSTAL-STRUCTURE ANALYSIS AND REFINEMENT OF CONSTITUTIVE C-PHYCOCYANIN FROM THE CHROMATICALLY ADAPTING CYANOBACTERIUM FREMYELLA-DIPLOSIPHON AT 1.66 A RESOLUTION

Constitutive phycocyanin from cyanobacterium Fremyella diplosiphon (Calothrix sp. PCC 7601) grown in green light, has been isolated and crystallized. The crystals belong to the space group R3 with cell constants a = b = 180.26 A ̊, c = 61.24 A ̊, α = β = 90 °, γ = 120 °. The crystal structure has been determined by Patterson search techniques using the molecular model of C-phycocyanin from the cyanobacterium Agmenellum quadruplicatum. The asymmetric unit of the crystal cell consists of two (αβ)-monomers related by a local dyad. Three asymmetric units are arranged around a crystallographic triad and form an (αβ)6-hexamer, the functional unit in the native antenna rod. The initial structure has been refined in a cyclic manner by energy-restrained crystallographic refinement and modelling until the conventional crystallographic R-factor converged at 18.1% with data to a resolution of 1.66 Å. The molecular structure resembles closely the C-phycocyanins of Mastigocladus laminosus and A. quadruplicatum. The conformation and configuration of the α-84 and β-84 chromophores is very similar to the corresponding chromophores in the trimeric C-phycocyanin of M. laminosus, whereas the β-155 chromophore differs in configuration with C(4)-Z, C(10)-Z and C(15)-Z compared to C(4)-Z, C(10)-Z, C(15)-Z, E. The stereochemistry of the β-155 chiral centres is C(2)-RC(3)-R and C(31)-S, respectively, whereas α-84 and β-84 have C(2)-R C(3)-R and C(31)-R. The amino acid sequences of constitutive and inducible phycocyanin differ mainly in residues located on the surface of the β-subunits that mediate the inter-hexameric contacts. © 1991.