REFINED 3-DIMENSIONAL STRUCTURE OF PHYCOERYTHROCYANIN FROM THE CYANOBACTERIUM MASTIGOCLADUS-LAMINOSUS AT 2.7-A

The structure of the phycobiliprotein phycoerythrocyanin from the thermophilic cyanobacterium Mastigocladus laminosus has been determined at 2.7 Å resolution by X-ray diffraction methods on the basis of the molecular model of C-phycocyanin from the same organism. Hexagonal phycoerythrocyanin crystals of space group P63 with cell constants a = b = 156.86 A ̊, c = 40.39 A ̊, α = β = 90 °, γ = 120 ° are almost isomorphous to C-phycocyanin crystals. The crystal structure has been refined by energy-restrained crystallographic refinement and model building. The conventional crystallographic R-factor of the final model was 19.2% with data to 2.7 Å resolution. In phycoerythrocyanin, the three (αβ)-subunits are arranged around a 3-fold symmetry axis, as in C-phycocyanin. The two structures are very similar. After superposition, the 162 Cα atoms of the α-subunit have a mean difference of 0.71 Å and the 171 Cα atoms of the β-subunit differ by 0.51 Å. The stereochemistry of the chiral atoms in the phycobiliviolin chromophore A84 is C(31)-R, C(4)-S. The configuration of the chromophore is C(10)-Z, C(15)-Z and the conformation C(5)-anti, C(9)-syn and C(14)-anti like the phycocyanobilin chromophores in phycoerythrocyanin and C-phycocyanin. © 1990.