Über die Bindungen zwischen Chromophor und Protein in Biliproteiden, V.Stereochemie von Modell-Imiden

Refined three-dimensional structures of two cyanobacterial C-phycocyanins at 2.1 and 2.5 A resolution. A common principle of phycobilin-protein interaction

The crystal structure of the light-harvesting protein-pigment complex C-phycocyanin (C-PC) from Mastigocladus laminosus (at 2.1 A resolution (1 A = 0.1 nm] has been refined by energy-restrained least-squares methods to a conventional R-factor of 21.7%. In the same way, the crystal structure of C-PC from Agmenellum quadruplicatum has been refined further (2.5 A, R = 18.4%); pyrrole rings C and D of the chromophore at position A84 have been corrected with respect to the previously reported structure. The two C-PC structures are very similar, 213 C alpha positions have a root-mean-square deviation of 0.49 A. Polar and ionic side-chain interactions are discussed in detail and the two subunits of C-PC from M. laminosus are compared to each other. All three chromophores are completely defined and their tetrapyrroles exhibit very similar geometry. The structure of a C-PC chromophore resembles a cleaved porphyrin which has been twisted roughly 180 degrees around the C-5-C-6 and C-14-C-15 bonds. Accordingly, the configuration/conformation of the chromophores is Z-anti, Z-syn, Z-anti (with the exception of the "configuration" of C-15 of chromophore B155, which is almost midway between Z and E). The three chromophores interact similarly with the protein. They arch around aspartate residues (A87, B87 and B39), and the nitrogens of pyrroles B and C are within hydrogen-bonding distance of one of the carboxylate oxygens. Most of the propionic side-chains of the chromophores form salt bridges with arginine and lysine residues. The updated relative chromophore distances and orientations confirm our conclusion that hexameric aggregates are probably the basic functional units, and that inter-hexameric energy transfer takes place preferentially via the central B84 chromophores.

Hydrophobic properties of phytochrome as probed by 8-anilinonaphthalene-1-sulfonate fluorescence

8-Anilinonaphthalene-1-sulfonate (ANS) complexes with phytochrome, exhibiting a higher affinity for the Pfr form of phytochrome than for the Pr form. ANS fluorescence is enhanced by the additional binding of ANS to Pfr upon transformation of phytochrome from Pr to Pfr. The specific site of ANS binding appears to be the hydrophobic surface area of the protein, which becomes at least partially exposed in the Pfr form. An exposed, hydrophobic surface area in the Pfr phytochrome has been confirmed by the effects of ANS on the phototransformation of phytochrome. ANS accelerates the Pr leads to Pfr phototransformation, and it inhibits Pfr leads to Pr photoreversion and dark reversion. These effects are interpretable in terms of competitive binding of ANS to the chromophore binding site. Binding of ANS results in a drastic bleaching of the chromophore's absorption bands at 660 and 730 nm, particularly of the latter. This can be attributed to the exposed chromophore, which tends to resume a cyclic conformation with concomitant blue shift and hypochromism of the Qy bands. Sodium dithionite counteracts the inhibitory effects of ANS on the dark reversion of Pfr to Pr, and its effect on the biphasic kinetics of the reversion has been discussed in terms of the Pfr model proposed.