B

Figure 11.1 (a) (aP) phycocyanin monomer. The monomer formation surface formed by the X-Y helices of both subunits is identified by the white oval. The white square identifies the position of two conserved usual CPK color code. (b) (aP)6 hexameric form of phycocyanin. In both panels the a and P subunits are depicted in yellow and blue cartoon helices, respectively. The PCB chromophores are shown in sphere representation: aPCB84 in cyan, PPCB84 in red and PPCB155 in orange. Figures 11.1, 11.2 and 11.4 were prepared using Pymol (pymol. sourceforge.net).

Figure 11.1 (a) (aP) phycocyanin monomer. The monomer formation surface formed by the X-Y helices of both subunits is identified by the white oval. The white square identifies the position of two conserved usual CPK color code. (b) (aP)6 hexameric form of phycocyanin. In both panels the a and P subunits are depicted in yellow and blue cartoon helices, respectively. The PCB chromophores are shown in sphere representation: aPCB84 in cyan, PPCB84 in red and PPCB155 in orange. Figures 11.1, 11.2 and 11.4 were prepared using Pymol (pymol. sourceforge.net).

phycobiliprotein modifications: the PThr77

residue (with non standard peptide dehydral angles) and PAsn72 which is methylated.

Amino acid residues are depicted in stick representation and colored according to the composed of two homologous subunits, a and P , known as the (aP) monomer, shown in Figure 11.1a. There are four major forms of PBPs, classified by the number and type of bilin cofactor to which they are associated. These are allophy-cocyanin (APC, Xmax = 652nm), phycocyanin (PC, Xmax = 620nm), phycoerythrin (PE, Xmjx = 560nm), and the infrequently found phycoerythrocyanin (PEC, = 575 nm). The molecular weights ofthe a and P subunits are 16-20 kDa, depending on the type of PBP. The major structural facet of all PBP subunits is a compact globular structure made up of six a-helices (A, B, E, F, F', and H) that are denoted according to their similarity to other members of the globin family (Figure 11.1a). All cofactors are covalently bound to these core helices or to additional loops that have arisen by insertion into the gene sequence. Two additional a-helices (X and Y) extend out from the core and serve as the assembly interface of the monomer and of additional levels of assembly (Figure 11.1a, white circle).

All PBPs, except for a special class of soluble PE found only in cryptomonads (see Section 11.2.3.5), further assemble into (aP)3 trimers. Further levels ofassem-bly of PC, PEC, and PE occur rapidly: (aP)3 trimers can associate into (aP)6 hexam-ers and these then associate further into extended structures called rods. Figure 11.1a and b show a representative PC monomer and hexamer (respectively) from the Thermosynechococcus vulcanus crystal structure at the highest resolution determined to date (1.4 A, deposition into the PDB in progress). The APC trimers associate in a somewhat different manner, forming cylinders containing four

Figure 11.2 Three-dimensional structures of bilin chromophores thio-ether linked to cysteine residues. The coordinates of each bilin structure were carved out of the crystal structures: phycocyanoblins (PCB) from T. vulcanus PC-[1KTP], phycoviolobilin (PVB) from M. laminosus PEC-[1C7L], phycoerythrobilin (PEB) and doubly linked phycourobilin (PUB) from G. monilis PE —[1 B8D]. The positions of the conjugated double bonds systems are denoted by black lines. Note that identical bilins can have different conformations (see two PCBs) and that the orientations of the propionic acid groups is highly variable.

Figure 11.2 Three-dimensional structures of bilin chromophores thio-ether linked to cysteine residues. The coordinates of each bilin structure were carved out of the crystal structures: phycocyanoblins (PCB) from T. vulcanus PC-[1KTP], phycoviolobilin (PVB) from M. laminosus PEC-[1C7L], phycoerythrobilin (PEB) and doubly linked phycourobilin (PUB) from G. monilis PE —[1 B8D]. The positions of the conjugated double bonds systems are denoted by black lines. Note that identical bilins can have different conformations (see two PCBs) and that the orientations of the propionic acid groups is highly variable.

trimers. Two to five of these cylinders pack into what is called the core of the phycobilisome.

All of the cofactors of the phycobilisome belong to the bilin family (Figure 11.2). These molecules are linear tetrapyrroles, which differ by the number and position of the conjugated double bonds that serve to tune the general region of light absorption. Both APC and cyanobacterial PCs contain only phycocyanobilin (PCB) cofactors, which have eight conjugated double bonds, while rhodophyte PC and all PECs and PEs can contain phycoviolobilins (PVB, seven conjugated double bonds), phycoerythrobilins (PEB, six conjugated double bonds), and phycoeurobi-lins (PUB, five conjugated double bonds). Some PVBs, PEBs, and PUBs can be covalently linked to two cysteine residues via a thio-ether linkage, while all PCBs are singly linked. The molecular structures shown in Figure 11.2 have been carved out of the PDP coordinates, as determined by X-ray crystallography of the isolated PBPs (two -dimensional representations of the bilins can be seen in (Grossman, Schaefer, Chiang and Collier, 1993)). In this way, the bent and twisted nature of the cofactors can be appreciated. It can be clearly seen that the degree of linearity, the positions of the propionic acids, and the relative orientation of the bilins with respect to the cysteines (to which each bilin is covalently bound) differ. These structural modifications are of course caused by the polypeptide environment surrounding each bilin. It is apparent that, by use of the open chain structure, the proteins can apply a second level of tuning by bending and twisting the molecules. Such tuning results in a broader range of absorption spectra and, potentially, serves to induce directed energy transfer from the outside of the phycobilisome down into the core and finally into the reaction center (see Section 11.4.2).

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