Plastocyanins, conformation

A solution structure of French Bean plastocyanin has been reported by Wright and co-workers,19 using nuclear magnetic resonance techniques described in Section 3.5 of Chapter 3. The structure, determined from a plastocyanin molecule in solution rather than in a solid-state crystal, agrees well with that of reduced poplar plastocyanin X-ray crystallographic structure reported above. Conformations of protein side chains constituting the hydrophobic core of the French bean plastocyanin are well-defined by the NMR technique. Surface side chains show... 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

The solution conformation of plastocyanin from French bean, spinach, and S. obliquus has now been determined from distance and dihedral angle constraints derived by NMR spectroscopy [37,40]. These two-dimensional NMR studies have indicated a well defined backbone conformation, which is very similar to that of poplar PCu in the crystalline state. However, in the case of S. obliquus there are deletions at positions S7 and 58 which influence the shape in the acidic region and in particular close to residues 59-61. The gap which is created is in effect repaired with consequent tightening of the loop 57-62 as indicated in Fig. 5. One of the pronounced bulges at the remote site of poplar and presumably other higher plant plastocyanins is not therefore present in S. obliquus (or plastocyanin from other green algae) [31, 32], as well as parsley... 

Parsley, spinach, French bean, poplar and S. obliquus (but not A. variabilis) conform extensively to the above criteria for reaction at the remote site. There is extensive evidence for cytochrome f reacting at the remote site on plastocyanin. The aromatic residue at 83 would seem to be a prime candidate as lead-in group for electron transfer. Desolvation at the surface around 83, and interaction with an aromatic component on the reaction partner, e.g. the porphyrin ring of cytochrome f, may be important. The exact manner of electron transfer has yet to be confirmed. The distance from the aromatic ring of Tyr83 to the Cu for electron transfer is 12 A. 

Crystallographic analysis has provided us with a detailed structure of hCp on the other hand, essentially all of the structure-function analyses have been done on FetSp. Also, except for the copper site structural homology, the two proteins are quite different. hCp is composed of six plastocyanin-like domains (plastocyanin is a type 1 copper-containing protein) that are arranged in a trigonal array (Zaitseva et al., 1996). One result of this domain replication is a conformational fold that produces a distinct, negatively charged patch on the protein surface adjacent to the catalytically active type 1 Cu(II). This copper atom is in domain 6. (Domains 2 and 4 contain type 1-like copper sites that do not participate in the ferroxidase reaction.) Lindley et al. (1997) have proposed that this... 

Reversible protonation and dissociation of the exposed His ligand have been observed in several BCP in the reduced metal-bound state. Since this protonation renders the proteins inactive, it has been characterized thoroughly (Sykes, 1985, 1991). An active site of 4.9 was determined by NMR for Cu(I) spinach plastocyanin (Markley et al., 1975). The occurrence of this process was conhrmed later by the crystal structure of reduced poplar plastocyanin at low pH (Cuss et al., 1986). Similar equilibria have been characterized in Achromobacter cycloclastes pseudoa-zurin (pA a 4.6) (Dennison et al., 1994b) and in Thiobacillus versutus ami-cyanin (pA a 6.7) (Lommen et al., 1988). In the latter system a lineshape analysis revealed that this His residue, on protonation and detachment from the copper(I) ion, fluctuates between two conformers (Lommen and Canters, 1990). 

Crystal structure information (4) for poplar plastocyanin in the Cud) state at low pH has indicated the existence of two conformers. The... 

In summary. Fig. 9 illustrates the pathway for donation of electrons from PC to P700 through His-87 surrounded by the hydrophobic patch, while PsaF, attached to the major PS-I protein subunits PsaA and PsaB, serves to stabilize plastocyanin through electrostatic interaction between the southern acidic patch of PC and the lysine residues in the N-terminal helix of PsaF and at the same time assures the correct conformation for efficient electron transfer. 

In all three proteins, the type 1 copper is coordinated in a distorted tetrahedron in which the Cu2+ ion is situated 0.35 (Pcy), 0.43 (Paz), and 0.40 A (Acy) above the plane formed by both histidines and cysteine, towards the methionine residue [20]. In azurin, the distance between the methionine ligand and copper ion is markedly larger than for the members of the plastocyanin family, resulting in a more trigonal pyramidal conformation [20,21], a ligand stereochemistry which also occurs in halocyanin [18,94], Other factors influence the redox potentials as well, e.g., the hydrophobicity of the region surrounding the copper center. However, so little is known about the collaboration between the various factors that it is currently not possible to predict accurately the probable redox potentials of type 1 copper proteins. 

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