Plastocyanin sequences

Fig. 3. Plastocyanin amino-acid sequences relevant to this review. Other sequence information is included. There are now 24 known sequences (18 formerly in Ref. [ I]). The 23 residues invariant throughout all 24 known higher plant and algal plastocyanin sequences are indicated ( ), 5 others (A) are invariant if A. variabilis is excluded, and a further 19 (o) if only the higher plant sequences are considered. Eieletions are indicated ( ), and residues which coordinate the Cu(V)...

Amicyanin appears to be more homologous with the plastocyanin sequence than with the pseudoazurin sequence. 

Plant plastocyanins are synthesized in the cytosol as 160-170-ammo acid precursor polypeptides consisting of a 60-70-residue transit peptide followed by a 97 99-amino acid mature protein. The transit peptide imports the precursor plastocyanin molecule across the chloroplast envelope and thylakoid membranes to its final destination in the thylakoid lumen, where it shuttles electrons by accepting them from the membrane bound cytochrome / (cyt /) of the cyt b6/f complex and donating them to the photooxidized reaction center P700-I- of photosystem I. Cyanobacterial plastocyanins use an 30-amino acid leader seqnence for thylakoid membrane translocation. Currently, there are more than 100 plant and cyanobacterial plastocyanin sequences that are available either by direct protein sequencing or deduced from the nucleotide sequences of their genes. 

From 13 completed amino-acid sequences and 54 partial sequences (>40 residues) of plastocyanins from higher plants it appears that sixty residues are invariant and 7 are conservatively substituted 02,7). With three algal plastocyanins included there are 39 invariant or conservatively substituted groups. It is believed that the same structural features apply to the whole family, and that highly conserved residues are an indication of functional sites on the protein surface. The upper hydrophobic and right-hand-side surfaces are believed to be particularly relevant in this context, the latter including four consecutive... 

D. S. German, R. P. Levine (1965) Cytochrome / and plastocyanin their sequence inthephotosynthetic electron transport chain of Chlamydomonas reinhardtii. Proc. Natl. Acad. Sci. USA, 54 1665-1669... 

It is interesting to note that the plastocyanin from the alga Anabaena variabilis has a similar structure to that isolated from poplar, while having a different amino acid sequence.58 This results in an overall positive charge of +2. Thus, as shown in Figure 32a, it affords a wellshaped response for the Cu(II)/Cu(I) reduction E° = + 0.32 V vs. NHE) using a perpendicular pyrolytic graphite electrode, without the presence of additives. 

A transit peptide consisting of a hydrophobic 66 amino acid long peptide interspersed with positively charged residues has been identified and sequenced [52]. This is initially attached to the 99 amino acids of the mature plastocyanin, and is responsible for taking the plastocyanin across membranes into the thylakoid region of the chloroplast. 

Table 6. Charge distribution on diflerent plastocyanins PCu(I) at pH 7.S, based on a count of acidic (Asp, Glu) and basic (Lys, His, Arg) residues with no allowance for H-bonding. Information obtained from sequences in Fig. 3...

There are surprisingly few examples and considerable care is required. A number of earlier reports have been checked out [90,95,108], and shown to be incorrect with effects certainly not as extensive as claimed. The first plastocyanin example (1978) was the [Co(phen)3] oxidation of parsley PCu(I) [39]. In this study first-order rate constants (k bs) obtained with [Co(phen)3] in > 10-fold excess of PCu(I) ( 10 M) give a non-linear dependence on [Co(phen)3 ] (concentrations of oxidant to 3 x 10 M), Fig. 7. Such behavior can be accounted for by the reaction sequence (2)-(3),... 

Although most authors believe that LHCI polypeptides are coded by a multigene family whose components have been described above, it has recently been reported that the product of psaF, an unrelated gene, previously indicated as plastocyanin binding protein is indeed a chla binding subunit [120] which is part of LHCI. The deduced sequence of the psaF [125] subunit is not related to that of the other antenna polypeptides. 

The salient features of A. faecalis pseudoazurin are that (1) it has a Cu-Met bond length shorter than that of either plastocyanin or azurin (see Table III) (2) it has only one NH - S bond, as does plastocyanin and (3) its overall architecture resembles plastocyanin (see Fig. 4), with an extended carboxy terminus folded into two a helices [a preliminary sequence comparison suggested that the folding would resemble plastocyanin (Adman, 1985)]. It retains the exposed hydrophobic face found in azurin and plastocyanin. Just how it interacts with nitrite reductase is still a subject of investigation. It is intriguing that the carboxy-terminal portion folds up onto the face of the molecule where the unique portions of other blue proteins are the flap in azurin, and, as we see below in the multi-copper oxidase, entire domains. 

Cucumber basic blue protein (Cbp) is a protein without known function, also known as cusacyanin or plantacyanin. Its structure (Guss et al., 1988) completes the repertoire of cupredoxins with known structures. The topology of its folding is similar (Fig. 5) to those of plastocyanin and azurin, as might have been expected from sequence similarities and... 

Amicyanin (Husain and Davidson, 1986 Groeneveld etal., 1988) spectroscopically resembles plastocyanin more than pseudoazurin and has about the same number of amino acids, so that its classiflcation has been changed from subgroup II to III (the plastocyanin group see Table II). However, its sequence is distinctly different than the plastocyanins, and the new function may indicate yet another class. 

The location of the copper with respect to the Greek key fold is interesting when compared to that of the cupredoxins. While the copper in the cupredoxins lies in the interior of the /8 barrel bound by three interior-facing residues of the carboxy-terminal loop in the )8 barrel, and by a histidine in an adjacent strand, the copper in SOD lies on the outside of its jS barrel, bound by one residue from the carhoxy-terminal loop and three from the adjacent strand (cf. Figs. 2c-5c with Fig. 8c.) A structural comparison of plastocyanin and SOD, coupled with sequence alignment of plastocyanin and ceruloplasmin (Ryden, 1988), showed that three of the SOD ligands correspond to putative copper ligands in ceruloplasmin. Why this is so will become more evident after the description of the ascorbate oxidase structure and its relationship to ceruloplasmin. 

It is noteworthy that the proximity of the copper sites in ceruloplasmin, and, indeed, the involvement of most of the correct ligand histidines, were predicted some time ago by Ryden (1982, 1984) strictly on the basis of sequence homologies to plastocyanin. A similar prediction was made for laccase based on sequence similarities around the cysteine regions (Briving et al, 1980). Proximity of the type II site to the type III site (e.g., a trinuclear site) was also predicted by Solomon and co-workers (Allen-dorf et al., 1985 Spira-Solomon et al, 1986) on the basis of spectroscopic analysis of azide binding to laccase. What could not have been foreseen... 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

Amino acid sequence data for many plastocyanins are now available. A large number of residues are conserved,910 particularly among the plastocyanins from higher plants, where about 50% of the residues are the same. Residues 31-44 and 84-93 are highly conserved in all the proteins, and provide the donor groups for copper. 

The azurins are electron-transfer proteins in the respiratory chains of certain bacteria. They have been particularly well studied from Pseudomonas aeruginosa and other pseudomonads, and contain one type 1 copper bound to a single polypeptide chain of molecular weight about 16 000. Amino acid sequence data for a number of azurins show that about one third of residues are conserved. All contain three cysteine residues. Three are also sequence homologies with the plastocyanins. 

Many of the reactions of the plastocyanins and azurins with other redox proteins follow Marcus behaviour.946 These reactions all show a single mechanism of electron transfer, with no kinetic selectivity and no specific interactions between the proteins. The notable exception to this behaviour is cytochrome / (c552), where a specific interaction occurs,934 appropriate for its natural redox partner. Equation (48) represents a probable sequence of electron carriers, although it is difficult to extrapolate conclusions to the membrane-bound proteins. 

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