Biliproteins

The biliprotein chloroplast pigments are found only in certain algae: the Rhodophyta, Cryptophyta and Cyanophyta. They are either red (phycoerythrins, phycoerythrocyanin) or blue (phycocyanins, allo-phycocyanins) in colour. They have been reviewed by Bogorad (1975), Gantt (1975, 1977), O'Carra and O'h Eocha (1976), Glazer (1981, 1985), MacColl and Guard-Friar (1987), Rowan (1989) and Mimuro and Kikuchi (2003).

The biliproteins of the red and blue-green algae are closely related and we shall consider them together. They are divided into four classes on the basis of the position of their absorption bands: these are, in order of increasing wavelength, the phycoerythrins, the phycoerythrocyanins, the

200 4iM SQO HO 71)0

Wavrlrnnrh Km)

Fig. 8.14 Absorption spectra of biliproteins (after O'h Eocha, 1965). (a) R-phycoerythrin from Ceramium rubrum. (b) R-phycocyanin from Porphyra laciniata.

200 4iM SQO HO 71)0

Wavrlrnnrh Km)

Fig. 8.14 Absorption spectra of biliproteins (after O'h Eocha, 1965). (a) R-phycoerythrin from Ceramium rubrum. (b) R-phycocyanin from Porphyra laciniata.

phycocyanins and the allophycocyanins. The position of the main absorption peaks are listed in Table 8.4, and absorption spectra of a red-algal phycoerythrin and phycocyanin are shown in Fig. 8.14.

The general principle governing the distribution of these pigments appears to be that allophycocyanin and one or other of the phycocyanins occur in all red and blue-green algal species: a phycoerythrin (or phycoer-ythrocyanin in some blue-green algae) may or may not, in addition, be present. Phycoerythrin is rarely absent in the red algae but is frequently absent in the blue-green algae. The typical situation is that in the red algae, phycoerythrin constitutes most of the biliprotein present, whereas in the blue-green algae, phycocyanin or (less commonly) phycoerythrin (or phycoerythrocyanin) is the major component: allophycocyanin is nearly always a minor component.

Phycoerythrobilin (Cysteine residue)

HOOC COOH

HOOC COOH

Phycocyanobilin (Cysteine residue)

Fig. 8.15 Structures, and probable mode of binding to protein, of the phycobilin chromophores of algal biliproteins.458'689 The systems of conjugated double bonds, upon which the spectral properties of the chromophores depend, are emphasized by heavy lines. Phycourobilin is likely to have CH2 rather than CH groups joining rings A to B and C to D. Linkage to protein cysteine can also occur through the vinyl/ethyl group on ring D.458

The chromophores to which biliproteins owe their colour are open-chain tetrapyrrole compounds known as phycobilins. There are four main chromophores - phycocyanobilin (blue), phycoerythrobilin (red), phycourobilin (yellow) and phycoviolobilin (purple) - also known as crypto-violin. Unlike chlorophylls and carotenoids, they are covalently bound to their proteins. Their structures and modes of linkage to the apoproteins are shown in Fig. 8.15.

All the algal biliproteins contain equimolar amounts of two kinds of subunit, referred to as a and b: some of the phycoerythrins contain in addition much lesser amounts (one molecule for every six of a or b) of a third class of subunit, g. The a and b subunits have molecular weights of about 17 to 22 kDa but the g subunit may be larger. Each a or b subunit in a biliprotein has at least one phycobilin chromophore attached. The a subunit can have one or two chromophores; the b subunit has one, two or three; and the g subunit has four. The distribution of chromophores among the sub-units in the different proteins is shown in Table 8.5. This distribution may not be immutable. In phycoerythrin from the red alga Callithamnion roseum it seems that the proportion of phycoerythrobilin to phycourobilin varies with light intensity during growth:1495 it is possible that at some sites within the protein, either of these two chromophores may be attached.

Biliproteins occur as aggregates of the basic ab structure. In the isolated, soluble state, various sizes are found of which the most common

Table 8.5 Spectroscopic properties of the different biliproteins and the distribution of phycobilin chromophores among their subunits.457'459'1002'1008'

17,1018,1442,458,841,842,844

Table 8.5 Spectroscopic properties of the different biliproteins and the distribution of phycobilin chromophores among their subunits.457'459'1002'1008'

17,1018,1442,458,841,842,844

Biliprotein

Algal typea

Absorption maxima in photosynthetic rangeb (nm)

a-subunitc

b-subunitc

g-subunitc

Red and blue-green

algae

Allophycocyanin

R, B

650, (620)

1 PCB

1 PCB

-

Allophycocyanin-B

R, B

671, 618

1 PCB

1 PCB

-

C-Phycocyanin

R, B

620

1 PCB

2 PCB

-

R-Phycocyanin

R

618, 553

1 PCB

1 PCB,

-

1 PEB

Phycoerythrocyanin B

(590), 568

1 CV

2 PCB

-

C-Phycoerythrin

B

562-565

2 PEB

3 PEB

-

CU-Phycoerythrin

B

490-500, 545-560

2 PEB

3 PEB,

-

1 PUB

b-Phycoerythrin

R

(563), 545

2 PEB

4 PEB

-

B-Phycoerythrin

R

(565), 546, (498)

2 PEB

3 PEB

2 PEB,

2 PUB

R-Phycoerythrin

R

568, 540, 598

2 PEB

2 PEB,

1 PEB,

1 PUB

3 PUB

Cryptophytes

Phycocyanin-615

615, 585

1 PCB

2 PCB,

-

1 CV

Phycocyanin-630

630, 585

n.d.

n.d.

-

Phycocyanin-645

645, (620), 585

1U

2 PCB,

-

1 CV

Phycoerythrin-544

(565), 544

2 PEB,

PEB

-

2 CV

Phycoerythrin-555

555

PEB

PEB

-

Phycoerythrin-568

568

PEB

PEB

-

a R, red algae; B, blue-green algae. b Minor maxima or shoulders are in brackets.

c PCB, phycocyanobilin; PEB, phycoerythrobilin; PUB, phycourobilin; CV, cryptoviolin; U, unknown phycobilin chromophore, distinct from the previous four, with peak at 697 nm. The absence of a number means that the number of chromophores is unknown. n.d., no data.

appear to be the hexamer a6 b6 and the trimer a3 b3. Those phycoerythrins that have a g subunit are likely to occur as a6 b6g aggregates in the isolated state. In vivo, biliproteins occur as much larger aggregates, the phycobilisomes, particles of diameter 30 to 40 nm attached to the outer surface of the thylakoids (Fig. 8.2). These are ordered, specific structures

Phycobilisome
Fig. 8.16 Schematic representation of phycobilisome structure (after Glazer, 1985; Gantt, 1986; Glazer and Melis, 1987).

rather than random aggregates, and it seems likely that the three or so different biliproteins present in the alga are all present in each phycobilisome, together with small amounts of some colourless proteins. Allophycocyanin, which as we shall see later transfers the energy harvested by the phycobilisome to chlorophyll, is believed to be located at the base of the particle, adjoining the membrane: the other, more plentiful proteins form a shell around the allophycocyanin, with the phycocyanin being next to the allophycocyanin and the phycoerythrin or phycoerythrocyanin forming the periphery of the particle. Electron microscope and biochemical studies have given rise to a model of phyco-bilisome structure along the lines of that shown schematically in Fig. 8.16. In the chlorophyll d-containing prokaryote, Acaryochloris marina, the biliproteins (phycocyanin and allophycocyanin) exist as rod-shaped structures located on the cytoplasmic side of the thylakoid membrane.594

Turning to the Cryptophyta we find some similarities to, as well as marked differences from, the situation in the Rhodophyta and Cyano-phyta. The cryptophytan pigments are either red or blue and accordingly are classified either as phycoerythrins or phycocyanins. Each cryptophyte species appears to contain only one biliprotein. No allophycocyanin-like pigments have yet been found in this group. The main absorption peaks are in the 612 to 645 nm range in the case of the phycocyanins, and the 544 to 568 nm range in the case of the phycoerythrins. The chromophores so far detected in these proteins are phycoerythrobilin, phycocyanobilin, cryptoviolin (phycoviolobilin) and an unknown phycobilin with an absorption peak (in acid urea) at 697 nm.580

The cryptophyte biliproteins appear to be organized on the same aft subunit principle as the red and blue-green algal proteins: in the isolated state they exist predominantly as a2 b2 dimers. The subunits, at 10 and 17.5 kDa, respectively appear, however, to be smaller than those in the other algae, and there are two types of a subunit so that the dimer composition is better represented as aa'b2.513 Where they differ most strikingly from the corresponding proteins in the other algae is their location in the photosynthetic apparatus: instead of being present as phycobilisomes on the surface of the thylakoids, the cryptophyte biliproteins occur as a dense granular matrix filling the interior of the thylakoid (Fig. 8.3).

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