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THEJOURNAL
OF BIOLOGICAL
CHEMISTRY
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Vol. 264, No. 1, Iseue of January 5, pp. 1-4, 1989
0 1989 by The American Society for Biochemistry and Molecular Biology, Inc.
Printed in U.S.A.
complex containing hundreds of chromophores to be delivered to a
reaction center with an efficiency exceeding 95%.
Light Guides
Bilins
DIRECTIONALENERGY TRANSFERIN A
PHOTOSYNTHETIC ANTENNA*
Red algae and cyanobacteria may appear blue-green, purple, red,
orange, or yellow in color. The pigmentation is a composite of contributions from the chlorophyll a and @-caroteneinvariably present
inthese organisms and of the predominating phycobiliprotein(s)
which they may contain. The distinctive colors of different phycobiliproteins are in turn dependent on the nature of the covalently
attached tetrapyrrole (bilin) prostheticgroups. The structures of the
bilins are shown in Fig. 1 in colors which approximate closely those
of neutral solutions of the appropriate bilin peptides. Two important
points emerge from examination of these structures. The compounds
shown in Fig. 1 are isomers differing only in the arrangement of
double bonds. The diverse colors generated in this simple manner
span almost the total range of visible wavelengths from a maximum
at 495 nm for phycourobilin (PUB)' to one a t 620 nm for peptidebound phycocyanobilin (PCB). The second point is that the bilins
are in each instance bound to cysteinyl residues through ring A and
that bilins linked through both rings A and D to cysteinyl residues
10 residues apart in the linear amino acid sequence are also present.
The most important way in which the diversity of phycobiliprotein
spectra is generated is by variation in the structure of the bilin
prosthetic groups.
Alexander N. Glazer
From the Department of Microbiology and Immunology,
University of California, Berkeley, California 94720
Light Harvesting in Photosynthesis
The light reactions of photosynthesis begin with the absorption of
a photon by a pigment-protein (antenna) complex. In a series of
radiationless transfers the excitation energy is conveyed from one
light-harvesting chromophore to another ending at a special pair of
chlorophyll (or bacteriochlorophyll) molecules within a transmembrane reaction center complex. Its arrival promotes a separation of
charge which is stabilized by several consecutive electron transfer
steps within the reaction center.
In general, each reaction center is associated with an antenna
complex containing several hundred light-harvesting pigment molecules. The requirement for a large antenna is evident from the
following considerations. The light reactions described above are
s, whereas about 0.02 s are required for the
complete in about
completion of the dark reactions of photosynthesis. The frequency of
absorption events ( n )for a molecule is determined by the equation n
= 4 X lo-'' EN,,., where t is the molar extinction coefficient/molecule
and Nhuis the light flux in quanta/(s .cm2). Indirect sunlight a t noon
the light flux is 1017visible quanta/(s. cm2).Assuming an average of
3 X lo' for visible light for antenna pigment molecules such as
chlorophylls and bilins, one molecule would absorb about 12 quantal
s. Since the dark reactions of photosynthesis require 8 quanta/0.02 s
(l),photosynthesis would be light-limited even under such very
intense light flux unless some 34 accessory pigment molecules were
able to supply a reaction center. Adaptations to low light intensity
can be dramatic. Green sulfur bacteria have over 2000 antenna
bacteriochlorophyll molecules associated with each reaction center.
Clayton (2) noted that a culture of these organisms ". . . growing in a
30-liter jug, illuminated only by a 25-watt tungsten lamp, soon resembles liquid spinach."
Different photosynthetic organisms have distinctive antenna and
reaction center structures (3). Clarification of many features of the
conversion of excitation energy to electron flow within reaction
centers has resulted from the elegant crystallographic and spectroscopic studies of such complexes from purple bacteria (4, 5). Comparable information is now accumulating on a light-harvesting complex, the phycobilisome,present incyanobacteria ("blue-green algae")
and red algae. The purple bacterial reaction center has served as a
paradigm for reaction centers of higher plants (6). Inthe same
manner, the understanding of the principles that govern the structure
and function of the phycobilisome can guide the study of other lightharvesting assemblies.
The greatest diversity of antenna pigments is seen among the
marine algae (2, 3, 7). For example, phycobiliproteins play this role
in red algae and in cyanobacteria, while brown algae, diatoms, and
dinoflagellates utilize protein complexes with carotenoids such as
fucoxanthin and peridinin. The value of these particular complexes
can be readily rationalized on an ecological basis. Their absorption
spectra lie in the region of maximum light transmission through
seawater (7). Phycobiliprotein-containing organisms show the greatest range of spectral variation in their antenna pigments and the
greatest adaptabilityto thequality and quantity of incident radiation.
In this reviewwe examine the structural features that permit an
excitationquantum absorbed at any location within anantenna
Phycobiliproteins
The majority of antenna complexes and all reaction center complexes are membrane proteins. In contrast, the phycobiliproteins are
highly water-soluble. They are extraordinary in yet another respect.
The chromophores of all other photosyntheticcomplexes are extractable by organic solvents, but, as noted above, those of the phycobiliproteins are covalently attached to the polypeptides. The unusual
properties of these intensely colored brilliantly fluorescent proteins
did not go unnoticed by early investigators. A comment made by
Sorby in 1877 (11) "It would be difficult to find another series of
colouring matters of greater beauty or with such remarkable and
instructive chemical and physical peculiarities" has not been challenged. The phycobiliproteins are an artifact of purification. In intact
cells they are components of a large macromolecular structure which
breaks down as soon as the cells are ruptured unless very special
precautions are taken to stabilize the phycobilisomes (12). Phycobiliproteins should therefore be regarded as highly ordered subassemblies which preserve to a varying degree the supramolecular structure
which they possessed in the intact phycobilisome (8).Certain properties of these building blocks can be clearly defined.
The phycobiliproteins each consist of two dissimilar polypeptide
chains, a and @,
of approximately 17 and 18 kDa, respectively. Each
polypeptide chain carries one or more covalently attached bilins. The
amino acid sequences of numerous phycobiliproteins have been determined either directly or from the DNA sequence of the relevant
genes. The sequences of the a and @ subunits of a given phycobiliprotein are related to each other and to those of the corresponding
subunits of other phycobiliproteins (13, 14). These proteins have all
evolved from a common ancestral gene. Purification of native phycobiliproteins by conventional methods frequently leads to the isolation of either a trimeric, ((Y@)B, or hexameric, (CY&, complex. Some
phycobiliproteins are isolated either as a@ or (a@)2
forms or as
equilibrium mixtures of varying aggregates. The outcome depends on
the purification protocol and on the organismal source of the protein
(15). Certain phycoerythrins are isolated as assemblies with the
composition (a@)67,where the y subunit has a molecular weight of
30,000. Phycobiliprotein trimers are disc-shaped with a thickness of
30 A and a diameter of 120 A the hexameric molecules are face-toface dimers of the trimeric assemblies. Evidence from electron microscopy and from a variety of spectroscopic studies indicates that
* This work wassupported in part by National Science Foundation
Grant DMB 85-18066 and National Institutes of Health Grant GM
28994.
' The abbreviations used are: PUB, phycourobilin; PCB, phycocyanobilin; PEB, phycoerythrobilin.
1
Minireview: Directional Energy Transfer in Phycobilisomes
L
A
Peptide-linked
PHYCOCYANOBILIN
possible to compare the position of the bilin attachment sites in
proteins with increasing numbers of bilins/subunit (see Fig. 2). Allophycocyanin has the smallest number of bilins, one PCB/subunit.
These are attached a t a-84 and 8-84 (C-phycocyanin residue numbering is used throughout, see Ref. 13). The three PCBgroups in Cphycocyanin are a t a-84, 8-84, and 8-155. C-phycoerythrin carries
five PEB groups a t or-84, a-l43a, 8-84, 8-155, and 8-50,61 (linked
through both rings A and D) (16). The a-143a attachment site is on
a short inserted sequence; the 8-50,61 site appears to have been
generated by amino acid substitutions. Conservation of sites of attachmentis seen in all of the phycobiliproteins. The important
finding is that the
location of the bilin residue in theprimary structure
is fmed whereas the chemical nature of the bilin a t a given site differs
from one phycobiliprotein to another.
Peptide-linked PHYCOBlLlVlOLlN
C
Phycobilisome Structure
and Assembly
)tide-
1
“
Peptide-linked
PHYCl
FIG. 1. Bilin structures and bilin-peptide linkages in phycobiliproteins (see Refs. 8-10 for complete references).
PROTEIN
TYPF OF R I U
BILINS/(ap)3
PCB
PEB
PXB
PUB
I
I
In intact cells, phycobilisomes form regular arrays on the cytoplasmic face of the thylakoid membranes. The morphology of phycobdisomes varies with the source organism (17-20). The particles
may be ellipsoidal, hemidiscoidal, or bundles of rod-shaped elements.
These differences in gross morphology do not reflect fundamental
differences in the placement of the major phycobiliproteins or in the
functional properties of this particle. Phycobilisomes range in size
from about 7 X lo6 to about 15 X lo6 daltons. The particles may be
detached intact from the thylakoid membranes with detergents such
as Triton X-100 in high concentrations of NaK-phosphate buffers,
0.65-1.0 M,pH 7-8, and purified by sucrose density gradient centrifugation (12). The particles are stable only in the presence of such
high concentrations of NaK-phosphate or sodium sulfate. Isolated
phycobilisomes retain full functional integrity as assessed by comparison of the kinetics of energy transfer from one phycobiliprotein
to the next in the purified particles and in intactcells (e.g. Refs. 21
and 22).
Hemidiscoidal phycobilisomes are common among cyanobacteria
and one such phycobilisome, that of Synechocystis PCC6701,is described here. This phycobilisome consists of two morphologically
distinct domains-a triangular core made up of three cylindrical
elements from which emana? six rods (Fig. 3). Each core element is
made up of four 30 X 115-A discs, whereas the rods are stacks of
60 X 120-A double discs. Detailed analyses of incomplete phycobilisomes from mutant cyanobacteria, of phycobilisome subassemblies
isolated by partial dissociation, and of structures formed in in vitro
reconstitution experiments have led to the picture of phycobilisome
structure shown in Fig. 4, which details the correspondence between
the morphological elements revealed by electron microscopy (Fig. 3)
ll
6
6
0
B
8
16
BILINS/(
34
34
FIG. 2. Bilin compositionandcontent
in variousphycobiliproteins. The colors in the fist column approximate closely
those of dilute solutions of each of the phycobdiproteins. The number
of bilins given in the vertical colored columns is per a8 monomer,
except for B- and R-phycoerythrii where the numbers given are per
(&)gy.
Allophycocyanin B and allophycocyanin have one bilin/subunit, while C- and R-phycocyanin and phycoerythrocyanin have one
bilin on the a subunit and two bilins on the 8 subunit. C-, B-, and Rphycoerythrins have two bilins on the a subunit and three on the 8
subunit. The y subunit of B- and R-phycoerythrins carries four bilins.
PXB, cryptoviolin.
these complexes share common structural featureswith those within
phycobilisomes.
Fig. 2 shows the number and type of bilins carried by various
phycobiliproteins. The color of a given phycobiliprotein is not determined solely by the chemical nature of ita bilins. For example, Cphycocyanin and allophycocyanin both carry only PCB groups, yet
C-phycocyanin has .a, X a t 620 nm and allophycocyanin at 650 nm.
The spectroscopic properties of each bilin within a phycobiliprotein
are strongly influenced by the conformation and environment imposed on the bilin by the native protein. Inter-a8 interactions also
make an important contribution. For example, monomeric (a8)allophycocyanin has a, X a t 615 nm whereas, X of ab)^ lies at 650
nm.
When the amino acid sequences of the polypeptide chains of the
phycobiliproteins are aligned so as to maximizehomology, it is
FIG. 3. Electronmicrograph of Synechocystis PCC6701
phycobilisomes negatively stainedwith uranyl formate. The
width of a rod substructure is 120 A.
Minireview: Directional Energy Transfer in Phycobilisomes
A
'
\
I
I
ROD COMPLEXES
C is mode up of 2 copies each of
FIG.5. Crystal structure of C-phycocyanin.Structure of the
Mastigoclodus laminosus C-phycocyanin trimer projected down the
crystallographic 3-fold axis. C" backbone (a-subunit, brown; &subunit, green) and PCB chromophores (a-84, violet; 8-84, pink; 0-155,
blue) are shown (courtesy of Dr. R. Huber; see Ref. 28).
CORE COMPLEXES
FIG.4. Schematic representation
of the phycobilisome from protein is long. In contrast, the preferred conformation of bilins in
Synechocystie PCC6701. A rod is made up of hexameric bilipro- free solution is cyclohelical (8, 30). The latter conformation is chartein complexes, each of which is attached through its specific linker
polypeptide to the component adjacent to it in the phycobilisome.
The abbreviations A P , PC, and PE are used for allophycocyanin,
p", etc. for the a and 8
phycocyanin, and phycoerythrin, and 8,
subunits of these proteins. Linker polypeptides are abbreviated L,
with a superscript denoting the apparent size X lo3 daltons, and a
subscript that denotes the location of the polypeptide: R, rod
substructure; RC, rod-core junction; C, core; CM, core-membrane region
(modified from Ref. 8).
and their polypeptide composition (for reviews, see Refs. 8, 14, 23,
and 24). Results of analyses of phycobilisomes assembled by organisms in which individual polypeptide components have been deleted
by insertional inactivation of structural genes are fully consistent
with this structure (25). The assembly of the phycobilisome is mediated by a group of polypeptides named "linker" polypeptides (26,
27). As indicated in Fig. 4, each of the trimeric or hexameric subassemblies of the phycobilisome contains at least one specific linker
polypeptide. Each of the four rod liiker polypeptides plays several
roles. It determines the type and location of the phycobiliprotein
within the rod, its aggregation state, andmodulates ita spectroscopic
properties (see below). Inspection of Fig. 4 shows that the order of
565 nm), phycocythe major phycobiliproteins, phycoerythrin (X,
anin (Amm 620 nm), and allophycocyanin (Arna 650 nm), corresponds
to that of spontaneous excitation energy flow (from higher to lower
energy transitions) with phycoerythrin a t the periphery of the rods
and allophycocyanin in the core.
Three-dimensionalStructure and Pathways of Energy Flow
in Phycobiliproteins
The determination of the crystal structure of C-phycocyanin in
both the trimeric (Fig. 5) and hexameric assembly forms has provided
a basis for the understanding of the functional properties of the
phycobiliproteins (28, 29). An extraordinary fiiding is that each of
the three bilins within an a8 monomer interacts with an aspartyl
residue in an identical manner. The protein modifies the spectroscopic properties of the bilins in two ways critical to their role in
light absorption and transfer: (a) the
bilins in C-phycocyanin are held
rigidly in extended conformations; (b)the excited state lifetime of the
acterized by a strong near-UV absorption band and weaker absorption
in the visible. The reverse is true for the extended conformations,
such as those seen in C-phycocyanin, where the strongest absorption
band is in the visible. Free bilins (and bilins in denatured phycobiliproteins) fluoresce very weakly. This indicates that the excitation
energy is lost rapidly by radiationless relaxation pathways. The
radiationless de-excitation pathways (twisting about bonds, photoisomerization, and intersystem crossing) are minimized by the environment and rigid framework in which the bilins are held in the
native protein. In consequence, native phycobiliproteins fluoresce
with high quantum yields.
The energy levels of the bilins in a phycobiliprotein arenot
equivalent. Consequently, whereas all of the bilins in a given phycobiliprotein absorb excitation energy, the fluorescence of the protein
originates from the bilins with the longest wavelength absorption
bands. Bilins which absorb light energy but transfer the excitation
energy to other bilins are called donors. Those bilins that both absorb
excitation energy and fluoresce are called acceptors. The intramolecular energy transfer within phycobiliprotein trimers and hexamers is
very fast, and in consequence the steady state fluorescence emission
originates almost exclusively from the acceptors. Synthesis of the
data from chemical, crystallographic, and spectroscopic studies on
numerous phycobiliproteins has led to theidentification of donor and
acceptor bilins in C-phycocyanin and, by less rigorous arguments, in
the various other phycobiliproteins. In C-phycocyanin, the biliis at
a-84 and 8-155 are donors and the bilin a t 8-84 is the acceptor (31,
32). PCB 8-84 extends into the center of the trimeric disc (Fig. 5),
whereas those a t 01-84and 8-155 lie toward the periphery. In consecutive double discs in the phycobilisome rod, the phycocyanin trimers
are stacked face-to-face (29). In the stack, the 6-84 bilins of consecutive discs are arranged vertically above one anotherand ina mutual
orientation that favors rapid transfer of energy from each of the three
acceptor bilins in one disc to the acceptor bilins of the disc below.
The phycoerythriis listed in Fig. 2 have five bilinslap. The sequences of the a and 8 subunits of B- and C-phycoerythrins and the
bilin attachment sites are known. These proteins are highly homologous to each other and to C-phycocyanin. If the assumption is made
that these phycoerythrins share a similar three-dimensional structure
with C-phycocyanin, the location of the additional bilins can be
assigned by model building. This leads to the finding that the addi-
4
Minireview: Directional Energy
Transfer
tional bilins lie at the periphery of the discs and to the conclusion
that they function as donors. Support for this conclusion has been
obtained from studies of the phycoerythrins of marine cyanobacteria
Synechococcus spp. (33). These phycoerythrins are very similar to Band C-phycoerythrins but have much higher phycourobilin contents.
Sequence studies on these phycoerythrins have shown that thepositions of the “additional” bilins in the phycoerythrins can be occupied
at 495 nm, whereas
by PUB groups (33). These groups have a, X
PEB groups have Amax at 530-565 nm. Consequently, a position
occupied by a PUB group in a phycoerythrin containing PEB groups
as well must be a donor position.
Picosecond energy transfer measurements show that theexcitation
energy absorbed by any of the bilins at the periphery of the disc is
rapidly localized on the centrally located acceptor bilins. The latter
bilins are the way stations in the energy transfer pathway through
the phycobilisome.
i n Phycobilisomes
bilins on the aAPB
and LCM”polypeptides is complete in <8 X 10”’ s
(37). In summary, the energy absorbed by any one of the bilins in the
phycobilisome localizes rapidly on the four terminal acceptor bilins
in the core. The emission of these bilins overlaps precisely the
absorption spectrum of the reaction center of photosystem 11, believed
to be the recipient of the energy harvested by the phycobilisome (38).
With the radiationless transfer of energy from the terminal acceptors
in the phycobilisome to the reaction center, the light guide function
of the phycobilisome is completed.
Acknowledgment-I am grateful to Phyllis Thompson Spowart for
her assistance with the illustrations for this paper.
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4. Deisenhofer, J., Epp, O., Miki, K., Huber, R., and Michel, H. (1984) J. Mol.
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Photosynthesis. III. Photosynthetic Membranes and Light Harvesting
S y s t e m (Staehelin, L. A,, and Amtzen, C. J., eds) Vol. 19, pp. 371-381,
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7. Yentscb, C. S. (1980) in The PhysiologicalEcologyofPhytoplankton(Morris,
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8. Glazer, A. N. (1985) Annu. Reu. Biophys. Biophys. Chem. 14,47-77
9. Bishop, J. E., Rapoport, H., Klotz, A. V., Chan, C. F., Glazer, A. N.,
Fiiglistaller, P., and Zuber, H. (1987) J. Am. Chem. SOC.109,875-881
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h o n n e l l , J. F., and Rapoport, H. (1988) J. Biol. Chem. 2 6 3 , 1297712985
11. Sorby, H. C. (1877) J. Linn. Soc. Land. Bot. 16,34-40
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13. Zuber, H. (1987) in The Light Reactions (Barber, J., ed) pp. 197-259,
Elsevier Scientific Publishing Co., Amsterdam
14. Glazer, A. N. (1984) Biochim. Eiophys. Acta 7 6 8 , 29-51
15. Glazer, A. N. (1981) in The Biochemistry of Plants (Hatch, M. D., and
Boardman, N. K., eds) Vol. 8, pp. 51-96, Academic Press, New York
16. Sidler, W., Kumpf, B., Rudiger, W., and Zuber, H. (1986) Biol. Chem.
Hop e Se ler 367,627-642
17. Morscfel,
Koller, K.-P., Wehrmeyer, W., andSchneider, H. (1977)
Cytobiologie 16,118-129
18. Bryant, D. A,, Guglielmi, G., Tandeau de Marsac, N., Castets, A,“., and
Cohen-Bazire, G. (1979) Arch. Microbiol. 1 2 3 , 113-127
19. Glazer, A. N., Williams, R. C., Yamanaka, G., and Schachman,H. K. (1979)
Proc. Natl. Acad. Sci. U.S. A. 76,6162-6166
20. Gantt, E. (1980) Int. Reu. Cytol. 66,45-80
21. Porter, G.. Tredwell, C. J., Searle, G. F.W., and Barber, J. (1978) Biochim.
Bioph s Acta 601,232-245
22. Searle, $.F. W., Barber, J., Porter, G., and Tredwell, C. J. (1978) Biochim.
Biophys. Acta 601,246-256
23. Glazer, A. N.(1983) Annu. Reu. Microbiol. 3 6 , 173-198
24. Glazer, A. N., Lundell, D. J., Yamanaka, G., and Williams, R. C. (1983)
Ann. Microbiol. Inst. Pasteur B134,159-180
25. Bryant, D. A. (1986) Can. Bull. Fish. Aqwt. Sci. 214,423-500
26. Lundell, D. J., Williams, R.C., and Glazer, A. N. (1981) J. BioI. Chem.
256,3580-3592
27. Yu, M.-H., Glazer, A. N., and Williams, R. C. (1981) J. Bid. Chem. 266,
13130-13136
28. Schirmer, T.,Bode, W., Huber, R., Sidler, W., and Zuber, H. (1985) J. Mol.
B ~ O 184,257-277
L
29. Schirmer, T., Huber, R., Schneider, M., Bode, W., Miller, M., and Hackert,
M. L. (1986) J. Mol. Bioi. 1 8 8 , 6 5 - 6 7 6
30. Scheer, H., and Kufer, W. (1977) 2. Naturforsch. Sect. C Biosci. 32,513-
Pathway of Energy Plow in Phycobilieomes
How is the excitation energy conveyed through the phycobilisome?
Each of the 12 C-phycoerythrin hexamers in acomplete Synechocystis
PCC 6701 phycobilisome (34) contains 30 bilins, and each of the 1 2
C-phycocyanin hexamers contains 18 bilins (Figs. 2 and 4). How is
random walk of the excitation energy among the 576 bilins in the
phycobilisome rods minimized and how is polarity of energy transfer
achieved? The phycoerythrin and phycocyanin hexamers are assembled into six separate rods. Consequently, excitation energy absorbed
by phycoerythrin, for example, is limited to a 60-PEB domain within
a single rod. If all of the phycoerythrin in a phycobilisome were
interconnected, the PEB domain size would be 360. Within a phycoerythrin hexamer, the transfer of energy to theacceptor bilins takes
place within <8 X 10”’ s (35). The fluorescence lifetime of a phycoerythrin hexamer is 2.5 X lo-’ s, whereas the time taken for the
energy absorbed by phycoerythrin to transfer to a proximal phycocyanin disc is about 2.5 X lo-” s (35). Consequently, the efficiency
of transfer is 100 X ((2.5 X lo-’ - 2.5 X 10-”)/2.5 X lo-’) = 99%.
Only three such transfers are needed to convey the energy to the core
complexes.
It is easy to see that the large energy difference between phycoerythrin and phycocyanin promotes directional energy transfer. How
is preferential polarity of transfer achieved between the phycocyanin
discs? As indicated above (Fig. 4), each disc contains a specific linker
polypeptide. Interaction with different linker polypeptides confers
distinctive spectroscopic properties on the acceptor bilins in the two
phycocyanin hexamers. The absorption and emission spectra of the
(~pcppc)~LRC’7complex are
shifted
red relative to those of
(~pcflpc)&R33’6
(26,27). In consequence, the favored direction of
transfer is from the distal disc to the phycocyanin disc proximal to
the core.
In the rods, the design of the discs ensures a rapid flow of energy
from the donor bilins to the acceptor bilins. The positioning of the
phycobiliproteins and the influence of the linker polypeptides contribute to rapid directional flow of the excitation energy from one
acceptor bilin to theone below it toward the core. The understanding
of the organization of the core is less complete, but features similar
to those seen in the rods govern the polar transfer of energy. Allophycocyanin trimers form the major building blocks of the core (Fig. 31.
4). The fluorescence properties of (apcflpC)6L~cz7
and the absorption
properties of allophycocyanin ensure that the latter functions as an 32.
efficient acceptor. The basal core cylinders of the phycobilisome each 33.
contain two copies of each of two distinct terminal energy acceptor
complexes with similar spectroscopic properties, (a1APBa2APf13AP)LC10
and (aAppAp)2@’8.6LcM99
(Fig. 4). Each of the polypeptides L C Mand
~
34.
aAPB
carries a single PCB group and confers a, ,X of 670 nm and a
35.
fluorescence emission maximum of 680 nm on the complexes in which
they are contained (8, 24, 36). Note that the same bilin (PCB) is 36.
present in phycocyanin, allophycocyanin, and the terminal acceptor
complexes. The spectrum of the bilins is in each instance “tuned” by 37.
the protein to a different A., . In the intact phycobilisome core, the
transfer of energy from allophycocyanin to the terminal acceptor 38.
2,
514
Si&;zknriibl, S., Fischer, R., and Scheer, H. (1987) 2.Naturforsch. Sect. C
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Berkeley
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