Page 159 - Algae Anatomy, Biochemistry, and Biotechnology

142 Algae: Anatomy, Biochemistry, and Biotechnology

photosystems. The apoproteins are three membrane-spanning a-helices and are nuclear-encoded.
LHCs are arranged externally with respect to the photosystems. In Cyanophyta, Glaucophyta, Rho-
dophyta, and Cryptophyta, no LHCs are present and the light-harvesting function is performed by
phycobiliproteins organized in phycobilisomes peripheral to the thylakoid membranes in the first
three divisions, and localized within the lumen of thylakoids in the latter division. The phycobili-
some structure consists of a three-cylinder core of four stacked molecules of allophycocyanin, close
to the thylakoid membrane, on which converge rod-shaped assemblies of coaxially stacked hex-
americ molecules of only phycocyanin or both phycocyanin and phycoerythrin, (cf. Chapter 2,
Figure 2.76). Phycobilisomes are linked to PSII but they can diffuse along the surface of the
thylakoids, at a rate sufficient to allow movements from PSII to PSI within 100 ms. Among prokar-
yotes, Prochlorophyta (Prochlorococcus sp., Prochlorothrix sp. and Prochloron sp.), differ from
cyanobacteria in possessing an external chlorophyll a and b antenna, like eukaryotic algae,
instead of the large extrinsic phycobilisomes.
PSII complex can be divided into two main protein superfamilies differing in the number of
membrane-spanning a-helices, that is, the six-helix protein superfamily, which includes the internal
antennae CP43 and CP47 (CP stands for Chlorophyll–Protein complex), and the five-helix proteins
of the reaction center core D1 and D2 (so-called because they were first identified as two diffuse
bands by gel electrophoresis and staining) where ETC components are located. External antenna
proteins of Prochlorophyta belong to the six-helix CP43 and CP47 superfamily and not to the
three-helix LHCs superfamily.
PSII is a homodimer, where the two monomers in the dimers are almost identical. The
monomer consists of over 20 subunits. All the redox active cofactors involved in the activity of
PSII are bound to the reaction center proteins D1 and D2. Closely associated with these two pro-
teins are the chlorophyll a binding proteins CP43 and CP47 and the extrinsic luminally bound pro-
teins of the oxygen evolving complex. Each monomer also includes one heme b, one heme c, two
2þ
plastoquinones, two pheophytins (a chlorophyll a without Mg ), and one non-heme Fe and con-
tains 36 chlorophylls a and 7 all-trans carotenoids assumed to be b-carotene molecules. Eukaryotic
and cyanobacterial PSII are structurally very similar at the level of both their oligomeric states and
organization of the transmembrane helices of their major subunits. The eukaryotic PSII dimer is
flanked by two clusters of Lhcb proteins. Each cluster contains two trimers of Lhcb1, Lhcb2,
and Lhcb3 and the other three monomers, Lhcb4, Lhcb5, and Lhcb6.
The reactions of PSII are powered by light-driven primary and secondary electron transfer pro-
cesses across the reaction center (D1 and D2 subunits). Upon illumination, an electron is dislodged
from the excited primary electron donor P 680 , a chlorophyll a molecule located towards the luminal
surface. The electron is quickly transferred towards the stromal surface to the final electron accep-
tor, a plastoquinone, via a pheophytin. After accepting two electrons and undergoing protonation,
plastoquinone is reduced to plastoquinol, and it is then released from PSII into the membrane
þ
matrix. The cation P 680 is reduced by a redox active tyrosine, which in turn is reduced by a Mn
ion within a cluster of four. When the (Mn) 4 cluster accumulates four oxidizing equivalents (elec-
trons), two water molecules are oxidized to yield one molecule of O 2 and four proton. All the redox
active cofactors involved in the electron transfer processes are located on the D1 side of the reaction
center.
PSI complex possesses only eleven-helix PsaA and PsaB protein superfamilies. Each 11 trans-
membrane helices subunit has six N terminal transmembrane helices that bind light-harvesting chlor-
ophylls and carotenoids and act as internal antennae and five C terminal transmembrane helices that
bind Fe 4 S 4 clusters as terminal electron acceptors. The N terminal part of the PsaA and PsaB proteins
are structurally and functionally homologues to CP43 and CP47 proteins of PSII; the C terminal part
of the PsaA and PsaB proteins are structurally and functionally homologues to D1 and D2 proteins of
PSII. Eukaryotic PSI is a monomer that is loosely associated with the Lhca moiety, with a deep cleft
between them. The four antenna proteins assemble into two heterodimers composed of Lhca1 and
Lhca4 and homodimers composed of Lhca2 and Lhca3. Those dimers create a half-moon-shaped