Identification of Carotenoids in Photosynthetic Proteins: Xanthophylls of the Light Harvesting Antenna - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 118

PSII membrane stacks granae Xanthophylls of LHCIIb

100 nm I

PSII complex

Major
antenna
Minor
antenna
Reaction
center core
complex
5nm

3

2

1

4

II

LHCII trimer

Lutein 1 Lutein 2

Neoxanthin

LHCII oligomer

Violaxanthin

50 nm

FIGURE 7.3 Structure of PSII membranes, macrocomplexes and LHCII antenna. Left from the top: elec-
tron microscopy of grana stacks, PSII macrocomplexes, LHCII trimers, and LHCII oligomers. Right from
the top: Atomic structure of LHCII monomer (I and II are side and top views). Bottom part displays LHCII
xanthophylls.

7.3.3 F UNCTIONS OF X ANTHOPHYLLS IN THE A NTENNA : A S TRUCTURAL P ERSPECTIVE
Xanthophyll functions in the LHCII antenna are believed to increase the spectral cross section,
complimenting chlorophyll absorption: photoprotection of chlorophyll against excess excitation
energy, assembly and stability of the complex, and participation in the conformational dynamics
of LHCII. The mechanisms behind these processes are still poorly understood, a major obstacle
being the lack of detailed structural and spectral information available in vivo . There are important
issues concerning xanthophylls in the LHCII antenna that remain unanswered. What is the purpose
of having variations in a number of conjugated double bonds? What is the reason for the presence
of the three types of xanthophylls in LHCII structure? How do differences in polarity and head
group orientation determine xanthophyll binding sites. Where is zeaxanthin bound? How do the
lumen-localized deepoxidase reach the violaxanthin epoxy group situated closer to the stromal side
of the membrane? These are only a few questions of many, which remain to be answered in order to
understand the role of xanthophylls in antenna function.