The Orange Carotenoid Protein of Cyanobacteria - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 3

1 The Orange Carotenoid

Protein of Cyanobacteria

Cheryl A. Kerfeld, Maxime Alexandre,
and Diana Kirilovsky

CONTENTS
1.1 Introduction .............................................................................................................................. 3
1.2 Recent Studies on the Function of the OCP ............................................................................. 4
1.3 The OCP: Primary to Quaternary Structure ............................................................................ 7
1.4 The Structure of the OCP in the Context of Function............................................................ 10
1.5 Conclusions and Prospects ..................................................................................................... 15
Acknowledgments ............................................................................................................................ 15
References ........................................................................................................................................ 15

1.1 INTRODUCTION
Molecular, spectroscopic, and functional genomics studies have demonstrated the remarkable simi-
larity among the components of the photosynthetic machinery of cyanobacteria, algae, and plants.
These organisms also share the need to balance the collection of energy for photosynthesis with the
threat of photodestruction. Carotenoids are central to attaining this balance.
The photoprotective processes of photosynthetic organisms involving the dissipation as heat of
the excess of absorbed energy in the antenna of the photosystem II are collectively known as non-
photochemical quenching (NPQ). In this mechanism, there is a decrease in the amount of energy
funneled to the reaction center (RC) with a concomitant reduction in the amount of the reactive
oxygen species generated. NPQ is well characterized in plants (Demmig-Adams 1990, Horton et al.
1996, Niyogi 1999, Muller et al. 2001). It relies on the same components used for light harvesting in
photosynthesis. The absorption of light is accomplished by light-harvesting complexes (LHCs) that
surround RCs; a RC and its LHC together form a photosystem (PS). There are two PSs in organisms
that carry out oxygenic photosynthesis, PSI and PSII. In eukaryotic PSs, the RCs and LHCs are inte-
gral membrane pigment protein complexes located in the thylakoid membranes. The carotenoids in
these complexes are thought to provide structural stability and act as accessory light-harvesting pig-
ments as well as mediate photoprotection. In plants, the carotenoid-based photoprotection in PSII is
triggered by acidii cation of the thylakoid lumen under saturating light conditions (Demmig-Adams
1990, Horton et al. 1996, Niyogi 1999, Muller et al. 2001). The drop of the lumen pH induces the
interconversion of specii c LHC carotenoids (Yamamoto 1979, Gilmore and Yamamoto 1993) and
the protonation of a PSII subunit (PsbS), a member of the LHC superfamily (Li et al. 2000, 2004).
This process also involves conformational changes in the LHCII, modifying the interaction between
chlorophylls and carotenoids (Ruban et al. 1992, 2007, Pascal et al. 2005). This thermal energy dis-
sipation is accompanied by a decrease of PSII-related l uorescence emission, known as high-energy
quenching (qE), one of the NPQ processes.

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