423
15.1 Transition Metals in Photosynthetic Cells
Transition metals play an important role in many biological processes (for reviews,
see Kaim and Schwederski 1994). In the photosynthetic apparatus, transition met-
als are required as cofactors in electron transport processes (Merchant and Sawa-
ya 2005). Among these are Fe containing cofactors such as iron-sulfor clusters,
cytochromes and non-heme iron (see Chaps. 12 and 13), Cu in plastocyanin (see
Chap. 21) and Mn in Photosystem II (see Chaps. 12 and 13). As a result, the demand
for some of these metals in cyanobacteria and plants far exceeds that of other organ-
isms. A most illustrative example is the evolutionary transition from non-oxygenic
to oxygenic photosynthesis which resulted in a ~100 times higher internal Mn quota
in the oxygen evolving cyanobacterium Synechocystis sp. PCC 6803 compared to
that of the photosynthetic bacterium Rhodobacter capsulatus which is unable to
evolve oxygen (Keren et al. 2002).
The “invention” of oxygenic photosynthesis was the key step for the evolution
of the whole biosphere because it provided the indispensable prerequisites for the
development and sustenance of all higher forms of life on earth (Des Marais 2000).
Nevertheless, oxygenic photosynthesis generates severe risks for living matter due
to the production of molecular dioxygen with its peculiar properties (for further
reading, see articles in Gilbert 1981). Unique problems emerge from the necessity
to transport, accumulate and assemble transition metal ions into functional cofac-
tors because these cations catalyze the formation of the extremely reactive hydroxyl
radicals (OH
•
) from superoxide and/or hydrogen peroxide via the Haber-Weiss re-
action (Haber and Weiss 1934). Reactive oxygen species (ROS) can be formed at a
number of junctions along the course of the photosynthetic electron transport chain.
G. A. Peschek et al. (eds.), Bioenergetic Processes of Cyanobacteria,
DOI 10.1007/978-94-007-0388-9_15, © Springer Science+Business Media B.V. 2011
Chapter 15
Mn Transport and the Assembly
of Photosystem II
Eitan Salomon, Gernot Renger and Nir Keren
N. Keren ()
Department of Plant and Environmental Sciences, The Silberman Institute of Life Sciences,
The Hebrew University of Jerusalem, Edmond Safra Campus, Givat-Ram, Israel
e-mail: nirkeren@vms.huji.ac.il