Page 157 - Algae Anatomy, Biochemistry, and Biotechnology

140 Algae: Anatomy, Biochemistry, and Biotechnology

stacked membranes to be utilized by ATP-synthase molecules that are located far away in
unstacked membranes. What is the functional significance of this lateral differentiation of the thy-
lakoid membrane system? If both photosystems were present at high density in the same membrane
region, a high proportion of photons absorbed by PSII would be transferred to PSI because the

energy level of the excited state P 680 relative to its ground state P 680 is higher than that of P 700 rela-

tive to P 700 . A lateral separation of photosystems solves this problem by placing P 680 more than
˚
100 A away from P 700 . The positioning of PSI in the unstacked membranes gives it a direct
þ
access to the stroma for the reduction of NADP . In fact the stroma-exposed surface of PSI,
which contains the iron-sulfur proteins that carry electron to ferredoxin and ultimately to
˚
þ
NADP , protrudes about 50 A beyond the membrane surface and could not possibly be accommo-
˚
dated within the stacks, where adjacent thylakoids are separated by no more than 40 A. It seems
likely that ATP-synthase is also located in unstacked regions to provide space for its large protrud-
ing portion and access to ADP. In contrast, the tight quarters of the appressed regions do not pose a
problem for PSII, which interacts with a small polar electron donor (H 2 O) and a highly lipid-soluble
electron carrier (plastoquinone). According to the model of Allen and Forsberg (2001), the close
appression of grana (stacks of thylakoids) membranes arises because the flat stroma-exposed
surfaces of LHCII form recognition and contact surfaces for each other, causing opposing surfaces
of thylakoids to interact. There is not steric hindrance to this close opposition of stacked grana
membranes, because similar to LHCII PSII presents a flat surface that protrudes not more than
˚
10–20 A beyond the membrane surface.
The functional significance of thylakoid stacking is presumably to allow a large, connected,
light-harvesting antenna to form both within and between membranes. Within this antenna both
the excitation energies can pass between chlorophylls located in LHCII complexes that are adjacent
to each other, both within a single membrane and between appressed membranes.
The degree of stacking and the proportion of different photosynthetic assemblies are regulated
in response to environmental variables such as the intensity and spectral characters of incident light.
The lateral distribution of LHC is controlled by reversible phosphorylation. At low light levels,
LHC is bound to PSII. At high light levels, a specific kinase is activated by plastoquinol, and phos-
phorylation of threonine side chains of LHC leads to its release from PSII. The phosphorylated form
of these light harvesting units diffused freely in the thylakoid membrane and may become asso-
ciated with PSI to increase its absorbance coefficient (Figure 3.3).
Central to the photosynthetic process is PSII, which catalyzes one of the most thermodynamically
þ
2
demanding reactions in biology: the photo-induced oxidation of water (2H 2 O ! 4e þ 4H þ O 2 ).
PSII has the power to split water and use its electrons and protons to drive photosynthesis. The
first ancestor bacteria carrying on anoxygenic photosynthesis probably synthesized ATP by
oxidation of H 2 S and FeS compounds, abundant in the environment. The released energy could
have been harnessed via production of a proton gradient, stimulating evolution of electron transport
chains, and the reducing equivalents (electrons) generated used in CO 2 fixation and hence bio-
synthesis. This was the precursor of the PSI. About 2800 million years ago the evolutionary
pressure to use less strongly reducing (and therefore more abundant) source of electrons appears
to have culminated in the development of the singularly useful trick of supplying the electrons
to the oxidized reaction center from a tyrosine side chain, generating tyrosine cation radicals
that are capable of sequential abstraction of electrons from water. Oxygenic photosynthesis,
which requires coupling in series of two distinct types of reaction centers (PSI and PSII)
must have depended on later transfer of genes between the evolutionary precursors of the
modern sulfur bacteria (whose single reaction center resembles PSI) and those of purple bacteria
(whose single reaction center resembles PSII). Thus the cyanobacteria appeared. They were the
first dominant organisms to use photosynthesis. As a by-product of photosynthesis, oxygen gas
(O 2 ) was produced for the first time in abundance. Initially, oxygen released by photosynthesis
was absorbed by iron(II), then abundant in the sea, thus oxidizing it to insoluble iron(III) oxide
(rust). Red “banded iron deposits” of iron(III) oxide are marked in marine sediments of ca. 2500