590
K. M. G. Mostofa et al.
O 2 -releases [H 2 O 2 + light or enzymes (catalases/peroxidases) → O 2 + 2H 2 O
or other components] and their reused in H 2 O 2 generation in photosynthetic
organisms (O 2 + Chl + H + + hυ → H 2 O 2 ) can balance the O 2 level in the
environments in new photosynthetic reaction. But this does not occur in old photosynthetic reaction. (iv) Conversion of H 2 O 2 to O 2 occurs at a higher extent in
biological systems than in photoinduced decomposition processes (Moffett and
Zafiriou 1990). For instance, the Cu(II) bathocuproinedisulfonic acid complex
(Cubc 2 ) can convert H 2 O 2 to O 2 via the reaction (Eq. 4.1) (Moffett et al. 1985):
Unconvincing evidence has been found for S 0 to S 4 transitions, and four successive transitions are needed for H 2 O decomposition (Rappaport and Diner 2008;
Kok et al. 1970; Joliot and Kok 1975; Krishtalik 1986, 1990).
(v) It is hypothesized that the O–O bond formation occurs when O5 in Mn 4 CaO 5
cluster provides one O atom via formation of hydroxide ion in the S 1 state (Umena
et al. 2011; Saito et al. 2012). A major issuer is then if it is possible to break down
O5 in the Mn 4 CaO 5 cluster. The problem is that, were it possible, probably the entire
PSII system would be broken down. Furthermore, each Mn atom in the Mn 4 CaO 5
cluster is in octrahedral form with six ligands, and it is also paramagnetic with 5
unpaired electrons in its outer d-orbitals (Mn 2+ = 1s 2 2s 2 2p 6 3s 2 3p 6 4s 0 3d 5 ). The
result is that Mn could carry out strong H-bonding with other components. Such an
effect enables a second coordination sphere by D1-Asp 61, D1-His 337 and CP43Arg 357, in addition to the direct ligands. Therefore, these three residues might be
responsible for maintaining the oxygen-evolving activity (Umena et al. 2011; Nixon
and Diner 1994; Chu et al. 1995; Hwang et al. 2007; Service RJ, Hillier W, Debus
RJ 2010). D1-Asp 61 is located at the entrance of a proposed proton exit channel
involving a chloride ion (Cl − ) in Mn 4 CaO 5 (Umena et al. 2011; Kawakami et al.
2009; Guskov et al. 2009; Murray and Barber 2007; Ho and Styring 2008). This residue may facilitate proton exit from the Mn cluster. Proton releases from Mn 4 CaO 5
may play a key role in the formation of H 2 O 2 via O 2
•− and HO 2
•
.
(vi) Finally, the occurrence of about 1,300 water molecules in the PSII monomer, located at the luminal and stromal sides (Umena et al. 2011), could allow
the inclusion of a lot of dissolved O 2 molecules. They could add electrons after
they are released from Chl molecules upon excitation by light. Crystal structures
of PSI or PSII do not include any information about dissolved O 2 , and issue that
will need further studies to be clarified.
5 Factors Affecting the Photosynthesis of Organisms
Cyanobacteria carry out oxygenic photosynthesis using a photosynthetic system
similar to that observed in chloroplasts of higher plants. Therefore, cyanobacteria can
be used in model studies to understand the effects of various environmental factors
(4.1)
2Cu(II)bc 2 + H 2 O 2 → 2Cu(I)bc 2 + O 2 + 2H
+
K. M. G. Mostofa et al.
O 2 -releases [H 2 O 2 + light or enzymes (catalases/peroxidases) → O 2 + 2H 2 O
or other components] and their reused in H 2 O 2 generation in photosynthetic
organisms (O 2 + Chl + H + + hυ → H 2 O 2 ) can balance the O 2 level in the
environments in new photosynthetic reaction. But this does not occur in old photosynthetic reaction. (iv) Conversion of H 2 O 2 to O 2 occurs at a higher extent in
biological systems than in photoinduced decomposition processes (Moffett and
Zafiriou 1990). For instance, the Cu(II) bathocuproinedisulfonic acid complex
(Cubc 2 ) can convert H 2 O 2 to O 2 via the reaction (Eq. 4.1) (Moffett et al. 1985):
Unconvincing evidence has been found for S 0 to S 4 transitions, and four successive transitions are needed for H 2 O decomposition (Rappaport and Diner 2008;
Kok et al. 1970; Joliot and Kok 1975; Krishtalik 1986, 1990).
(v) It is hypothesized that the O–O bond formation occurs when O5 in Mn 4 CaO 5
cluster provides one O atom via formation of hydroxide ion in the S 1 state (Umena
et al. 2011; Saito et al. 2012). A major issuer is then if it is possible to break down
O5 in the Mn 4 CaO 5 cluster. The problem is that, were it possible, probably the entire
PSII system would be broken down. Furthermore, each Mn atom in the Mn 4 CaO 5
cluster is in octrahedral form with six ligands, and it is also paramagnetic with 5
unpaired electrons in its outer d-orbitals (Mn 2+ = 1s 2 2s 2 2p 6 3s 2 3p 6 4s 0 3d 5 ). The
result is that Mn could carry out strong H-bonding with other components. Such an
effect enables a second coordination sphere by D1-Asp 61, D1-His 337 and CP43Arg 357, in addition to the direct ligands. Therefore, these three residues might be
responsible for maintaining the oxygen-evolving activity (Umena et al. 2011; Nixon
and Diner 1994; Chu et al. 1995; Hwang et al. 2007; Service RJ, Hillier W, Debus
RJ 2010). D1-Asp 61 is located at the entrance of a proposed proton exit channel
involving a chloride ion (Cl − ) in Mn 4 CaO 5 (Umena et al. 2011; Kawakami et al.
2009; Guskov et al. 2009; Murray and Barber 2007; Ho and Styring 2008). This residue may facilitate proton exit from the Mn cluster. Proton releases from Mn 4 CaO 5
may play a key role in the formation of H 2 O 2 via O 2
•− and HO 2
•
.
(vi) Finally, the occurrence of about 1,300 water molecules in the PSII monomer, located at the luminal and stromal sides (Umena et al. 2011), could allow
the inclusion of a lot of dissolved O 2 molecules. They could add electrons after
they are released from Chl molecules upon excitation by light. Crystal structures
of PSI or PSII do not include any information about dissolved O 2 , and issue that
will need further studies to be clarified.
5 Factors Affecting the Photosynthesis of Organisms
Cyanobacteria carry out oxygenic photosynthesis using a photosynthetic system
similar to that observed in chloroplasts of higher plants. Therefore, cyanobacteria can
be used in model studies to understand the effects of various environmental factors
(4.1)
2Cu(II)bc 2 + H 2 O 2 → 2Cu(I)bc 2 + O 2 + 2H
+