BioPHYSICAL chemistry

The hydrogen-abstraction model was highly innovative but appears to

be somewhat over-simplified compared to the actual mechanism of water

oxidation as derived from spectroscopic studies. For example, there is still

discussion about the oxidation states of the manganese cluster and proton

release at each of the S states. These results have led to modifications of

the hydrogen-abstraction model, which are being subjected to ongoing

experimental testing.

436 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY

H

YZ

H







H

Mn

Base Light

Lumen

P680 Protein

H

YZ

(a) Oxidation (b) Oxidation of TyrZ
of P680

H

H

e

Mn

Base

Lumen

P680 Protein

H

YZ

(c) H transfer to base

H

H

Mn

Base

Lumen

P680 Protein

H

YZ

(f ) Proton abstraction

H

Mn

Base

Lumen

P680 Protein

YZ

(e) S-state advancement

H

H

Mn

Base

Lumen

P680 Protein

YZ

(d) Neutral TyrZ radical

H

H

H

Mn

Base

Lumen

P680 Protein

Figure 20.15The hydrogen-abstraction model in which water oxidation is facilitated by the
transfer of the hydrogen to the phenolic ring of the tyrosyl radical after oxidation. Modified from
Britt (1996).

References and further reading

Allen, J.P., Feher, G., Yeates, T.O., Komiya, H., and
Rees, D.C. (1987) Structure of the reaction center
from Rhodobacter sphaeroidesR-26: I. The cofactors.
Proceedings of the National Academy of Sciences USA 84 ,
5730 – 4.
Blankenship, R.E. (2002) Molecular Mechanisms of
Photosynthesis. Blackwell Publishing, Oxford.
Blankenship, R.E. and Hartman, H. (1998) The
origin and evolution of oxygenic photosynthesis.
Trends in Biochemcal Sciences 23 , 94 –7.
Blankenship, R.E., Madigan, M.T., and Bauer, C.E. (eds)
(1995) Anoxygenic Photosynthetic Bacteria. Kluwer
Academic Publishers, Dordrecht, The Netherlands.

Britt, R.D. (1996) Oxygen evolution. In Oxygenic

Photosynthesis: the Light Reactions(Ort, D.R. and

Yocum, C.F., eds). Kluwer Academic Publishers,

Dordrecht, The Netherlands, pp. 137– 64.

Britt, R.D., Peloquin, J.M., and Campbell, K.A. (2003)

Pulsed and parallel-polarization EPR character-

ization of the photosystem II oxygen-evolving

complex. Annual Review of Biophysics and Biomolecular

Structure 29 , 463 –95.

Debus, R.J. (2001) Amino acid residues that modulate

the properties of tyrosine YZand the manganese

cluster in the water oxidizing complex of photo-

system II. Biochimica Biophysica Acta 1503 , 164 – 86.