Architecture of a Charge-Transfer State Regulating Light Harvesting in a Plant Antenna Protein

Tae Kyu Ahn, et al.

Science 320, 794 (2008)

Miyasaka Lab.

Yuji Morii

Contents

Introduction　　　・ Photosynthesis

　　　・ Photoprotective process Results and discussion　　　・ Scheme of the energy dissipation mechanism

　　　・ Measurement of the NIR transient absorption

　　　・ Specification the position of energy dissipation Conclusion

Photosynthesis

Light

Reaction Center Antenna

Chlorophyll

Zeaxanthin

Excess Light

Energy Dissipation

Photosynthesis

Convert light energy into electric energy Light harvest

Scheme of the energy dissipation mechanism

hν

Chl* *

Chl-Zea +

Chl ・－

Zea ・＋

Chl-Zea

photosynthetic reaction center

~~

Energy dissipation

N. E. Holt et al., Science 307, 433 (2005).

OH

OH

OH

OH

OH

OH

O

O

O

Violaxanthin

Antheraxanthin

Zeaxanthin

Excesslight

Lowlight

Xanthophyll cycle carotenoids

Energy dissipation is regulated by excess or limiting light.

B.Demmig- Adams, W. W. Adams , Ⅲ Tends Plant Sci. 1, 21 (1996).

Zeaxanthin

Violaxanthin

Chl

Chl*

E

CP29 homology structure model

A1-A5 : Chlorophyll a

B3,B5,B6 : Chlorophyll b

L1,L2 : Carotenoid-binding site

NIR transient absorption kinetics

Red : CP29-ZeaBlack : CP29-VioBlue : Subtraction of Red from Black

NIR transient absorption spectrum

Max : 980 nm

The spectrum is in good agreement with the established Zea ・

+ absorption characteristics.

NIR transient absorption kinetics

The different profile is indicative of transient Zea ・ + formation.

Energy dissipation

hν

Chl* *

Chl-Zea +Chl ・－

Zea ・

＋

Chl-Zea

photosynthetic reaction center

~~

Sample ~ a series of mutant CP29 complexes ~

L1 site : LuteinL2 site : Zea or Vio

・ CP29 each lacking specific chlorophylls

CP29-A1 (unstable)

CP29-A2

CP29-A3

CP29-A4

CP29-A5 (also loss of B5)

CP29-B3

CP29-B5 (lacking B5 only)

CP29-B6

lackFar away

Kinetic profiles of CP29-A2 , -A3 , -A4, –B6

These kinetic profiles indicate the Zea ・ + evolution.

Energy dissipation is active.

Kinetic profiles of CP29-B3 , -A5 , –B5

Energy dissipation is active.

No measurable Z ・ + formation signal

Energy dissipation is inactive.

Molecular detail of the CT quenching site

The molecular site of CT quenching in CP29 comprises Z and a strongly coupled chlorophyll pair (A5 and B5).

Strongly coupled to each other

Zeaxanthin

Conclusion

The primary event of CT quenching in CP29 involves electron transfer Zea to a strongly coupled chlorophyll dimer in the A5-B5 pocket of CP29, rather than from Zea to a monomeric chlorophyll molecule.

Controlling the coupling strength between chlorophylls A5 and B5 in CP29 would modulate the reduction potential of the chlorophyll dimer and therefore could be used to switch ON and OFF the CT quenching.