Further details of the two photosystems:

Photosystem II

Photosystem II is a transmembrane assembly of more than 10 polypeptides! The sequence of reactions is outlined below:

a. P680 + hn ----> P680*

b. P680* ----> pheophytin (chlorophyll with 2 H+ instead of Mg2+)

c. pheophytin ----> PQ (mobile plastiquinones)

d. PQ ----> cyto b559 ----> cyto b552 (these two cytochromes are known as cyto bf)

e. cyto bf ----> plastocyanin (PC) ----> P700 (photosystem I)

This sequence of reactions leaves P680 deficient in electrons, making it a very strong oxidant, (there are 4 Mn involved), the manganese center is a strong enough oxidant to oxidize the oxygen in water.

P680 <---- 4 Z <---- 4 Mn (electrons are moving from Mn to P680)

4 Mn+ + 2 H2O ----> 4 Mn + 4 H+ + O2

This is the oxygen-evolving center of photosynthesis.

Photosystem I:

Photosystem I is made up or 13 polypeptides. Light energy is absorbed by the P700 antennae chlorophyll and transferred to reaction center P700.

a. e- (from plastocyanin, above) ----> P700* ----> A0 ----> A1 (phylloquinones)

b. A1 ----> (3 bound ferredoxins) Fe4S4 (called FAFBFx)

c. FAFBFx ---> soluble ferredoxin2 (called Fd)

d. ferredoxinred + NADP+ ----> NADPH + ferredoxinox

This last reaction is catalyzed by the enzyme ferredoxin-NADP+ oxidoreductase (a flavoprotein; abbreviated as FNR).

Summary of Stoichiometry:

2 H2O + hn ------> 4 H+ + 4 e- + O2

4 e- + 2 H+ + 2 NADP+ --------> 2 NADPH

The Thylakoid ATPase

The thylakoid discs make ATP in a manner similar to mitochondria (by capturing the energy of protons as they pass down an electrochemical gradient). The difference is that the pH is low (the difference is 3.5 pH units) on the inside of the thylakoid discs (lumenal space) and higher on the outside. ATP is synthesized as protons flow out of the thylakoid disc. This ATPase complex is called CF1-CFo (which is analogous to the mitochondrial F1-Fo).

Cyclic photophosphorylation

To balance the production of NADPH and ATP the light reactions have a bypass in photosystem I, which results in a process called cyclic photophosphorylation.

P 700* ---> ferredoxin ----> NADP+

|

----> cytochrome bf ---> plastocyanin ---> P 700

This shunt uses light energy to pump additional protons into the thylakoid discs; extra ATP is synthesized as these protons flow out of the discs.

Bacterial Photosystem:

a. the first X-ray structure (at atomic resolution) of a membrane protein

b. group studied purple sulfur bacterium -- Rhodopseudomonas viridis

c. Deisenhofer, Michel, and Huber received Nobel Prize in Chemistry in 1988

d. completed X-ray structure of this bacteria's reaction center (which is similar to system II in plants)

Four polypeptides: absorb light at 870 nm (vs 680 for plant system)

4 bacteriochlorophylls

4 bacteriopheophytins

2 quinones + Fe