Photosystem II (Thermosynechococcus elongatus)

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I. Introduction

This dimer from T. elongatus is responsible for absorbing light energy, oxidizing H₂O to O₂, and sending the excited electrons through an electron transport chain (ETC). Through this ETC, plants produce the energy required to produce glucose.

For more information see Biochemistry Online: Chapter 8D - Photosynthesis: The Light Reaction

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II. General Structure

Wireframe
Cartoon Protein

Notice the pink region: it contains several transmembrane helices spanning the thylakoid membrane. The gold region and part of the pink are located in the lumen of the thylakoid.

Photosystem II 12 kDa extrinsic protein - PsbU- (green) in one monomer

PsbU is an extrinsic protein of the photosystem II complex. PsbU stabilizes the oxygen-evolving machinery of the photosystem II complex against heat-induced inactivation.

Core Light Harvesting Protein (cyan) and CP43 (yellow) in one monomer

These proteins bind the chlorophyll, which absorb light

Heteroatoms structures in PSII.

• OEC - oxygen evolving complex

• CLA - chlorophyll A (CPK)

• PHO - pheophytin A

• HEM - heme

• PL9 - plastoquinone 9

• BCR - beta carotene

These proteins bind the chlorophyll, which absorb light

α Chlorophyll a

Responsible for absorbing light in the visible region and exciting electrons derived from H2O.

Detailed Oxygen Evolving Centers (OEC) - Mn cluster

The PSII dimer contains two OECs: OEC-1 and OEC-2001.  Mn₃CaO₄--Mn (linked via mono-µ-oxo bridge).  One is shown here.  At these sites, water is oxidized to H2O, "one of the most thermodynamically demanding reactions in biology1." The oxygen evolving center locations are located at the yellow points