18. Photosystem II functions as a light dependent water plastoquinone oxidoreductase. What are the names of two reaction center protein that bind electron transfer prosthetic group, such as P680, pheophytin and plastoquinone?
(1) CP43 and CP47  (2) D1 and D2
(3) 33 kDa and 23 kDa      (4) F_A and F_B

The correct answer is (2) D1 and D2. These two reaction center proteins form the heterodimeric core of Photosystem II and bind all the major electron transfer prosthetic groups, including P680, pheophytin, and plastoquinone.​

Photosystem II Reaction Center Proteins: D1 and D2

Introduction

Key phrase: Photosystem II reaction center D1 and D2 electron transfer

Photosystem II (PSII) is a multi-subunit complex in the thylakoid membrane, catalyzing the light-driven oxidation of water and reduction of plastoquinone. The electron transfer prosthetic groups (P680, pheophytin, plastoquinone) are coordinated by its key reaction center proteins D1 and D2, which are essential for initiating the charge separation and sequential electron transport of photosynthesis.​

Explanation of Each Option

• (1) CP43 and CP47

  • Incorrect. CP43 and CP47 are chlorophyll-binding antenna proteins that transfer excited energy to the reaction center, but they do not directly bind or coordinate P680, pheophytin, or plastoquinone for electron transfer.​

• (2) D1 and D2

  • Correct. The D1 and D2 proteins form the reaction center core of PSII; they bind P680 (special chlorophyll pair), pheophytin (primary electron acceptor), and plastoquinone, acting as the fundamental platform for initial electron transfer and charge separation in photosynthesis.​

• (3) 33 kDa and 23 kDa

  • Incorrect. These refer to extrinsic proteins of the oxygen-evolving complex but are not involved in binding the primary electron transfer molecules of the reaction center.​

• (4) FA and FB

  • Incorrect. FA and FB are iron-sulfur centers found in Photosystem I, not in Photosystem II, and are involved in electron transfer there.​

Quick Facts: D1 and D2 in Photosystem II

• D1 (PsbA) and D2 (PsbD) are transmembrane subunits forming a heterodimer.

• They coordinate the redox-active cofactors for photochemical charge separation: P680, pheophytin, and plastoquinone.​

• Their interaction ensures efficient energy conversion and water-splitting.​