Question

The chemical identity of Oxygen Evolving Centers (OEC) present in the Photosytem-II of plants and cyanobacteria is:

• A. Oxygenated carotenoids
• B. Oxylipins
• C. Metal-oxygen clusters
• D. Fe-S clusters

Hint:

Manganese is uniquely suited for the water-splitting complex because it can stable exist in multiple oxidation states ($+2, +3, +4$, and $+5$), allowing it to accumulate the four oxidizing equivalents needed to split water.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
During the light reactions of photosynthesis, Photosystem II (PSII) absorbs light energy to drive the oxidation of water molecules, producing molecular oxygen, protons, and electrons.
This water-splitting reaction is catalyzed by a specialized inorganic cofactor complex called the Oxygen-Evolving Center (OEC) or Water-Oxidizing Complex (WOC).

Step 2: Detailed Explanation:
The Oxygen-Evolving Center (OEC) is located on the lumenal side of the thylakoid membrane, associated with the reaction center proteins of Photosystem II.
Chemically, the catalytic core of the OEC is a highly organized, inorganic metal-oxygen cluster with the composition:
\[ \text{Mn}_4\text{CaO}_5 \]
This cluster is composed of four manganese ($\text{Mn}$) ions, a single calcium ($\text{Ca}$) ion, and five bridging oxygen ($\text{O}$) atoms.
The structure resembles a distorted "chair" or "cubane" arrangement, where three manganese ions and the calcium ion are linked by oxygen bridges to form a cube, with the fourth manganese ion attached externally.
During the S-state cycle (Kok cycle), the manganese ions undergo sequential oxidation steps (accumulating positive charges) driven by light-induced electron transfers. This oxidation state changes until the complex can extract four electrons from two water molecules, releasing one $\text{O}_2$ molecule.
Therefore, the chemical identity of the OEC is a metal-oxygen cluster.
The other options are incorrect: carotenoids are accessory pigments, oxylipins are lipid-derived signaling molecules, and Fe-S clusters are involved in electron transfer in Photosystem I and the respiratory chain.

Step 3: Final Answer:
The chemical identity of the Oxygen Evolving Center is a metal-oxygen cluster, which corresponds to option (C).