Plastocyanin Properties Overview

Plastocyanin is a small copper-containing protein involved in electron transfer in photosynthesis. It belongs to the family of blue copper proteins, characterized by their intense blue color and unique spectroscopic properties. Let's analyze the given statements about plastocyanin.

Analyzing Plastocyanin Statements

We will evaluate each statement to determine its accuracy regarding the properties of plastocyanin.

• Statement A: Coordination geometry of Cu^II-ion is distorted tetrahedron.

  In the oxidized state (Cu^II), the copper ion in plastocyanin is typically coordinated by two histidine nitrogen atoms, one cysteine sulfur atom, and one methionine sulfur atom. This coordination environment leads to a distorted tetrahedral or distorted trigonal pyramidal geometry around the Cu^II ion. Thus, this statement is generally considered correct, describing the characteristic geometry of the type 1 copper site in plastocyanin.

• Statement B: The geometry around Cu^II changes to square-planar on reduction.

  Reduction of Cu^II in plastocyanin results in Cu^I. Copper(I) ions usually prefer geometries with lower coordination numbers like tetrahedral or linear, not square-planar. Square-planar geometry is more common for certain d⁹ ions like Cu^II, especially with strong field ligands. Upon reduction, the Cu^I center in plastocyanin adopts a more tetrahedral-like geometry, becoming less distorted compared to the Cu^II state. It does not become square-planar.

• Statement C: In the EPR spectrum, the A_|| values are smaller in plastocyanin than in \(\rm [CuCl_{4}]^{2-}\).

  EPR spectroscopy is used to study paramagnetic species like Cu^II. Type 1 copper centers in blue copper proteins like plastocyanin are known for their unusually small hyperfine coupling constant (A_||) in the EPR spectrum compared to typical Cu^II complexes with similar ligands. This small A_|| value is a characteristic feature of the highly covalent copper-cysteine bond in the active site. The complex \(\rm [CuCl_{4}]^{2-}\) exists in distorted tetrahedral or square planar forms depending on the environment, and its EPR spectrum shows larger A_|| values compared to plastocyanin. Therefore, this statement is correct.

• Statement D: The active site contains the amino acids histidine and cysteine only.

  As mentioned in the analysis of Statement A, the copper ion in the active site of plastocyanin is coordinated by ligands from two histidine residues, one cysteine residue, and one methionine residue. The statement claims it contains only histidine and cysteine, which is incorrect as methionine is also a coordinating ligand.

Conclusion on Correct Statements

Based on the analysis:

• Statement A is correct.
• Statement B is incorrect.
• Statement C is correct.
• Statement D is incorrect.

The statements that best describe plastocyanin among the given options are A and C.