GATE 2026 CY – Question 24

Inorganic Chemistry · Bioinorganic Chemistry: Ion (Na+ and K+) transport, oxygen binding, transport and utilization, electron transfer reactions, nitrogen fixation, metalloenzymes containing magnesium, molybdenum, iron, cobalt, copper and zinc. · 1 mark · Multiple select

The correct statement(s) regarding plastocyanin is(are)

It is an electron transfer protein
It is intensely colored in its oxidised form
Its central metal ion has a distorted tetrahedral geometry
It contains S2− bridged metal cluster

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Correct answer: (A) It is an electron transfer protein; (B) It is intensely colored in its oxidised form; (C) Its central metal ion has a distorted tetrahedral geometry

Explanation

**Plastocyanin** is a small blue copper protein found in plants and cyanobacteria.

**A. An electron transfer protein.** True: it carries electrons between photosystem II and photosystem I. ✓
**B. Intensely coloured in its oxidised form.** True. Cu(II) in plastocyanin is deep blue, owing to an intense $S(\text{Cys})\to\text{Cu(II)}$ ligand-to-metal charge-transfer band near 600 nm. ✓
**C. Its metal ion has a distorted tetrahedral geometry.** True. Copper is bound to two histidine N atoms, a cysteine S and a (long) methionine S, in a distorted tetrahedral site that suits both Cu(I) and Cu(II). ✓
**D. It contains an $S^{2-}$-bridged metal cluster.** False. That description fits iron-sulfur proteins (such as ferredoxins). Plastocyanin has one copper ion.

Answer **A, B and C**.