# DBT JRF 2020 — Question 83

Exam: DBT JRF 2020 · Section: B · Topic: Biochemistry · Difficulty: Hard

## Question

The comparison of the structures of haemoglobin and myoglobin shows that they have

## Options

- **A.** different primary structure but similar tertiary structure
- **B.** similar primary and tertiary structures
- **C.** different primary and tertiary structures
- **D.** similar primary structure but different tertiary structure

## Answer

Option A

## Explanation

Haemoglobin ( $\alpha_2\beta_2$ tetramer, MW ~64 kDa per subunit) and myoglobin (monomer, MW ~17 kDa) are both members of the globin protein superfamily. Primary structure comparison: Human haemoglobin $\alpha$ -chain and myoglobin share only ~25–30% amino acid sequence identity — DIFFERENT primary structures. 3D structure comparison: Despite divergent sequences, both fold into the characteristic globin fold: 8 $\alpha$ -helices (designated A–H) with interconnecting loops, a central hydrophobic core, and a haem (iron-protoporphyrin IX) binding pocket. The fold is conserved by function (oxygen binding). This is an outstanding example of structural convergence/conservation despite sequence divergence — different primary structures but similar tertiary (globin fold) structures.

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