Question

Two proteins with totally different primary structures are capable of binding calcium ions with very high affinity and selectivity. Give an explanation as to why two such proteins, with very different primary structures, would have this capability in common.

Answer 1

Ans. Two protein with quite different primary structure and similar affinity and selectivity for a ligand (here, Ca²⁺ ions) are most likely to share a common (or similar) ligand binding domain. During evolution, the two proteins diverged in their primary structure to fit the same function of ligand (Ca²⁺)-binding in different types of cells (prokaryotes, mammalian, plant, fungal cells, etc.) and/or tissues by conserving the ligand-binging domain/motif.

For example, several Ca²⁺ binding proteins have a conserved EF loop consisting of helix-loop-helix motif. The change in primary structure aids the proteins in simultaneous interaction with molecules other than Ca²⁺. For example, both protein 1 and 2 binds Ca²⁺ with same affinity and selectivity because they share a conserved EF loop; but difference in their primary structure ensures that protein 1 also binds to another molecule A and proteins 2 binds another molecule B simultaneously with Ca²⁺ binds. Thus, both proteins can respond to and generate subsequent signals after binding the same ligand (Ca²⁺).