Question

Which of the pairs of amino acids can form a salt bridge at physiological pH?

lysine and glutamic acid

two cysteines

arginine and tyrosine

glutamic acid and aspartic acid

aspartic acid and histidine

Answer 1

Correct answer is A) Lysine and Glutamic acid

Reason: Lysine is a basic amino acid. Its side chain amino group is protonated at physiological pH resulting in positive charge.

Glutamic acid is an acidic amino acid. Its side chain carboxyl is deprotonated at physiological pH resulting in negative charge.

Hence, these two will form salt bridges.

Incorrect choices:

Two cysteines : These two form a disulfide bond rather than a salt bridge.

Arginine and Tyrosine: Arginine bears positive charge at physiological pH but Tyrosine has unionized side chain.

Glutamic acid and aspartic acid : Both these are negatively charged at physiological pH and hence cannot form salt bridge with each other.

Aspartic acid and histidine : Aspartic acid is negatively charged while histidine is uncharged at physiological pH. Hence, it does not forms salt bridges.