# DBT JRF 2020 — Question 77

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

## Question

Two amino acids with negatively charged side chains are

## Options

- **A.** aspartic acid and glutamic acid
- **B.** aspartic acid and glycine
- **C.** lysine and glutamic acid
- **D.** aspartic acid and lysine

## Answer

Option A

## Explanation

Amino acids with negatively charged side chains (acidic amino acids, pKa of side chain ~2–4, so deprotonated and negatively charged at physiological pH 7.4): Aspartate (Asp, D) — side chain: $-\text{CH}_2-\text{COO}^-$ , pKa = 3.65. Glutamate (Glu, E) — side chain: $-\text{CH}_2-\text{CH}_2-\text{COO}^-$ , pKa = 4.25. Both carry a net $-1$ charge at pH 7.4. In contrast: Glycine has no ionisable side chain; Lysine (pKa ~10.5) and Arginine (pKa ~12.5) are positively charged. These acidic amino acids are critical for enzyme active sites (Asp in the catalytic triad of serine proteases; Glu in lysozyme), metal-binding (Ca $^{2+}$ coordination), and signal transduction.

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