11. Protein ‘X’ contains 192 amino acids and mono-ubiquinated at two residues. What could be the
probable molecular weight of the protein? [Molecular weight of ubiquitin is 8 kDa]
a. 11 kDa
b. 20 kDa
c. 37 kDa
d. 28 kDa

Introduction

Protein mono-ubiquitinated at two residues molecular weight calculation involves adding the core protein mass (~20 kDa for 192 amino acids) to two ubiquitin molecules (8 kDa each). Ubiquitination adds ~16 kDa total, yielding ~36-37 kDa observed mass, a common CSIR NET question testing protein modification calculations. This reflects how post-translational modifications alter apparent molecular weight in SDS-PAGE analysis.​

Protein Molecular Weight Basics

Average amino acid residue mass is ~110 Da, so 192 amino acids yield ~21.1 kDa core protein (192 × 110 Da). Each ubiquitin (76 amino acids, ~8.5 kDa actual) is given as 8 kDa, adding covalently via isopeptide bond with minimal mass loss (~0.2 kDa). Mono-ubiquitination at two lysine residues means two separate ubiquitin molecules attached, totaling +16 kDa modification.​

Detailed MCQ Solution

Question: Protein ‘X’ contains 192 amino acids and mono-ubiquitinated at two residues. What could be the probable molecular weight of the protein? [Ubiquitin = 8 kDa]

Calculation: Core protein + 2 × ubiquitin = (192 × 110 Da ≈ 21 kDa) + 16 kDa = ~37 kDa

a. 11 kDa – Too low; even unmodified 192 aa protein (~20-21 kDa) exceeds this. Represents ~100 aa only. Incorrect.​

b. 20 kDa – Matches unmodified core protein mass (192 × ~110 Da). Ignores both ubiquitinations (+16 kDa). Incorrect.​

c. 37 kDa – Correct: ~21 kDa (192 aa) + 16 kDa (2 × 8 kDa ubiquitin) = ~37 kDa. Matches observed SDS-PAGE shift for di-mono-ubiquitinated protein. Correct answer.​

d. 28 kDa – ~21 kDa + 8 kDa (only one ubiquitin). Underestimates by missing second modification site. Incorrect.​

Ubiquitination Mass Calculation Guide

• Core protein: 192 residues × 110 Da average = 21,120 Da ≈ 21 kDa​

• Each mono-ubiquitin: 8 kDa (given)​

• Two sites: 2 × 8 kDa = 16 kDa added​

• Total MW: 21 + 16 = 37 kDa​

• Key concept: “Mono-ubiquitinated at two residues” = two single ubiquitins, not one di-ubiquitin chain.

MW Comparison Table