# DBT JRF 2020 — Question 144

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

## Question

A mixture of three proteins (X, Y and Z) was loaded on a size exclusion column.
| Protein | MW (KD) | pI |
| --- | --- | --- |
| X | 140.75 | 5.5 |
| Y | 22.3 | 10.1 |
| Z | 88.6 | 2.8 |
 The correct order of elution of the proteins from the column is

## Options

- **A.** X, Z, Y
- **B.** Y, X, Z
- **C.** X, Y, Z
- **D.** Y, Z, X

## Answer

Option A

## Explanation

Size exclusion chromatography (SEC/GFC) elution order: Larger molecules elute FIRST (excluded from pores; shorter path through column); smaller molecules elute LAST (enter pores; longer effective path). Given proteins from the table: X = 140.75 kDa (largest), Z = 88.6 kDa (medium), Y = 22.3 kDa (smallest). Elution order (first to last): X → Z → Y. Column elution: X, Z, Y. This is the reverse of molecular weight order in denaturing SDS-PAGE. In native SEC, the hydrodynamic radius ( $R_h$ , Stokes radius) — not just MW — determines elution; however for globular proteins of similar shape, $R_h \propto M_r^{1/3}$ , and larger MW → larger $R_h$ → earlier elution.

Source: https://mypyqbuddy.com/questions/a-mixture-of-three-proteins-x-y-and-6a05a8a02adafec4702932a8