27. The following table provides information about four proteins.
Protein                                             Native mol. wt. (Da)                                  pI                               Type
P                                                             32000                                                     6.4                           monomer
Q                                                            40000                                                     8.5                           homodimer
R                                                            25000                                                      4.9                            monomer
S                                                            45000                                                       8.5                           homotrimer
Which one of the following options correctly identifies the order of elution in size exclusion
chromatography and the increasing order of mobility in SDS polyacrylamide gel?
(A) Chromatrography: SQPR; Electrophoresis: RPQS
(B) Chromatrography: RPQS; Electrophoresis: SQPR
(C) Chromatrography: PRQS; Electrophoresis: PRQS
(D) Chromatrography: SQPR; Electrophoresis: PRQS

Size exclusion chromatography separates proteins by native size, with larger complexes eluting first, while SDS-PAGE denatures proteins and ranks them by subunit molecular weight, where smaller subunits show higher mobility. For the given proteins—P (32 kDa monomer), Q (40 kDa homodimer, 20 kDa subunits), R (25 kDa monomer), S (45 kDa homotrimer, 15 kDa subunits)—option (D) is correct.

Protein Data Overview

The table lists native molecular weights, pI values (irrelevant here), and oligomeric states:

Homodimer Q has 40,000 / 2 = 20 kDa subunits; homotrimer S has 45,000 / 3 = 15 kDa subunits.

Size Exclusion Chromatography Order

Size exclusion (gel filtration) separates native complexes: larger hydrodynamic volumes elute first (SQPR).

• S (45 kDa trimer) elutes first.

• Q (40 kDa dimer) next.

• P (32 kDa monomer).

• R (25 kDa monomer) last.

SDS-PAGE Mobility Order

SDS-PAGE with reducing agents denatures to individual subunits, coats with SDS for charge uniformity, and smaller subunits migrate faster (higher mobility: PRQS).

• S subunits: 15 kDa (fastest).

• Q subunits: 20 kDa.

• R: 25 kDa.

• P: 32 kDa (slowest).

Option Analysis

• (A) Chromatography: SQPR; Electrophoresis: RPQS—Chromatography correct, but electrophoresis wrong (R 25 kDa faster than P 32 kDa, but S/Q smaller).

• (B) Chromatography: RPQS; Electrophoresis: SQPR—Both reversed (ignores native sizes and subunit order).

• (C) Chromatography: PRQS; Electrophoresis: PRQS—Wrong; chromatography needs largest-first, electrophoresis subunit-based.

• (D) Chromatography: SQPR; Electrophoresis: PRQS—Correct on both techniques.

This matches standard biochemistry principles for native vs. denatured separations.