A protein has an apparent mass of 800 kD by gel filtration...

A protein has an apparent mass of $800 \mathrm{kD}$ by gel filtration chromatography, but SDS-PAGE shows a single band at a position corresponding to $200 \mathrm{kD} .$ In an ultracentrifuge, will the protein exhibit a sedimentation coefficient corresponding to $200 \mathrm{kD}$ or $800 \mathrm{kD} ?$

Key Concepts

Protein Quaternary Structure

The quaternary structure refers to the assembly of multiple protein subunits into a functional complex. In this question, the protein appears to be monomeric by SDS-PAGE but exhibits a larger size in native conditions, indicating that multiple subunits (oligomerization) form the mature, functional complex.

Ultracentrifugation and Sedimentation Coefficient

Analytical ultracentrifugation measures the sedimentation behavior of macromolecules under high centrifugal forces. The sedimentation coefficient depends on the molecule's mass, shape, and density. Under native conditions, it reflects the behavior of the intact oligomeric assembly rather than the individual denatured subunits.

SDS-PAGE

SDS-PAGE is an electrophoretic method where proteins are denatured and coated with SDS, giving them a uniform negative charge. This causes them to be separated primarily by their molecular weight as single polypeptide chains. It provides the mass of individual subunits since the quaternary structure is lost during sample preparation.

Gel Filtration Chromatography

This technique separates molecules based on their size and shape in their native state. Larger molecules or complexes elute earlier than smaller ones because they are excluded from entering the pores of the gel matrix. In the context of this question, the apparent mass measured by gel filtration reflects the size of the oligomeric assembly rather than the mass of a single monomer.

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