Question

A student isolates a protein from anaerobic bacteria and analyses the protein by polyacrylamide gel electrophoresis containing SDS (PAGE-SDS). Following protein staining, a single band appears, which excites the student's supervisor. To be certain, the supervisor suggests that the student run a second electrophoresis under native conditions (i.e. non-denaturing, or without SDS). This gel shows two bands after staining. Assuming no errors were committed during these experiments, explain the observations.

A. In the absence of SDS, protein subunits separate into two peptide with charges, leading to two peaks.
B. In the absence of SDS protein subunits separate into two peptides with different masses, leading to two peaks.
C. In the presence of SDS two proteins with identical charges but different molecular weights will show a single band, while in the absence of SDS different molecular weights will create separation.
D. In the presence of SDS two proteins with identical molecular weight but different isoelectric points will show a single band, while in the absence of SDS different charges will create separation.

Answer 1

The student observed a single band under PAGE-SDS because SDS denatured the proteins, allowing separation based on size alone. Under native conditions, two bands appeared, indicating that the protein may consist of differently sized subunits that are able to separate when not denatured by SDS.

Step-by-step explanation:

When the student ran the protein sample through PAGE-SDS, the sodium dodecyl sulfate (SDS) detergent denatured the proteins and gave them a uniform negative charge, so they were separated based solely on their size. This is why only a single band appeared after staining.

However, when the sample was run under native conditions without SDS, proteins maintained their native shape and charge, which could have led to different mobilities for the protein subunits. Therefore, the two bands in the native PAGE suggest that the protein may be a multimer consisting of subunits with different masses.

These subunits migrated separately in native conditions because without SDS, the proteins were not uniformly charged and could hence separate based on their native charge and size differences.

Answer 2

The correct answer is option D.

Step-by-step explanation:

In the presence of SDS single band appeared, while in the absence of SDS two bands appeared. SDS or sodium dodecyl sulfate refers to an anionic detergent, which combines with amino acid side chains providing the protein a net negative charge. It dissociates the non-covalent bonds.

In SDS-PAGE, the separation of proteins takes place on the basis of their molecular weight. Options A and B are incorrect as only in the presence of SDS, the separation of protein subunits takes place. Option C is also incorrect as a protein containing distinct molecular weight cannot show single band.

Option D is correct as the presence of SDS supplements a bunch of negative charges to the protein, thus, charge is not the factor. Therefore, the proteins are distinguished on the basis of the molecular weight. Thus, identical molecular weight demonstrates a single band. In the non-presence of SDS, charge performs a function along with the molecular weight, therefore, two bands appear.