Proteins A, B, C, and D are dimers. They were run on a reducing SDS gel, shown below: What is the order of molecular weights of the proteins in their native state, from largest to smallest?

Question

Proteins A, B, C, and D are dimers.  They were run on a reducing SDS gel, shown below:  What is the order of molecular weights of the proteins in their native state, from largest to smallest?

Quizplus · Accepted Answer

11ecba2d_0ef0_ceec_a3bf_5f510ed6e8bb_MD0008_00 In their native (ie, properly folded) form, many proteins consist of multiple polypeptide chains known as subunits. These proteins, called multimers, may consist of two subunits (dimers), three subunits (trimers), four subunits (tetramers), or more. When a multimer runs through a reducing SDS gel, the sodium dodecyl sulfate (SDS) causes proteins to unfold, and the reducing agent breaks any disulfide bonds that may link subunits together. Each subunit migrates through the gel by itself, and subunits of different sizes are separated. For multimers in which all subunits are identical (homomultimers), all subunits migrate the same distance and appear as a single band. If the subunits are not identical (heteromultimers), they migrate different distances according to molecular weight and appear as separate bands. According to the question, each protein in the gel is a dimer. Lanes A and C each show single bands, indicating that these proteins are homodimers. In contrast, Proteins B and D separate into two distinct bands, indicating that they are heterodimers. In their native (biologically active) form, the dimers have a molecular weight equal to the sum of the weights of their subunits. For heterodimers, the weights of each band (indicated by the marker lane) can be added. For homodimers, the weight of the single band can be doubled to account for both subunits. From largest to smallest, the order of the proteins is Protein C (80 kDa) → Protein D (65 kDa) → Protein A (60 kDa) → Protein B (55 kDa). (Choices A and C) Protein C is the largest of the proteins shown at 80 kDa and should be the first in order, followed by D, A, and B. (Choice D) Proteins A and D each have a 25-kDa subunit. However, protein D also has a 40-kDa subunit, whereas Protein A has a 30-kDa subunit. Therefore, protein D is larger than protein A. Educational objective: In their native form, many proteins consist of multiple polypeptide chains (subunits) and are known as multimers (eg, dimers, trimers, tetramers). Reducing SDS gels separate multimer subunits by size. Identical subunits migrate the same distance and form a single band, whereas distinct subunits migrate different distances and form separate bands.

Proteins A, B, C, and D Are Dimers