Which two subunits of Na+-NQR can be separated by gel filtration but not by ion exchange chromatography?

• A. NqrA and NqrB
• B. NqrE and NqrF
• C. NqrC and NqrD
• D. NqrG and NqrH

Answer: (B)

The correct answer highlights a specific interaction between Na+-NQR subunits that allows for their separation by gel filtration chromatography but not by ion exchange chromatography. Gel filtration chromatography separates proteins based on their size; smaller proteins pass through the columns while larger proteins are retained longer, effectively allowing the distinction based on molecular weight.

In contrast, ion exchange chromatography separates proteins based on their net charge at a given pH, with oppositely charged groups being attracted to the stationary phase and allowing for the elution of unbound proteins. For two subunits to be separated effectively by gel filtration but not by ion exchange, they must be similar in their charge characteristics, allowing them to interact with the ion exchange matrix to a similar extent.

The selection of NqrE and NqrF indicates that these two subunits may have comparable charges at the experimental pH used in ion exchange chromatography, rendering them unable to distinguish between one another based solely on charge interactions. However, their size difference could be significant enough so that they can be differentiated by gel filtration. Understanding the properties of the subunits—such as their sizes and charge states—helps clarify why they would behave differently under these two types of chromatographic techniques.