Which of the following describes a commonly used method of determining protein subunit mass?

Options:
explanation: This describes SDS-PAGE, a method of determining the mass of a protein accurately. The SDS addition to a denatured protein ensures a standardised polypeptide chains mass-to-charge ratio so the movement induced by the current accurately reflects the mass of each protein subunit when compared to the standard ladder.
name: Denature the protein by boiling with SDS, add Coomassie blue stain, load into a well of polyacrylamide gel, apply a current, and compare the test well to a well with a standard ladder
label: a
answer: yes
explanation: Although the structures resolved by X-ray cryo-crystallography could technically be used retrospectively to work out the mass of a protein subunit, to resolve the structure of a protein, the primary structure must already be known which means the mass of the protein must already be known. Structural techniques are used after the determination of a protein subunit's mass. They are also highly laborious, long and expensive compared to methods for determining mass such as SDS-PAGE.

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name: A solution of a protein is concentrated and a precipitant is added to cause crystallisation, then the crystals are rapidly frozen in a loop by liquid nitrogen and X-rays from a synchrotron are passed through the repeating 'cells' of protein causing a scatter pattern. From this pattern, the structure and therefore mass of a protein can be determined using Fourier transformations once the phase problem is accounted for by repeating with alterations of the protein structure through synthesis with heavy atoms added such as selenium in selenomethionine, or by molecular replacement models using similar previously resolved protein structures
label: e
answer: no
explanation: Flow cytometry is used to identify subsets of cells that express different amounts of a protein of interest, but has no function in the determination of protein subunit mass.
name: Add a fluorescent antibody complementary to the protein to cells expressing it then pass the cells through a flow cytometer
label: b
answer: no
explanation: This answer choice describes enzyme-linked immunosorbent assay (ELISA), which is a sensitive method used to measure the presence and concentration of a specific protein in a solution rather than its molecular mass.
name: Addition of different dilutions of a solution of the protein to a 96-well plate to which antibodies complementary to the protein have been attached. This is followed by washing and the addition of a second complementary antibody to which a reporter enzyme is conjugated. Further washing is performed and a reporter solution is added after which the intensity of the signal received is analysed through comparison to a standard curve
label: c
answer: no
explanation: This is a Bradford assay which is a method used to measure the presence and concentration of all proteins in a solution rather than the molecular mass of a specific protein.
name: Addition of different dilutions of a solution of the protein to a 96-well plate, followed by the addition of Coomassie blue stain to each well and then analysis of the intensity of the signal received through comparison to a standard curve
label: d
answer: no

Correct answer: a