Consider the following scenario: You have two related proteins, the wild-type pr
ID: 174596 • Letter: C
Question
Consider the following scenario: You have two related proteins, the wild-type protein which exists as a homodimer, and a mutant protein that exists as a monomer in solution. Unfortunately, your sample tube labels washed off, and you have no idea which sample is which, (a) How could Guinier plots help to distinguish between these two samples? (b) Of course, the example above is a simplification - the wild-type protein exists primarily as a dimer but there is also some wild-type monomer present in the sample, and in the mutant sample there is also a minor amount of mutant homodimers present in that sample. How will this affect the Guinier plots or your analysis?Explanation / Answer
For such study, the forward scattering intensity at q=0 is given by:
I0= N (p V) 2
In case of dimers, the N (number of scatters) is halved whereas V (volume) per scatter doubles which results in overall doubling of I0 compared to a monomer. The molecularity of a sample is found to be proportional to the forward scattering intensity. This can be studied by comparison with a standard monomer. Therefore, the difference in such sample containing a wild type homodimer protein and a mutant monomer protein can be differentiated based on such study.
In case of sample with primarily wild type homodimer protein containing some wild type monomer protein, the plot will reflect a small shift in population from homodimer towards monomer. Whereas, in case of sample containing mutant monomer protein primarily with some mutant homodimer, there is a slight shift in population from monomer towards homodimer.