Question

Scientist interested in designing protein-based biosensors determined the three-dimensional structure of a bacterial membrane protein that allows mainly hydrophobic solutes to enter the periplasmic space. The Ramachandran plot based on the experimental structure is shown below. The colored regions indicate allowed conformational space while the observed data points are shown by small dots.

180 135 90 45 -45 -135 180 -135 0 5 45 90 135 180 Phi (degrees)

The same protein was also characterized by CD measurements. Unfortunately, the graduate student assigned to the task measured CD spectra of three different proteins on the same day and forgot to label spectra. Identify which spectrum most likely belongs to this bacterial membrane protein:

Select one:

Answer 1

The points in the Ramachandran plot are majorly concentrated near the top left corner. This is the typical pattern when the protein considered consists largely of beta sheets. Hence, the bacterial membrane protein can be inferred to be majorly in the beta sheet form, forming a beta barrel through the membrane.

Circular dichroism sprectroscopy is a light based method used to identify the secondary structures of proteins, apart from other things. The different secondary structures show characteristic curves. The first curve (optio A) is typically observed when the protein is composed majorly of alpha helical segments. The second curve (option B) indicates a globular soluble protein (mixed). The third option shows a curve most closesly similar to the curve observed for proteins forming beta sheets, although the variance of the curve is very high in case of proteins having mainly beta sheets.

Hence, the correct option is C.