Beneath some environmental conditions the halophile Halobacterium halobium synthesizes a membrane protein (MW 26,000) bacteriorhodopsin, which is purple as it comprises retinal. Molecules of this protein aggregate into purple.
Patches in the cell membrane. Bacteriorhodopsin acts as a light activated proton.
Pump which gives energy for cell functions. X-ray analysis of this protein
Reveals that it comprises of seven parallel alpha-helical segments, each of which transverses the bacterial cell membrane (thickness 4.5 nm = 45Å).
a) Compute the minimum number of amino acids essential for one segment of alpha-helix to traverse the membrane completely.
b) Estimate the fraction of the bacteriorhodopsin protein which takes place in alpha-helical form.