The thickness of a typical bilayer membrane is about 75 A (7.5 nm). If a helical polypetide with pitch 0.54 nm

Question

The thiness of a typical bilayer membrane is about 75 A (7.5 nm). If a helical polypeptide with pitch 0.54 nm having 3.6 residues per turn spans the membrane how many amino acids are within the bilayer?

Answer 1

Tough one, but i think prev poster is right. But they probably want an exact number. You need to know how long each turn is, ie what is the height of the helix. What is the pitch anyway? Okay, from this web page (below), pitch is the turn of the helix through 360 degrees. So that's 5.4 A in this example. So 3.6 amino acids will fit in one pitch, and thats 5.4A. So 75/5.4A is 13.9 turns of the helix, so total # of aa is 13.9*3.6=50. It's a theoretical question, they are more interested in how you got the answer, not 'all membranes have 20 or so aa possible embedded'.

Answer 2

Numbers aside, it takes 20 amino acids to span a lipid bilayer in helical form. It's a trick question meant for you to do a whole bunch of unnecessary calculation.