Suppose a cell membrane is 45 Å thick & an embedded protein has 7 parallel transmembrane α-helical segments. Calculate the minimum # of aa-residues required for all 7 α-helical segments to traverse the membrane.

• A. 125 residues.
• B. 39 residues.
• C. 71 residues.
• D. 210 residues.

Answer: (D)

Verified step by step guidance

1. Understand that each α-helix in a transmembrane protein typically spans the lipid bilayer, which is approximately 45 Å thick in this case.
2. Recall that an α-helix has a rise of about 1.5 Å per amino acid residue.
3. Calculate the number of amino acid residues required for one α-helix to span the membrane by dividing the membrane thickness (45 Å) by the rise per residue (1.5 Å/residue).
4. Determine the total number of residues required for all 7 α-helical segments by multiplying the number of residues per helix by 7.
5. Verify the calculation to ensure that the total number of residues matches the given correct answer of 210 residues.