Question

In an alpha-helical protein that spans the cell membrane, __ would likely be in the center of the membrane while __ would likely be associated with the polar head groups and __ would likely be associated with the aqueous environment.

(a) Trp, Asn, Ala

(b) Leu, Gln, Asp

(c) Tyr, Glu, Glu

(d) Ile, Arg, Gln

I believe the answer to be B or D but I need to explain it. I would think it would be D because the positive charge of Arg would interact with the negative polar heads....... I don't know if this logic holds up. Please help.

Answer 1

Ans. Your notion is correct.

The hydrophobic core (the center of membrane) exerts favorable hydrophobic interactions with hydrophobic (non-polar) amino acid like leucine and isoleucine here. So, either of options B and D are true for this part.

# The polar head groups of the membrane are negatively charged due to presence of phosphate. To exert favorable ionic interactions with negatively charged heads, the amino acid must be positively charged at physiological pH. Among the options B and D, only arginine is positively charged whereas glutamine is neutral. So, among options B and D, option D is most appropriate.

# Glutamine is a polar, uncharged residue, and can interact with the polar aqueous environment through H-bonds.

# So, the correct option is- D.