Submit reasoning and process, not merely answers! 3.4. Many proteins that form c

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Submit reasoning and process, not merely answers!

3.4. Many proteins that form channels through membranes pass through the membrane multiple times. For example, rhodopsin, the light receptor protein in the rod cells of the retina, passes through the membrane seven times as alpha-helical chains. Below is a cartoon showing the side view of part of a hypothetical channel-forming protein call it rhodopsin. The circles are amino acid residues, the number of each corresponding to the amino acid's position in the chain. The roman numerals refer to membrane- spanning alpha-helical segments of the protein (only the first two are shown here). The top view shows how the seven a-helices participate in the formation of a pore through the membrane. The pore serves as the means by which protons can pass the membrane in response to light. Congenital retinitis pigmentosa is a genetic disease leading to night-blindness. The disease exhibits a variety of symptoms of different severities, which, in many cases, have been linked to specific mutations in rhodopsin. For each given molecular outcome, choose one or more plausible amino acid mutations that could account for it. In each case, explain, briefly, why your choice(s) would lead to the outcome. a. Rhodopsin found in cytoplasm, fails to A. Insertion of three glutamates between Thr insert in membrane and Glu23- b. Radical change in structure of rhodopsin. B. Insertion of three glutamates between Phes0 c. Overall structure of rhodopsin normal, but d. Structure and function of rhodopsin normal. and Leu3 C. Glu3s mutated to arginine D. Aspiss mutated to leucine. E. Mutation in amino acid not found in mature Channel doesn't form properly channel does not conduct protons. I Top View Side View inside cell 16 II outside cell Trp tryptophan, Tyr tyrosine, Val valine Problem Set 3 (Protein structure/function) 2

Explanation / Answer

Rhodopsin is the integral membrane protein that is held in the membrane through hydrophobic interactions between non polar amino acids and fatty acyl groups of the phospholipids.
A: Rhodopsin found in cytoplasm: (B) if there is insertion of three glutamates (polar negatively charged amino acid) between Phe30 and Leu31 both of which are the non polar amino acids. Their non polar side chain form hydrophobic interactions with non polar fatty acyl chains of lipids of membrane to form the ion channels. So, insertion of a polar amino acid would fail to form such interactions and the protein would not be able to insert itself in the membrane.
B: Radical changes in structure of rhodopsin. (B) Channel does not form properly insertion of three glutamates (polar negatively charged amino acid) between Phe30 and Leu31 both of which are non polar amino acids would not allow the establishment of hydrophobic interactions whichn in turn forms the channels.
C: Overall structure of rhodopsin normal but channel does not conduct protons: (C and D) The amino acids present in lining of the channels should be polar and charged to allow the movement of the protons (charged particles) through them. Here, since protons are positively charged, so the amino acids forming the lining of the channels should be negatively charged (glutamate and asparatate). Presence of positively charged amino acids (such as arginine) would not allow the proton to be transported due to same charge repulsion. Similarly, mutation that replace the positively charged aminoacids with non polar ones also would not allow the membrane protein to transport the protons.
D: Structure and function of rhodopsin normal: (A and E) Insertion of positively charged amino acid glutamate in region where the cahnnel amino acids interact with surrouding water would not affect its structure and function. Mutations in amino acids that are not found in mature rhodopsin ( the amino acids of intron) also would not affect the protein's structure and function.

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