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Chapter 9: Problem 22

In the description of the mechanism of proton transport by bacteriorhodopsin, we find that light-driven conformation changes promote transmembrane proton transport. Suggest at least one reason for this behavior. In molecular terms, how could a conformation change facilitate proton transport?

Short Answer

Expert verified
Light triggers conformation changes in bacteriorhodopsin which reposition certain amino acids in the protein structure to create a pathway for proton transport. The charged amino acids then drive the proton movement across the pathway, by means of electrostatic interactions, effectively transporting the proton across the membrane.

Step by step solution

01

Understand Bacteriorhodopsin and Proton Movement

Bacteriorhodopsin is a protein used by certain bacteria to capture light energy and use it to move protons across their cellular membranes. This process is vital for the production of ATP, the energy currency of life. This protein exists on the cellular membrane and changes its shape when hit by light - this is its conformation change.
02

Identify the Role of Light-driven Conformation Changes

The process starts when the bacteriorhodopsin absorbs light, causing the retinal molecule (which is part of its structure) to change from a cis to a trans configuration, triggering a conformation change in the whole protein. This conformation change opens a pathway for protons to move from one side of the cellular membrane to the other.
03

Explain How Conformation Change Facilitates Proton Transport

The change in shape of bacteriorhodopsin upon light absorption aligns the amino acids in such a way as to create a pathway for proton movement. Certain amino acids in the protein structure can attract or repel protons, due to their charge, orchestrating a pathway across the membrane. When a proton enters this pathway, it can be 'pushed' from one amino acid to the next, facilitated by the attraction and repulsion forces of the amino acids, until it emerges on the other side of the membrane. Thus a conformation change in the protein can physically 'pump' a proton across the membrane.

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Most popular questions from this chapter

As described in this chapter, proline introduces kinks in transmembrane \(\alpha\) -helices. What are the molecular details of the kink, and why does it form? A good reference for this question is von Heijne, G. 1991\. Proline kinks in transmembrane \(\alpha\) -helices. Journal of Molecular Biology \(218: 499-503 .\) Another is Barlow, D. \(\mathrm{J}\)., and Thornton, J. M., \(1988 .\) Helix geometry in proteins. Journal of Molecular Biology \(201: 601-619\)

Sucrose gradients for separation of membrane proteins must be able to separate proteins and protein-lipid complexes having a wide range of densities, typically 1.00 to \(1.35 \mathrm{g} / \mathrm{mL}\) a. Consult reference books (such as the CRC Handbook of Biochemistry \()\) and plot the density of sucrose solutions versus percent sucrose by weight (g sucrose per 100 g solution), and versus percent by volume (g sucrose per \(100 \mathrm{mL}\) solution). Why is one plot linear and the other plot curved? b. What would be a suitable range of sucrose concentrations for separation of three membrane-derived protein-lipid complexes with densities of \(1.03,1.07,\) and \(1.08 \mathrm{g} / \mathrm{mL} ?\)

(Integrates with Chapter 3 .) Fructose is present outside a cell at \(1 \mu M\) concentration. An active transport system in the plasma membrane transports fructose into this cell, using the free energy of ATP hydrolysis to drive fructose uptake. What is the highest intracellular concentration of fructose that this transport system can generate? Assume that one fructose is transported per ATP hydrolyzed; that ATP is hydrolyzed on the intracellular surface of the membrane; and that the concentrations of ATP, ADP, and \(P_{i}\) are \(3 \mathrm{m} M, 1 \mathrm{m} M,\) and \(0.5 \mathrm{m} M,\) respectively. \(T=298 \mathrm{K}\). (Hint: Refer to Chapter 3 to recall the effects of concentration on free energy of ATP hydrolysis.)

singer and Nicolson's fluid mosaic model of membrane structure presumed all of the following statements to be true EXCEPT: a. The phospholipid bilayer is a fluid matrix. b. Proteins can be anchored to the membrane by covalently linked lipid chains. c. Proteins can move laterally across a membrane. d. Membranes should be about 5 nm thick. e. Transverse motion of lipid molecules can occur occasionally.

Proline residues are almost never found in short \(\alpha\) -helices; nearly all transmembrane \(\alpha\) -helices that contain proline are long ones (about \(20 \text { residues }) .\) Suggest a reason for this observation.
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