Module 7 questions

Question 1

The core principle ;linking all the function of all passive transporters (Gramicidin A, Porins, K+ channels, and Aquaporins is that transport specificity is determined by

Answer 1

The precise positioning of amino acid residues in the channel

Question 2

The peptide ________________forms membrane-spanning channel that facilitates the passive diffusion of Na+ and K+. This channel is unique because it is formed by a linear peptide with alternating L- and D- amino acids

Answer 2

Gramicidin A

Question 3

T/F: The K+ channel protein displays a selectivity of approximately 100- fold for K+ ions over Na+ ions

Answer 3

False, 10,000 fold not an 100 fold

Question 4

What is the molecular basis for the high selectivity of the K+ channel that allows it to exclude Na+?

Answer 4

The channel’s carbonyl oxygen atoms interact favorably with desolated K+ ions, but not with Na+ ions

Question 5

To pass through the selectivity filter of the K+ channel, the K+ ion must first perform what critical step regarding its structure?

Answer 5

The K+ ion must shed its hydration layer (or desolvate ) to favorable interact with the backbone carbonyl oxygens of the channel

Question 6

Aquaporins provide an efficient means for H2O molecules to pass through membranes. This transport occurs solely in response to:

Answer 6

An osmotic gradient

Question 7

T/F: aquaporin selectivity is achieved primarily by two short alpha-helices that for a physical restriction point, preventing any molecule larger than water from passing

Answer 7

True
While the Asn residues are also critical, the two short alpha helices form the core physical restriction

Question 8

The two short alpha helices that form the constriction point in the aquaporins contain a conserved __________________________ motif

Answer 8

Asn-Pro-Ala (NPA)

Question 9

Besides physical restriction, how do the Asn residues within the aquaporin channel’s constriction point contribute to water selectivity and the exclusion of H+ (protons)

Answer 9

The ASN residues hydrogen bond with the H2O molecules, ensuring single-file movement and disrupting the H2O hydrogen bonding chain that is essential for proton transport or proton hopping, thus excluding H+

Question 10

Porin proteins such as the VDAC in mitochondria are structurally characterized by what common motif that spans the membrane?

Answer 10

A transmembrane beta-barrel structure