Question

1. View the side groups in the model, with attention to the water channel. What do the numerous aromatic rings on the surface of the channel suggest about the chemical properties of the channel?

The channel will have a net negative charge.

The channel will have significant hydrophobic properties.

The channel will have significant hydrophilic properties.

The channel will have a net positive charge.

2. Conserved histidine, asparagine (blue), and cysteine (yellow) residues are present on the surface of the water channel. Which of the following is LEAST likely to hydrogen-bond with a water molecule?

the R group (side chain) of the cysteine

the R groups (side chains) of the two asparagines

the backbone carbonyl groups of amino acids on the surface of the channel

the R group (side chain) of a histidine

3. What prevents glycerol from passing through the water channel?

the backbone carbonyl groups of amino acids in the channel

the chemical properties of the cysteine in the channel

the chemical properties of the two conserved asparagines in the channel

the narrow diameter (~2.8 Å) of the selectivity channel

4. Aquaporins allow water to pass through the channel but prevent other solutes and ions from also crossing the membrane. Protons can move with water using the same hydrogen bonding facillitated movement. What PREVENTS protons from passing through the water channel?

the backbone carbonyl oxygen groups of amino acids in the channel

the chemical properties of the amine groups of the two conserved asparagines in the channel

the narrow diameter (~2.8 Å) of the selectivity channel

the chemical properties of the sulfur within the R group of the conserved cysteine in the channel

Answer 1

Ans. 1. B. The channel will have significant hydrophobic properties.

Note: The aromatic residues present on the surface of AQP make ‘the surface’ hydrophobic. The information is not enough to predict the polarity of the interior core/ channel.

Ans. 2. A. The R groups of the cysteine.

The formation of H-bonds requires highly electronegative atoms (N, O) and H-atoms. Histidine (has N-atoms in imidazole ring) and asparagine (N, O in side chain) can form H-bonds between same type of different types of molecules. Residues with carbonyl croup (-C=O) also form H-bonds. Cysteine side chain does not N or O-atoms but S-atom. Though cysteine also forms H-bonds but with very low frequency than His and Asn due to higher electronegativity of S-atom.      

Ans. 3. D. The narrow diameter (~ 2.8A⁰) of the selectivity channel.

Glycerol is also polar like water because of have -OH groups. But it’s much larger the pore diameter of the aquaporin, thus excluded from passing through AQP.

Ans. 4. A. the backbone carbonyl oxygen of amino acids in the channel.

The interior polar lumen of the AQP proteins inhibits the translocation of H⁺ ions through the lumen. Protons are generally present as H₃O⁺ ions in aqueous solution and rapidly move from one to another point through Grotthuss mechanism. However, the polar lumen (provide dielectric properties of the lumen) of the AQP resists proton translocation through this mechanism along the lumen.

Note: The mechanism inhibiting proton translocation through AQP has not yet been fully elucidated. The one presented above is the most probably one.