AKProblemSet8

AKProblemSet8 - Problem Set 8 Chem 114A; 2007 1. Explain...

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Problem Set 8 Chem 114A; 2007 1. Explain why oxygen can cross a red blood cell membrane without the aid of a transport protein. O 2 is small and nonpolar and therefore experiences little thermodynamic barrier to diffusing across the lipid bilayer. 2. Gramicidin A is a 15-residue polypeptide capable of forming a helical structure. But an α helix doesn't have a hollow core. How does gramicidin A mediate transport? The presence of both L- and D-amino acids in gramicidin A renders to form an helix that require more residues per turn. This results in a wider helix with a big ‘hole’ at the center. 3. Describe the features of the KcsA channel from Streptomyces lividans that allow it to transport K + ions selectively. Anionic amino acid side chains at the entrance of the channel attract cations and repel anions. In this manner, cations are selected over anions at the entrance of the channel. The central pore of the channel initially accommodates a hydrated K + ion, but the channel then narrows (the selectivity filter), forcing the K + ion to shed its waters of hydration, and interact with carbonyl groups on the protein instead. The dimensions of the selectivity filter are such that neither a hydrated nor non-hydrated Na + ion is accommodated. 4. What are the essential features of gated ion channels? Gated ion channels must be able to transport ions specifically and rapidly. They must also have the ability to open and close in response to specific stimuli, in order to perform a specific task for the cell. 5. Water molecules transiting an aquaporin might be expected to form a hydrogen- bonded chain that snakes through the pore. What deleterious effects on the cell would occur if water molecules were transported in this fashion? A proton jump would form a gradient of ion across the membrane, an event detrimental to cell. 6.
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AKProblemSet8 - Problem Set 8 Chem 114A; 2007 1. Explain...

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