Quiz1_s - MIT Department of Biology 7.014 Introductory Biology Spring 2005 Solutions to 7.014 Quiz I Class Average = 72 Median = 73 Grade A B C D F

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MIT Department of Biology 7.014 Introductory Biology, Spring 2005 Solutions to 7.014 Quiz I Class Average = 72 Median = 73 Grade Range % A 82 - 100 22 B 70 - 81 39 C 57 – 69 28 D 41 – 56 10 F 0 – 40 0
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2 Question 1 (36 points) Below is a ribbon representation of the K + channel, a membrane spanning protein made up of four copies of a single polypeptide. The K + channel allows K + ions to be shuttled through the membrane. a) What protein secondary structure is part of the K + channel protein as shown above? K + channel contains only α -helices. b) Does the K + channel have quaternary structure? If yes, describe it. Yes, the channel has quarternary structure. It consists of four interacting polypeptides. It appears that each polypeptide interacts with its two neighboring polypeptides, such that the overall shape of the channel is a barrel. c) Using as a schematic of a phospholipid, draw a cross-section of the membrane around the K + channel shown above. d) What type(s) of amino acids do you expect to find on the K + channel polypeptides i) next to the tails of the membrane lipids? (Circle all that apply) Polar Nonpolar Positively charged Negatively charged Why? The tails of the membrane lipids are hydrophobic, and it is energetically favorable for the protein to have hydrophobic amino acids in the hydrophobic environment. ii) next to the heads of the membrane lipids? (Circle all that apply) Polar Nonpolar Positively charged Negatively charged Why? The heads of the membrane lipids have negatively charged phosphate groups, and it is energetically favorable for the protein to have positively charged amino acids that can form ionic bonds, or polar amino acids that can form hydrogen bonds in that environment. Inside the cell Outside the cell
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Question 1, continued e) If you were trying to estimate the volume occupied by this protein, would the picture above provide all the information you need? Why or why not? No, the picture above is of a ribbon representation, i.e. backbone trace. It does not show any of the sidechains, and thus does not give us a clear picture of how much space the protein occupies. In addition, for the atoms in the backbone, the vdW radii are not shown. The positively charged K + ion is a very small soluble molecule. f) Explain why K + cannot come across the membrane without a channel protein. The tails of the membrane lipids are hydrophobic. It is not energetically favorable for a charged molecule, such as K + to be in a hydrophobic environment. Thus, K + by itself can not come across the membrane. You isolate a mutant of the K + channel that transports less K + than normal. You run both the wild-type and mutant proteins on a denaturing gel and get the following result: Wildtype Mutant g) From the gel data we can conclude that (circle all that apply): i) Each subunit of the mutant protein is shorter than longer than primary secondary tertiary the same length as in the wild-type protein. Justify your answer(s)
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This note was uploaded on 05/02/2009 for the course BIOL 7.014 taught by Professor Walker during the Spring '05 term at MIT.

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Quiz1_s - MIT Department of Biology 7.014 Introductory Biology Spring 2005 Solutions to 7.014 Quiz I Class Average = 72 Median = 73 Grade A B C D F

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