hmk9ANS_07 - NAME_ T.A.name & sect.no._ Page 1...

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NAME_____________________________ T.A.name & sect.no._________________________ Page 1 p. 1 Biochemistry 462a -- Problem Set #9 (ANSWERS) October 26, 2007 (due November 2, 2007) 15 points READING: Lehninger Principles of Biochemistry , 4th ed., Chapter 6 Show all work, including units where appropriate. 1. (3.5 pts) Suppose that an enzyme uses a catalytic mechanism in which a functional group on the enzyme has to accept a proton from the intermediate formed in the first chemical step. The pK a of that basic functional group on the enzyme is 8.0, and is the only pK a apparent on a plot of k cat vs. pH for the activity of this enzyme. The k cat of the fully active form of the enzyme is 1 x 10 5 sec –1 . A. (1 pt) On the axes at right, sketch the plot of k cat vs. pH to show the effect of pH on the activity of this enzyme, using the information given above. B. (0.5 pt) The functional group responsible for the pH dependence of the activity of this enzyme is acting as a __ GENERAL BASE ___ (term for the catalytic mechanism involved). C. (0.5 pt) What type of functional group might be responsible for the pH effect on k cat ? Justify your answer based on the "generic" pK a values of functional groups in peptides and proteins. Best answer is cysteine (Cys, C, thiol group) or N-terminal α -amino group of whole polypeptide chain. A lysine (Lys, K, ε -amino) or tyrosine (Tyr, Y, aromatic OH) with an unusually low pK a , or even a histidine (His, H, imidazole) with an unusually high pK a are possible answers if the unusual pK a is noted . D. (0.5 pt) Draw the structure of the functional group in its active form. Justify your answer based on the "generic" pK a values of amino acid residues' functional groups (R groups) in peptides/proteins. Conjugate base form is Cys thiolate
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hmk9ANS_07 - NAME_ T.A.name & sect.no._ Page 1...

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