Chapter 2 3. Measurement of Acetylcholine Levels by pH Changes The concentration of acetylcholine (a neurotransmitter) in a sample can be determined from the pH changes that accompany its hydrolysis. When the sample is incubated with the enzyme acetylcholinesterase, acetylcholine is quantitatively to yield acetate and a hydrogen ion: (有图) In a typical analysis, 15mL of an aqueous solution containing an unknown amount of acetylcholine had a pH of 7.65. When incubated with acetylcholinesterase, the pH of the solution decreased to 6.87. Assuming that there was no buffer in the assay mixture, determine the number of moles of acetylcholine in the 15mL sample. 8. pH and drug absorption Aspirin is a weak acid with a pKa of 3.5 It is absorbed into the blood through the cells lining the stomach and the intestine. Absorption requires passage through the plasma membrane, the rate of which is determined by the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. The pH of the stomach contents is about 6. Is more aspirin absorbed into the bloodstream from the stomach or from the small intestine? Clearly justify your choice. 16. Control of Blood pH by Respiration Rate (a) The partial pressure of CO2 in the lungs can be varied rapidly by the rate and depth of breathing. For example, a common remedy to alleviate hiccups is to increase the concentration of CO2 in the lungs. This can be achieved by holding one's breath, by very slow and shallow breathing (hypoventilation), or by breathing in and out of a paper bag. Under such conditions, the partial pressure of CO2 in the air space of the lungs rises above normal. Qualitatively explain the effect on the blood pH. (b) A common practice of competitive short-distance runners is to breathe rapidly and deeply (hyperventilate) for about half a minute to remove CO2 from their lungs just before running in, say, a 100m dash. Blood pH may rise to 7.60. Explain why the blood pH increases. (c) During a short-distance run the muscles produce a large amount of lactic acid (CH3CH(OH)COOH, Ka=1.38*10^(-4)) from their glucose stores. In view of this fact, why might hyperventilation before a dash be useful? Chapter 3 10. Net Electric Charge of Peptide A peptide has the sequence Glu-His-Trp-Ser-Gly-Leu-Arg-Pro-Gly (a) What is the net charge of the molecule at pH 3,8, and 11? (Use pKa values for side chains and terminal amino and carboxyl groups as given in Table 3-1.) (b) Estimate the pI for this peptide. 12. The Isoelectric Point of Histones Histones are proteins found in eukaryotic cell nuclei, tightly bound to DNA, which has many phosphate groups. The pI of histones is very high, about 10.8. What amino acid residues must be present in relatively large numbers in histones? In what way do these residues contribute to the strong binding of histones to DNA? 14. Purification of an Enzyme A biochemist discovers and purifies a new enzyme, generating the purification table below. Total protein Activity Procedure (mg) (units) ______________________________________________________________________ 1.Crude extract 20.000 4.000.000 2.Precipitation(salt) 5.000 3.000.000 3.Precipitation(pH) 4.000 1.000.000 4.Ion-exchange chromatography 200 800.000 5.Affinity chromatography 50 750.000 6.Size-exclusion chromatography 45 675.000 (a) From the information given in the table, calculate the specific activity of the enzyme solution after each purification procedure. (b) Which of the purification procedures used for this enzyme is most effective (i.e., gives the greatest relative increase in purity)? (c) Which of the purification procedures is least effective? (d) Is there any indication based on the results shown in the table that the enzyme after step 6 is now pure? What else could be done to estimate the purity of the enzyme preparation? 很高兴 终於学会转寄信件了 以後就可以把批万和批兔的东西互相传送了 -- 当我们以为地球不转时 其实是我们自己的心停止了 只要我们让心动起来 你会发现地球随时都在跳舞 by 623 --

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