Essay on Midterm Key

NAME:_____________________________

STUDENT ID #:______________________________

Multiple choice. Select the ONE best answer for each question. Two points each. 1. Which of these statements about hydrogen bonds is not true? A) Hydrogen bonds account for the anomalously high boiling point of water. B) In liquid water, the average water molecule forms hydrogen bonds with three to four other water molecules. C) Individual hydrogen bonds are much weaker than covalent bonds. D) Individual hydrogen bonds in liquid water exist for many seconds and sometimes for minutes. E) The strength of a hydrogen bond depends on the linearity of the three atoms involved in the bond. 2. Hydrophobic interactions make important energetic contributions to: A) B) C) D) E) binding of a hormone to its receptor protein. enzyme-substrate interactions. membrane structure. three-dimensional folding of a polypeptide chain. all of the above are true.

3. The pH of a sample of blood is 7.4, while gastric juice is pH 1.4. The blood sample has: A) B) C) D) E) 0.189 times the [H+] as the gastric juice. 5.29 times lower [H+] than the gastric juice. 6 times lower [H+] than the gastric juice. 6,000 times lower [H+] than the gastric juice. a million times lower [H+] than the gastric juice.

4. The chirality of an amino acid results from the fact that its α carbon: A) B) C) D) E) has no net charge. is a carboxylic acid. is bonded to four different chemical groups. is in the L absolute configuration in naturally occurring proteins. is symmetric.

5. Two amino acids paired below both contain hydroxyl groups. They are: A) B) C) D) E) cysteine and serine. cysteine and threonine. methionine and cysteine methionine and serine threonine and serine.

NAME:_____________________________

STUDENT ID #:______________________________

6.

All of the amino acids that are found in proteins, except for proline, contain a(n): A) B) C) D) E) amino group. carbonyl group. carboxyl group. ester group. thiol group.

7. Titration of valine by a strong base, for example NaOH, reveals two pK’s. The titration reaction occurring at pK2 (pK2 = 9.62) is: A) B) C) D) E) —COOH + OH− —COOH + —NH2 —COO− + —NH2+ —NH3+ + OH− —NH2 + OH− → → → → → —COO− + H2 O. —COO− + —NH2+. —COOH + —NH2. —NH2 + H2 O. —NH− + H2O.

8. In a highly basic solution, pH = 13, the dominant form of glycine is: A) B) C) D) E) NH2—CH2—COOH NH2—CH2—COONH2—CH3+—COONH3+—CH2—COOH. NH3+—CH2—COO-

9. For amino acids with neutral R groups, at any pH below the pI of the amino acid, the population of amino acids in solution will have: A) B) C) D) E) a net negative charge. a net positive charge. no charged groups. no net charge. positive and negative charges in equal concentration.

10. The peptide AEGAL has: A) B) C) D) E) a disulfide bridge. five peptide bonds. four peptide bonds. no free carboxyl group. two free amino groups.

11. At the isoelectric pH of a tetrapeptide: A) B) C) D) E) only the amino and carboxyl termini contribute charge. the amino and carboxyl termini are not charged. the total net charge is zero. there are four ionic charges. two internal amino acids of the tetrapeptide cannot have ionizable R groups.

NAME:_____________________________

STUDENT ID #:______________________________

12. By adding SDS (sodium dodecyl sulfate) during the electrophoresis of proteins, it is possible to: A) B) C) D) E) determine a protein’s isoelectric point. determine an enzyme’s specific activity. determine the amino acid composition of the protein. preserve a protein’s native structure and biological activity. separate proteins exclusively on the basis of molecular weight.

13. To determine the isoelectric point of a protein, first establish that a gel: A) contains a denaturing detergent that can distribute uniform negative charges over the protein’s surface. B) exhibits a stable pH gradient when ampholytes become distributed in an electric field. C) is washed with an antibody specific to the

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