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Name:_____________________
Introduction to Biological Chemistry
September 18, 2015
Exam #1
I. Short Answer/Problems: Please write complete but concise answers to questions. Show all work for
calculations. Partial credit will be given based on the work or answers provided.
1. (24 pts) When acetylcholine is incubated with the enzyme acetylcholinesterase, acetylcholine is
quantitatively converted to choline and acetic acid (which dissociates to yield acetate and a hydrogen
ion). In a typical analysis, 15 mL 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 there was no buffer in the assay mixture, determine the number of
moles of acetylcholine in the 15 mL sample.
Name:_____________________
2. (32 pts) Given 0.10 M solutions of acetic acid (pK a = 4.76) and sodium acetate, describe how you
would go about preparing 1.0 L of 0.10 M acetate buffer at pH 4.00.
Name:_____________________
3. The first reaction in glycolysis is the phosphorylation of glucose to produce glucose-6-phosphate.
This is a thermodynamically unfavorable process, with ∆Gº’ = 14 kJ/mol.
(a) (20 pts) In a liver cell at 37 ºC, the concentrations of both phosphate and glucose are normally
maintained at about 5 mM each. What would the equilibrium concentration of glucose-6phosphate be, according to the above data?
(b) (4 pts) In fact, this reaction is coupled to ATP hydrolysis to give the overall reaction, ATP +
glucose ↔ glucose-6-phosphate + ADP. What is the ∆Gº’ for this overall reaction?
Name:_____________________
4. Calculate the isoelectric points of:
(a) (10 pts) arginine (R-group pK a = 12.5)
(b) (10 pts) glutamate (R-group pK a = 4.1)
5. (18 pts) Draw the chemical structure of glutathione (γ-Glu-Cys-Gly) at physiological pH. Note that
the γ indicates that the peptide bond between Glu and Cys is formed between the γ-carboxyl of Glu
and the amino group of Cys.
Name:_____________________
6. (12 pts) The mass of a protein was determined by gel filtration chromatography to be 60 kd.
Chromatography in the presence of 6 M urea yields a 30 kd species. When the chromatography is
repeated in the presence of 6 M urea and 10 mM β-mercaptoethanol, a single molecular species of 15
kd results. Explain these results and the structure of the protein.
II. Multiple choice: Circle the letter of the best answer. Each question is worth two points. There is no
partial credit on this portion of the exam.
1. The major precursors for the formation of biomolecules include all EXCEPT:
a. nitrate and dinitrogen.
b. water.
c. carbon dioxide.
d. ammonium ion.
e. none, all are major precursors.
2. Which are the four most common elements in the human body?
a. hydrogen, calcium, oxygen and sodium
b. hydrogen, oxygen, iron and carbon
c. hydrogen, oxygen, carbon and nitrogen
d. oxygen, carbon, iron and nitrogen
e. oxygen, silicon, calcium and nitrogen
Name:_____________________
3. Grapefruit juice at pH 3.2 contains about ____ times as much H+ as orange juice at pH 4.3.
a. 0.9
b. 10−7.5
c. 10−2
d. 12
e. 101
4. Buffers have all of the following characteristics EXCEPT:
a. they have relatively flat titration curves at the pH(s) where they buffer.
b. they resist changes in their pH as acid or base is added.
c. they are typically composed of a weak acid and its conjugate base.
d. they buffer best for polyprotic acids half-way between the two pK a values.
e. buffer where the amounts of conjugate base are nearly equivalent to the amounts of
weak acid.
5. Which of the following statements is true for the Second Law of Thermodynamics?
a. Systems tend to proceed from disordered states to ordered states.
b. The entropy of the system plus its surroundings is unchanged by reversible
processes
c. The entropy of the system plus its surroundings decreases for irreversible
processes.
d. Naturally occurring processes never proceed toward equilibrium.
e. All of the above are true
6. ____ and ____ are a small family of universal biomolecules mediating the flow of energy from
exergonic reactions to the energy requiring processes of life.
a. Reduced coenzymes, caffeine
b. High-energy phosphate compounds, caffeine
c. Chlorophyll, caffeine
d. Hemoglobin, chlorophyll
e. Reduced coenzymes, high-energy phosphate compounds
7. Which statement MOST COMPLETELY EXPLAINS the large ∆G° for the hydrolysis of
phosphoenolpyruvate?
a. Hydrolysis of the phosphate bond along with the conversion of the enol to the keto
form of pyruvate.
b. The hydrolysis of the phosphoanhydride bond.
c. Conversion from the enol to the keto form of pyruvate.
d. Strong bond energy along with a change in the stereochemistry of the molecule.
e. The large ∆G° of the phosphoester bond.
Name:_____________________
8. All of the amino acids EXCEPT ____ have both free α-amino and free α-carboxyl groups.
a. aspartic acid
b. proline
c. asparagine
d. lysine
e. valine
9. Using a cation exchange resin, a mixture of four amino acids is separated using an elution gradient
of increasing NaCl solution. What would be the correct elution sequence?
a. Asp, Arg, Ser, Lys
b. Arg, Asp, Lys, Ser
c. Lys, Arg, Asp, Ser
d. Asp, Ser, Lys, Arg
e. Ser, Asp, Lys, Arg
10. A common reaction of two cysteine residues in proteins results in the formation of ____.
a. thioester bonds
b. disulfide bonds
c. dithiol bonds
d. thioether bonds
e. none of the above
Name:_______Key______________
Introduction to Biological Chemistry
September 18, 2015
Exam #1
I. Short Answer/Problems: Please write complete but concise answers to questions. Show all work for
calculations. Partial credit will be given based on the work or answers provided.
1. (24 pts) When acetylcholine is incubated with the enzyme acetylcholinesterase, acetylcholine is
quantitatively converted to choline and acetic acid (which dissociates to yield acetate and a hydrogen
ion). In a typical analysis, 15 mL 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 there was no buffer in the assay mixture, determine the number of
moles of acetylcholine in the 15 mL sample.
Ans.
Given that pH = -log[H+], we can calculate [H+] at the beginning and at the end of the reaction.
At pH 7.65, log[H+] = -7.65, so [H+] = 10-7.65 = 2.24x10-8 M
At pH 6.87, log[H+] = -6.87, so [H+] = 10-6.87 = 1.35x10-7 M
The difference in [H+] is 1.35x10-7 M - 2.24x10-8 M = 1.13x10-7 M.
Since the reaction is quantitative, the same amount of acetylcholine is initially present.
(0.015L)(1.13x10-7 M) = 1.7x10-9 mol of acetycholine
2. (32 pts) Given 0.10 M solutions of acetic acid (pKa = 4.76) and sodium acetate, describe how you
would go about preparing 1.0 L of 0.10 M acetate buffer at pH 4.00.
Ans.
pH = pK a + log ([A-]/[HA])
4.00 = 4.76 + log ([A-]/[HA])
-0.76 = log ([A-]/[HA])
0.17 = ([A-]/[HA])
We know that [A-] + [HA] = 0.10 M; 0.17[HA]=[A-]
Substituting in, we get: 0.17[HA] + [HA] = 0.10 M; [HA] = 0.0854 M
Knowing that, we know [A-] = 0.10 M - 0.0854 M = 0.0146 M
To make 1 L, mix 854 mL acetic acid with 146 mL sodium acetate.
3. The first reaction in glycolysis is the phosphorylation of glucose to produce glucose-6-phosphate.
This is a thermodynamically unfavorable process, with ∆Gº’ = 14 kJ/mol.
(a) (20 pts) In a liver cell at 37 ºC, the concentrations of both phosphate and glucose are normally
maintained at about 5 mM each. What would the equilibrium concentration of glucose-6phosophate be, according to the above data?
Name:_______Key______________
(b) (4 pts) In fact, this reaction is coupled to ATP hydrolysis to give the overall reaction, ATP +
glucose ↔ glucose-6-phosphate + ADP. What is the ∆Gº’ for this overall reaction?
Ans.
(a) ∆G = ∆Gº’ + RTln (([G6P]/([Pi][G]))
0 = 14000 J/mol + (8.31 J/molK)(310 K)ln ([G6P]/(0.005M*0.005M)
-5.43 = ln [G6P]/0.000025
0.0044 = [G6P]/0.000025
[G6P] = 1.1x10-7 M
(b) 14 kJ/mol – 30.5 kJ/mol = -16.5 kJ/mol
4. Calculate the isoelectric points of:
(a) (10 pts) arginine (R-group pK a = 12.5)
(b) (10 pts) glutamate (R-group pK a = 4.1).
Ans.
(a) The states of Arg are +2, +1, 0, and -1. The pKa values surrounding the neutral species are 9.0
and 12.5. The average, or pI, is 10.75.
(b) The states of Glu are +1, 0, -1, and -2. The pKa values surrounding the neutral species are 2.0
and 4.1. The average, or pI, is 3.1.
5. (18 pts) Draw the chemical structure of glutathione (γ-Glu-Cys-Gly). Note that the γ indicates that
the peptide bond between Glu and Cys is formed between the γ-carboxyl of Glu and the amino group
of Cys.
Ans
By convention, peptides are designated from the N-terminus to the C-terminus; therefore, Glu is the
N-terminus and Gly is the C-terminus.
+
H 3 NC(COO-)H-CH 2 CH 2 CO-NH-CH(CH 2 SH)-CO-NH-CH 2 -COO- (see Horton, p. 770)
6. (12 pts) The mass of a protein was determined by gel filtration chromatography to be 60 kd.
Chromatography in the presence of 6 M urea yields a 30 kd species. When the chromatography is
repeated in the presence of 6 M urea and 10 mM β-mercaptoethanol, a single molecular species of 15
kd results. Explain these results and the structure of the protein.
Name:_______Key______________
Ans.
Treatment with urea will disrupt noncovalent bonds. Thus, the original 60 kd protein must be made
of two 30 kd subunits. When these subunits are treated with urea and β-mercaptoethanol, a single 15
kd species results, suggesting that disulfide bonds link the 30 kd subunits.
II. Multiple choice: Circle the letter of the best answer. Each question is worth two points. There is no
partial credit on this portion of the exam.
1. The major precursors for the formation of biomolecules include all EXCEPT:
a. nitrate and dinitrogen.
b. water.
c. carbon dioxide.
d. ammonium ion.
e. none, all are major precursors.
ANS:
E
2. Which are the four most common elements in the human body?
a. hydrogen, calcium, oxygen and sodium
b. hydrogen, oxygen, iron and carbon
c. hydrogen, oxygen, carbon and nitrogen
d. oxygen, carbon, iron and nitrogen
e. oxygen, silicon, calcium and nitrogen
ANS:
C
3. Grapefruit juice at pH 3.2 contains about ____ times as much H+ as orange juice at pH 4.3.
a. 0.9
b. 10−7.5
c. 10−2
d. 12
e. 101
ANS:
D
4. Buffers have all of the following characteristics EXCEPT:
a. they have relatively flat titration curves at the pH(s) where they buffer.
b. they resist changes in their pH as acid or base is added.
c. they are typically composed of a weak acid and its conjugate base.
d. they buffer best for polyprotic acids half-way between the two pK a values.
e. buffer where the amounts of conjugate base are nearly equivalent to the amounts of
weak acid.
ANS:
D
Name:_______Key______________
5. Which of the following statements is true for the Second Law of Thermodynamics?
a. Systems tend to proceed from disordered states to ordered states.
b. The entropy of the system plus its surroundings is unchanged by reversible
processes
c. The entropy of the system plus its surroundings decreases for irreversible
processes.
d. Naturally occurring processes never proceed toward equilibrium.
e. All of the above are true
ANS:
B
6. ____ and ____ are a small family of universal biomolecules mediating the flow of energy from
exergonic reactions to the energy requiring processes of life.
a. Reduced coenzymes, caffeine
b. High-energy phosphate compounds, caffeine
c. Chlorophyll, caffeine
d. Hemoglobin, chlorophyll
e. Reduced coenzymes, high-energy phosphate compounds
ANS:
E
7. Which statement MOST COMPLETELY EXPLAINS the large ∆G° for the hydrolysis of
phosphoenolpyruvate?
a. Hydrolysis of the phosphate bond along with the conversion of the enol to the keto
form of pyruvate.
b. The hydrolysis of the phosphoanhydride bond.
c. Conversion from the enol to the keto form of pyruvate.
d. Strong bond energy along with a change in the stereochemistry of the molecule.
e. The large ∆G° of the phosphoester bond.
ANS:
A
8. All of the amino acids EXCEPT ____ have both free α-amino and free α-carboxyl groups.
a. aspartic acid
b. proline
c. asparagine
d. lysine
e. valine
ANS:
B
9. Using a cation exchange resin, a mixture of four amino acids is separated using an elution gradient
of increasing NaCl solution. What would be the correct elution sequence?
a. Asp, Arg, Ser, Lys
b. Arg, Asp, Lys, Ser
c. Lys, Arg, Asp, Ser
d. Asp, Ser, Lys, Arg
e. Ser, Asp, Lys, Arg
Name:_______Key______________
ANS:
D
10. A common reaction of two cysteine residues in proteins results in the formation of ____.
a. thioester bonds
b. disulfide bonds
c. dithiol bonds
d. thioether bonds
e. none of the above
ANS: B