(1 pt) write your exam id (a-1) in the blank at the upper...

Chem 153A, Winter 2009, Midterm Exam2, ID: A-1

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1. (1 pt) Write your exam ID (A-1) in the blank at the upper right of your answer sheet.

2. (10 pts) True or False

a. The free energy of the highest-energy intermediate directly determines the rate

of a reaction.

b. KI is equivalent to the Kd of an enzyme-inhibitor complex.

c. Some enzymes are regulated by how much substrate is present.

d. In cumulative feedback inhibition, a pathway product can partially inhibit the

activity of an enzyme in the pathway.

e. In the concerted model of homotropic positive cooperativity, the binding of

substrate yields a higher-affinity (or more active) state of the enzyme.

3. (3 pts) Which one of the following is not true?

a. Enzymes act on and produce specific stereoisomers.

b. Enzymes can be subject to multiple types of regulation.

c. Enzymes function over a wide range of reaction conditions.

d. Enzymes can potentially catalyze many different types of reactions.

e. Enzymes can achieve higher rates of catalysis than chemical catalysts.

4. (10 pts) List all possible results that can be produced by each type of inhibitor:

a. Competitive

b. Mixed

c. Noncompetitive

d. Uncompetitive

Choose from:

1. An apparent decrease in Km

2. An apparent increase in Km

3. No apparent change in Km

4. An apparent decrease in Vmax

5. An apparent increase in Vmax

6. No apparent change in Vmax


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5. (22 pts) You have discovered a new enzyme, metamorphase, which catalyzes the one-

to-one conversion of caterpillic acid into mothrate. You analyze the kinetics of the

enzyme by combining 0.8 nM metamorphase with varying concentrations of

caterpillic acid, and measuring the rate of production of mothrate, as shown in the

plots below. The equations on each plot describe the dotted line that traces the initial

slope of each curve.


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(Question 5 continued)

a. Draw a Lineweaver-Burk plot for metamorphase acting on caterpilic acid.

Label the axes with numerical values, labels, and units. Label your line ‘M’.

b. What is the Km of metamorphase for caterpilic acid? Show your work.

c. What is the Vmax of metamorphase in these experiments? Show your work.

d. What is the kcat of metamorphase? Show your work.

e. What is the catalytic efficiency of metamorphase? Show your work.

f. The kinetic experiments with metamorphase are repeated, this time with the

addition of 5 mM frigidol, an inhibitor. The apparent Km and Vmax are 20 mM

and 5 µM/s, respectively. Draw a line representing the results of this

experiment on your plot, and label it ‘M + F’.

g. What type of inhibitor is frigidol (when acting on metamorphase)?

h. What is the KI of the frigidol-metamorphase complex? Show your work.

i. Does metamorphase have a higher affinity for caterpillic acid or frigidol?

(Assume the conversion of caterpillic acid to mothrate is much slower than the

binding and release of caterpillic acid to metamorphase.)


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6. (15 pts) Below is an enzyme mechanism

(with reaction arrows omitted):

O

OO

OO

HOH2C

CH2OHNH

O

NH

HO

CH

H3C COO-

C O

CH3

CO

CH3

O

OO

OO

CH2OH

CH2OHNH

O

NH

HO

CH

H3C COO-

C O

CH3

CO

CH3

OO

Glu-57

OO

Glu-35

H

OO

Asp-52Enzyme surface

Enzyme surface

HOO

O

CH2OH

NH

HO

CO

CH3

OO

CH2OH

NH

O

CH

H3C COO-

C O

CH3

OO

Glu-57

OO

Glu-35

OO


Enzyme surface

H2O

H

O

H

OO

Glu-57

OO

Glu-35

H

OO


Enzyme surface

OO

HOH2C

NH

O

CH

H3C COO-

C O

CH3

OH

a. Name the enzyme.

b. Name the biological substrate

of this enzyme and the

substrate’s function.

c. Name the enzyme class.

d. List, in order, the catalytic

mechanisms used by this

enzyme (in the mechanism

shown).


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7. (12 pts) Shown below are several O2 binding curves. The curve in bold (#3)

represents O2 binding by red blood cells of an average person living at sea level.

Which curve below best represents O2 binding:

a. For a resident of Tibet, who has a higher-than-average BPG concentration.

b. For an elite athlete, who has a higher-than-average red blood cell count.

c. For an altered hemoglobin that can only adopt the R state.

d. For hemoglobin with half of its binding sites bound by carbon monoxide.

8. (3 pts) What functional groups of hemoglobin bind directly to O2?

9. (7 pts) An alien race on a far-away planet has evolved to resemble earth’s dolphins.

Because of the different air composition, the aliens’ oxygen-binding molecules, called

flipperglobin and goglobin, have different O2 binding affinities and transfer

efficiencies than myoglobin and hemoglobin. Using the information below, draw the

O2 binding curves for these two molecules, and label them ‘G’ for goglobin, and ‘F’

for flipperglobin.

- The pO2 in the aliens’ air-exchange organs is 200 torr.

- The pO2 in the aliens’ internal tissues is 80 torr.

- Goglobin fills 90% of its binding sites in the air-exchange organs, and

transfers oxygen to the tissues with 70% efficiency.

- Flipperglobin, found in the alien’s tissues, has a P50 of 20 torr.


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10. (4 pts) The ∆G°' of the one-to-one conversion of X to Y is 4.9 kJ/mol. The

concentration of X in human brain cells is 100 µM. At what cellular concentrations

of Y is the conversion of X to Y spontaneous? Show your work.

11. (2 pts) The ∆G°' of the reaction X � 2Z is 7.1 kJ/mol. Write the expression that

relates the ∆G of this reaction in a cell to that of standard conditions.

12. (8 pts) The structure of inorganic phosphate (Pi) is shown:

a. Does Pi act as a nucleophile or electrophile in biochemical

reactions?

b. Briefly explain your answer for part a (15 words or less).

c. The hydrolysis of phosphorylated compounds is generally

spontaneous:

What property of Pi favors this reaction? Briefly explain (15 words or less).

13. (4 pts) A cell has 70% of its adenylates in triphosphate form, 20% in diphosphate

form, and 10% in monophosphate form. Calculate the energy charge in this cell,

showing your work.

P

O

O

OH

O


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Extra Credit – earn up to 5 points

The following reactions occur in bacteria as initial steps in the biosynthesis of certain

amino acids.

For each step (1, 2, 3), list the class and subclass (if applicable) of enzyme that catalyzes

the reaction; choose the corresponding letter from the list provided below. Also, for steps

1 and 2, name the compounds that would most likely fill in the blank boxes. Assume that

the hydrolysis of compound b → a would be favorable in the cell.

C

CH2

O

CH

COO

NH3O

C

CH2

O

CH

COO

NH3O

2-O3P

C

CH2

H

CH

COO

NH3O

CH

CH2

CH2

CH

COO

NH3OH

C

OOC

O

Pi

CH3

C-OOC

O

1

3

2

Choose from:

A. aminotransferase

B. carboxyltransferase

C. dehydrogenase

D. glycosidase

E. hydrolase

F. kinase

G. isomerase

H. ligase

I. lyase

J. mutase

K. oxidase

L. oxidoreductase

M. oxygenase

N. peptidase

O. phosphatase

P. phosphorylase

Q. protease

R. reductase

S. synthase

T. synthetase

U. transferase

V. none of the above

a

b

c

d

Name (Last, First): ______________________________________ First two letters of last name: ___ ___

Student ID Number: ___________________________ Lecture (circle): 9am 12pm

Chemistry and Biochemistry 153A, Winter 2009 Midterm Exam 2, ID:_________

Page 1 (24 points)

2. a. True False

b. True False

c. True False

d. True False

e. True False

3. _____

4. List all that apply:

a. ___________________

b. ___________________

c. ___________________

d. ___________________

Page 1:

Page 3:

Page 4:

Page 5:

Page 3 (22 points)

5. a. & f.

b. d. g. ____________________________

h.

c. e.

i. _____________________________

/ 24

/ 22

I have read and agree to all instructions and guidelines for this exam.

Signed: Date:

Total score

/ 15

/ 21

Scores

/ 18

Page 6:

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E.C.:

Chemistry and Biochemistry 153A, Winter 2009 Midterm Exam 2

Page 4 (15 points)

6. a. ______________________________________ d.

b. ______________________________________

______________________________________

c. ______________________________________

Page 5 (21 points)

7. a. _____ 9.

b. _____

c. _____

d. _____

8. _______________________________

Page 6 (18 points)

10.

11. _________________________________________

12. a. ______________________________________

b.

c.

13.

E.C. 1. ___________

____________ → ____________

2. ___________

____________ → ____________

3. ___________

(b and c each: 15 words or less)

(1 pt) write your exam id (a-1) in the blank at the upper...

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