Download practice mid-term 1

PRACTICE MID-TERM 1
01. Mark each statement below TRUE or FALSE and give reasons. You will get credit ONLY if
your reasoning is correct.
A)
At equilibrium, there is no conversion of S  P
B)
The rate of a reaction depends only on its G0
C)
An enzyme’s binding energy determines the rate of the catalyzed reaction
D)
The energy of the E.S complex can sometimes be higher than the energy of the E.TS
complex
E)
Increasing the temperature of a reaction will change the G0
02. Consider the two enzyme-catalyzed reactions below:
Reaction 1: G0 for formation of the ES = -2 kJ mole-1
Reaction 2: G0 for formation of the ES = -4 kJ mole-1
Can you predict which of these reactions will have a higher rate? Explain your answer.
03. A reaction has a G0 = 10 kJ mol-1. At equilibrium, what is the ratio of [Products]:[Reactants]?
04. In a cell, Keq for a reaction is 344. Is this a spontaneous reaction in the cell? Give reasons for
your answer.
05. A mutation in a protein converts a Cys  Ala. What are all the possible effects this mutation
might have on the protein’s structure? Mention the effects on secondary, tertiary and
quaternary structure. Give reasons for your answer.
06. A mutation from Gly  Phe has no effect on the activity of an enzyme. What are some reasons
to account for this?
07. Which of the following statements is TRUE about secondary structure (mark all that apply)?
A)
B)
Stabilized by hydrophobic interactions
Without constraints on bond rotation angles, no ordered secondary structures would
form
C)
A given protein cannot contain both parallel and anti-parallel -sheets
D)
Bends and loops are not important in forming secondary structures
08. Two proteins have a pI of 7.6. Therefore:
A)
They must have the same amino acid seqence
B)
They must have the same 3° structure
C)
They must have the same molecular mass
D)
They will have not move in an electric field at pH 7.6
PRACTICE MID-TERM 1
09. For each molecule shown, in the box below the molecule, write in the amino acid it depicts. If
the molecule shown is not a standard amino acid, write in “NS.”
10. With regards to the Keq, the effect of an enzyme on a chemical reaction is BEST described as:
A)
Increasing the Keq in less time
B)
Decreasing the Keq in less time
C)
Maintaining the Keq for less time
D)
Achieving the Keq in less time
11. A schematic of an enzyme active site is shown. Based on this, which of the
following will be the MOST LIKELY tripeptide substrate for this enzyme.
Explain your answer.
A) Ile-Glu-Ala
B) Val-Leu-Lys
C) Ile-Ile-Asp
D) Glu-Glu-Tyr
12. Protein X by itself tends to form large aggregates in solution. However,
addition of only a very small amount of another protein, Y causes Protein
X to become soluble, without forming any aggregates. What is the MOST LIKELY explanation
for these observations?
A)
Protein Y is a chaperone, which refolds Protein X, causing hydrophobic regions of X to be
buried internally
B)
Protein Y causes the release of more water molecules from the solvation shell, increasing
entropy of the solution
C)
Protein Y causes an increase in the number of hydrophilic residues of Protein X,
increasing its solubility
D)
Protein Y adds amino acids to Protein X, changing its size and structure
PRACTICE MID-TERM 1
13. A protein is made of an ɑ-helix, consisting almost exclusively of hydrophobic amino acids.
Where in the cell are you MOST LIKELY to find this protein? Why?
A)
In the plasma membrane where the hydrophobic R-groups can interact with the lipids
B)
In the ER, where these is less water to interact with the hydrophobic R-groups of the
protein
C)
In the lysosome where the acidic pH means the protein can fold differently because of
different charges on the R-groups
D)
In the nucleus where the R-groups can form hydrophobic interactions with the
nucleotides in DNA
14. Consider the reaction depicted in the graph. Based on this graph,
can I conclude that this reaction MOST LIKELY does not have a
transition state? Why or why not?
Can I conclude that MOST LIKELY this reaction has reached
equilibrium? Why or why not?
(HINT: Note the labels on the axes)
15. Consider the uncatalyzed reaction A  B. If I start with 1 mole of A, at equilibrium, I am left
with 0.8 moles of A. It takes 30 minutes for this reaction to reach equilibrium. For the same
reaction catalyzed by an enzyme:
A)
MOST LIKELY, what can you predict about how long the reaction will take to reach
equilibrium?
B)
MOST LIKELY, how much of A will be left at the end of the reaction if I started with 1
mole of A?
16. Consider the reaction catalyzed by enzyme “E” shown in the
graph. I take 1 mole of “R” and add the enzyme “E” to it. What
is the MOST LIKELY outcome?
A)
The R  Q reaction will reach equilibrium faster than the
uncatalyzed reaction
B)
Nothing will happen since the R  Q reaction is nonspontaneous
C)
The uncatalyzed reaction will proceed at the same rate as
the catalyzed reaction since E does not catalyze the R  Q
reaction
D)
Nothing will happen since E does not catalyze the R  Q reaction
PRACTICE MID-TERM 1
17. I compare the sequence of
Protein “G” from humans and
gorillas. The differences are
shown schematically in the
figure (amino acid residues
that are the same in both species are NOT labelled; amino acids present in the active site are
labelled with a “*”). Based on this, MOST LIKELY what can you conclude about the function of
Protein “G” in the two species?
A)
The amino acids changed are similar in their hydrophobicity and hydrophilicity;
therefore, the proteins will function the same
B)
The amino acids in the active site are different; therefore, the functions will be different
C)
We cannot predict anything about the function, since we do not know the overall
structure of the two proteins
D)
The active site residues have opposite charges; therefore, the overall function will be
conserved, but the protein will interact with an oppositely charged substrate
18. Consider the molecule shown. It has a pK1 of 2.3 and a pK2 of 3.6. What is the
overall charge on this molecule at pH 1?
19. Select the BEST choice regarding the statement below:
Active sites of enzymes cannot have amino acids with hydrophobic R-groups.
A)
TRUE: hydrophbic residues will be on the inside of proteins, while active sites are on the
outside
B)
TRUE: hydrophobic residues will prevent the substrate from binding the active site
C)
FALSE: weak interactions with hydrophobic residues can stabilize the transition state
D)
FALSE: hydrophobic residues cause water to be excluded from active sites, thereby
allowing the substrate to bind
20. The measured activity of 1 mg of a protein in water is 455 units. The measured activity of 1 mg
of the same protein in benzene (a non-polar solvent) is 2 units. What is the BEST explanation
for this data?
A)
Non-polar solvents disrupt H-bonds required for secondary structure
B)
Non-polar solvents disrupt hydrophobic interactions required for tertiary structure
C)
Covalent peptide bonds are broken in non-polar solvents
D)
H-bonding and hydrophobic interactions are disrupted in non-polar solvents
21. Based on the data in the graph, MOST LIKELY, which line
represents the catalyzed reaction? Why?
A)
Solid line is catalyzed reaction because it has lower
activation energy
B)
Dotted line is catalyzed reaction because [Products] is
higher
C)
Solid line is catalyzed reaction because it has a lower slope
D)
Dotted line is catalyzed reaction because it reaches equilibrium faster
PRACTICE MID-TERM 1
22. Refer to the point marked “A” in the figure for the previous Question. What is your BEST
conclusion about “A”?
A)
Reaction equilibrium; no change in [Products]
B)
Reaction equilibrium; no change in G
C)
Transition state; highest energy of reaction
D)
Transition state; extremely short-lived intermediate
23. Two enzymes that catalyze different reactions contain the same domain. Based on this
observation, what is your BEST conclusion about these two enzymes?
A)
Extremely unlikely. Enzymes with similar domains should catalyze similar reactions
B)
The common domain is likely non-functional in these enzymes
C)
The two enzymes could have evolved from a common protein
D)
The common domain will most likely be mutated in one of the proteins
24. The figure depicts the primary structure of an
enzyme. The amino acids that make up the active
site are marked for the wild type enzyme (top
amino acids) and a mutant enzyme (bottom
amino acids). Based on this, what is your BEST prediction about the mutant enzyme?
A)
Mutant enzyme will have lower activity; Ile will not bind substrate as well as Leu, and will
not stabilize the transition state
B)
Mutant enzyme will have lower activity; mutation will cause protein to unfold
C)
Mutant enzyme will have the same activity as wild type; Leu and Ile have similar Rgroups with similar weak bonds
D)
Mutant enzyme will have higher activity; Ile will accommodate the substrate better and
stabilize the transition state better
25. The data in the graph was obtained by measuring the product
formed over time with a wildtype and mutant enzyme. Based
on these data, can you determine which line represents the
wildtype and mutant enzyme? Why?
A)
Solid line is mutant enzyme; [Products] is lower
B)
Dotted line is mutant enzyme; reaches equilibrium faster
C)
Solid line is mutant enzyme; reaches equilibrium slower
D)
Cannot tell which is mutant or wild type; mutations can increase or decrease enzyme
activity
26. Based on the data in graph, can you predict which reaction is
spontaneous under these reaction conditions (1, 2 or both)? Why?
A)
Only Reaction 1 is spontaneous, Keq > 1
B)
Both Reaction 1 and 2 are spontaneous, Keq is positive
C)
Only reaction 1 is non-spontaneous, G is positive
D)
Both reaction 1 and 2 are non-spontaneous, G is positive
PRACTICE MID-TERM 1
27. A -> X + Y G = -29 kCal mol-1; Activation energy = 5 kCal mol-1
X -> D + E G = +3 kCal mol-1;
Activation energy = 10 kCal mol-1
We can couple these two reactions to carry out the overall reaction: A -> Y + D + E. Which of
the following statements is TRUE about these reactions?
A)
A -> X+Y will occur more slowly since its activation energy is lower
B)
The coupled reaction will be very slow since its activation energy will be 15 kCal mol -1
C)
The coupled reaction will have a very low KM (<0.5) since its rate is very high
D)
X -> D+E is an endergonic reaction, that might be reversible
28. A protein is composed only of the amino acids Ala, Gly, Val, Leu, Ile, and Phe. This protein is
insoluble in water, and forms large aggregates when placed in pure water. What is the BEST
explanation for all this?
29. The G for the reaction D  E is -23 kJ mol-1. I start off with 1M D and
measure the [E] over time at constant temperature. The maximal solubility
of D is 1M. My data is shown in the graph. MOST LIKELY, what did I do at
the point marked “X” in the graph? Explain your answer.
X