Download Sample exam 1

Chemistry 255
Sample exam 1 (Background and proteins)
The exam is open book, notes, class handouts, case studies and exercises. You may
attach additional sheets if needed for your answers, but clearly label what problem(s)
they are for. There is to be no collaboration. You have 80 minutes to complete the exam.
Multiple choice:
1. When considering the thermodynamics of transferring toluene from a toluene/water
mixture to pure toluene, which one of the following statements is correct?
a. The ΔH of the process is negative, therefore the ΔG is negative.
b. The ΔS of the process is negative, therefore the ΔG is negative.
c. The ΔH of the process is zero, the ΔS of the process is positive and therefore the ΔG is
negative.
d. The ΔH of the process is zero, the ΔS of the process is negative and therefore the ΔG is
negative.
e. The ΔH of the process is negative, the ΔS of the process is negative and therefore the
ΔG is negative.
2. In the figure below, part of a polypeptide sequence is shown. What is the amino acid
sequence of residues 94 – 96?
a. SAY
b. YVS
c. YAT
d. TGY
e. YAS
3. Below is shown the oxygen binding curves for myoglobin and hemoglobin. Which one
of the following statements is correct?
a. The shape of curve II is characteristic of a cooperative binding process.
b. Curve I is sigmoidal in shape.
c. Curve II is hyperbolic in shape.
d. Curve I is the binding curve for hemoglobin.
e. Curve II is the binding curve for myoglobin
4. Examine the figure below. Which one of the following statements is correct?
a. The two β-strands in B run antiparallel to each other.
b. The two β-strands in A run parallel to each other.
c. The N-terminus of the upper β-strand in A is on the left hand side.
d. All side chains of both A and B are above the plane of the β-strands.
e. None of these statements is correct.
5. Which one of the following statements is correct regarding the figure below?
a. In the left figure (A), the inhibitor I is an uncompetitive inhibitor.
b. In the left figure (A), the inhibitor I is an competitive inhibitor.
c. In the right figure (B), the inhibitor I is an uncompetitive inhibitor.
d. Both figures are Michaelis-Menten graphs.
a. In the right figure (B), the inhibitor I is a mixed (noncompetitive) inhibitor.
Essay questions: Answer these in paragraph form or drawings, as requested.
6. The titration curve of a solution of the tetrapeptide Glu-Gly-Ala-Lys is shown
below.
a. Draw the structure of the tetrapeptide at pH 1.
b. Identify the Roman numeral point at the isoelectric point. Draw a predominant
structure or otherwise explain your choice.
7. The protein myoglobin is found in numerous organisms, and the amino acid residue
sequence of the protein from a wide variety of organisms has been determined. Recall
that the word “conserved” is applied to a particular region of the protein if the
sequence of that region is regular between several organisms. If the sequence is altered
slightly in this region but the protein still functions, the changes are said to be
“conservative substitutions”.
a. Which substitution would be considered conservative: Val replaced by Ala or Val
replaced by Phe? Briefly explain your choice.
b. Explain why replacing a Val by a Lys might result in a defective protein. As part of
your response, define what a “defective” protein is!
c. It is observed that insertions of extra residues, deletions of residues and substitutions
of amino acid residues in proteins occur more often in random coils than in the main
elements of secondary structure like alpha helices and beta-pleated sheets. In the
context of secondary structures and how they relate to the protein’s function, explain
why this makes sense.
8. From the following data on an enzymatic reaction, determine (a) the type of
inhibition, (b) KM for the substrate, and (c) KI for the inhibitor-enzyme complex.
Substrate concentration
(mM)
2.0
3.0
4.0
10.0
15.0
Initial product production rate, µg/hr
No inhibitor
with inhibitor
139
88
179
121
213
149
313
257
370
313
9. The enzyme phosphoserine phosphotase (PSP) is a key regulator of neurotransmitter
levels in the brain. The natural substrate of PSP is phospho-L-serine, shown below,
which is converted into serine. PSP can be inhibited by two molecules, 2-amino-3phosphonopropionic acid (AP3) and L-serine. These molecules inhibit the enzyme in
different ways.
phospho-L-serine
2-amino-3-phosphonopropionic acid
serine
From the structures alone, classify each inhibitor as either competitive or
uncompetitive. Give a reason why you labeled each inhibitor what you did, and be
sure to reference the chemical structures of the normal substrate and the inhibitors.