C485 Exam III Fall ‘10
Name___________________
Legible please!
Do not use acronyms. Use structures whenever they are asked for, or appropriate. Your
explanations should be brief. Overly lengthy answers with irrelevant or erroneous
material will receive deductions. GOOD LUCK
1. (14 pts total) Complete the chart.
a.) 10 pts. Fill in the boxes with the 20 amino acids according to their precursors (name
or structure acceptable).
b.) 4 pts. Circle the essential amino acids. Each correct circle is worth 0.5 points, and for
each incorrect circle, 0.5 points will be deducted.
Oxaloacetate
Pyruvate
α-Ketoglutarate
Phosphoenolpyruvate
and erythrose 4phosphate
3-Phosphoglycerate
Ribose 5-phosphate
2. (11 pts) List the 20 amino acids under the correct heading (name or structure).
Glucogenic
Ketogenic
Both
3. (12pts) Outline the degradative pathway for leucine. What steps in this pathway are
akin to steps involved in fatty acid oxidation or synthesis?
4. (8 Pts) What are the requirements for biological nitrogen fixation in terms of
electrons/nitrogen molecule and ATP consumed /nitrogen molecule fixed. Does the
reaction require the same amount of electrons as chemical fixation? If not, how do these
numbers differ and why are they different?
5. ( 8 pts) What is/are the ultimate biological molecule(s) that accept(s) ammonia in
higher organisms? What enzymes mediate this(ese) transformation(s)? What are the
energetic requirements for this(ese) reaction(s)?
6. (12 Pts) Draw the pathway used for the biosynthesis of isoleucine. Show starting
molecule(s) and all stable intermediates. Note the participation of any cofactors in
specific steps. Show the mechanism (curly arrows and structures please) of the first step
of the pathway. Both of the starting molecules in this pathway can be derived from
amino acids. Give an example for each starting molecule.
7. (10 pts total )
A) 6 pts Three coenzymes are involved in carrying activated one carbon units, Match
the activated group in the right column with the appropriate coenzyme in the left column.
a) tetrahydrofolate
b) S-adenosylmethionine
c) biotin
1) –CH3
2) –CH2_
3) –CHO
4) –CHNH
5) –CH=
6) –COO-
B) 4 Pts Which of the following answers completes the sentence correctly? The major
source of one-carbon units for the formation of the tetrahydrofolate derivative N5,N10methylenetetrahydrofolate is the conversion of (circle your answer)
a) methionine to homocysteine
b) deoxyuridine 5’-phosphate to deoxythymidine 5’-phosphate
c) 3-phosphoglycerate to serine
d) serine to glycine
8. (8 Pts) Draw the structure of histidine and show the biosynthetic derivation of each
heavy atom (C and N). You should show which atom of histidine corresponds to which
atom of the starting material. (You don’t need to draw the pathway, just show where
things come from.)
9. (8 Pts) Leucine is initially derived from the same pathway that generates another
branched chain amino acid. What subsequent additional carbon source is used in this
biosynthesis? The later stages of leucine’s biosynthetic pathway are similar to what
other pathway?
10. (9 Pts) There are two branch points in aromatic amino acid biosynthesis. Draw the
structure of the molecule at this branch point, and draw the subsequent molecule that is
the first intermediate in the unique pathway to each aromatic amino acid.
EC 1 15 pts total The degradation of cystein proceeds through the intermediate cysteine
sulfinate. Please answer the following questions about this molecule:
a) 3 pts draw the structure of cysteine sulfinate
b) 2 pts what type of enzyme is responsible for the conversion of cysteine into cysteine
sulfinate?
c) 10 pts The first step in the degradation of cysteine sulfinate involves the loss of the
sulfur moiety. What cofactor is likely involved in this transformation? Draw a likely
mechanism for this step. (Hint: you have not seen this mechanism, but initially, it
resembles a gamma-elimination.)
EC2 (10 pts) The breadown of methionine produces homocysteine. Draw the immediate
precursor of homocysteine. What cofactor is involved in the conversion of this molecule
to homocysteine? Draw a notional mechanism for this transformation (curly arrows and
structures please).
Point breakdown
Possible
actual
1)
14
________
2)
11
________
3)
12
________
4)
8
________
5)
8
________
6)
12
________
7)
10
________
8)
8
________
9)
8
________
10)
9
________
EC1 15
________
EC 2 10
________
TOTAL 125
________