Amino Acid Biosynthesis
Student Companion Ch 24 Self Test: Problems 3, 8, 10-18, 20, 21, 23, 24
1) What enzyme is responsible for the fixation of nitrogen? Describe it.
2) How many electrons are required to reduce nitrogen gas to ammonia? How many electrons
are required in the biological reduction?
3) What are the ultimate biological molecules that act as acceptors of ammonia? What enzymes
mediate this transformation? What are the energetic requirements for these reactions?
4) What nine amino acids cannot be synthesized in humans?
5) Alpha-ketoglutarate provides the carbon skeleton for which amino acids?
6) Two different amidation methods are used to install side chain amides in amino acids.
Describe these two methods and match them to the relevant amino acid.
7) Which amino acids derive their carbon skeletons completely from oxaloacetate?
8) Serine is derived from which glucogenic precursor?
9) The carbons of phenylalanine and tyrosine are derived from what glucogenic molecules?
10) The branchpoint for aromatic amino acid biosynthesis is chorismate. What is the structure
of chorismate? What are the three immediate products derived from chorismate that constitute
the first unique steps in the synthesis of the three aromatic amino acids?
11) From where are the two carbons of the five-membered ring of tryptophan derived? Be
specific. (Show the molecule and highlight the carbons that are used.) The carbons that are not
used are converted into what compound?
12) The nonaromatic carbons of tryptophan are derived from what precursor? What cofactor is
involved in this reaction? Show the mechanism of this transformation.
13) Roundup (glyphosate) inhibits biosynthesis of aromatic amino acids. Which step does it
inhibit? Why does this inhibitor have little effect on humans? Considering that glyphosate is an
incredibly effective herbicide, can you give a likely reason for why this compound is not an
effective antibiotic.
14) Which transformation in aromatic amino acid biosynthesis requires NAD+, but does not
yield NADH as a product. Explain the role of this cofactor. (Hint: this is similar to the reaction
catalyzed by S-adenosylhomocysteine hydrolase).
15) What are the carbon-containing precursors used for the synthesis of valine and isoleucine?
What cofactor is common to both these biosynthetic pathways? Explain its role and draw a
mechanism for the transformation.
16) The biosynthesis of valine and isoleucine have a rearrangement step that is very similar.
Draw this rearrangement (curly arrows please).
17) Leucine is derived from the same pathway that generates valine. What additional carbon
source is used in this synthesis? The later stages of leucines biosynthetic pathway are similar to
what other pathway?
18) Threonine is derived from what non-proteinacious amino acid? Which common amino acid
is the ultimate source of this molecule?
19) The methyl group of methionine is derived from what common amino acid? What is the
byproduct of this one carbon transfer? What cofactor catalyzes this carbon cleavage? What is
the acceptor of this carbon? The actual agent that ultimately transfers the carbon is which
cofactor?
20) Methionine may be synthesized from two complementary sets of amino acids. What are
these two sets?
21) Which amino acid is derived from oxaloacetate and pyruvate? What carbon piece is lost in
this process? What cofactor plays a role in this process? Draw a general mechanism for this
reaction. What molecule plays a role as a protecting group in this pathway?
22) Inhibition of a key enzyme activity by the end product of a biosynthetic pathway is known as
what?
23) Why is it useful to have multiple isozymes of enzymes that comprise common pathways to
multiple amino acids?
24) What molecules provide the carbon and nitrogen sources for histidine. Please delineate
which atoms in the starting materials wind up in which places in histidine.