glycolysis
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Site Title:
Principles of Biochemistry, 4th Edition CW
Book Title:
Principles of Biochemistry
Book Author:
Horton
Location on Site:
Chapter 11 > Chapter Quiz
Date/Time Submitted
:
November 5, 2010 at 9:52 PM (EDT)
Summary of Results
32% Correct of 28 Scored items:9 Correct: 32%
19 Incorrect: 68%
More information about scoring
1.Level I
In order for a radioactive CO2 to be released during anaerobic glycolysis that leads to ethanol formation, the label needs to be located at the ____ position of glucose.
Your Answer:
(a) or (d)
Correct Answer:
c) 3
2.Level I
Which of the following metabolites does not regulate glycolysis in liver cells?
Your Answer:
glucose-6-phosphate
Glucokinase is insensitive to glucose-6-phosphate but sensitive to both fructose monophosphates. The fructose bisphosphate regulates PFK-1.
3.Level I
Which of the following are substrates for protein kinase A?
Your Answer:
(blank)
4.Level I
Which of the following is not a substrate for hexokinase?
Your Answer:
fructose
Correct Answer:
galactose
5.Level I
Which of the following is not a potential end product of anaerobic glycolysis?
Your Answer:
(b) and (d)
Both ethanol and lactate use up the reducing equivalent (NADH) derived from earlier in the pathway.
6.Level I
Arsenate is poisonous because it acts as a _______ analogue in ________.
Your Answer:
phosphate; phosphoenolpyruvate
Correct Answer:
phosphate; 1,3-bisphosphoglycerate
7.Level I
Which of the following sugars cannot be fermented and yield energy?
Your Answer:
galactose
Correct Answer:
none of these
8.Level I
Which of the following enzymes catalyzes a substrate-level phosphorylation reaction?
Your Answer:
(d) phosphoglycerate kinase
Correct Answer:
(b) and (d)
9.Level I
During kinase reactions, the role of magnesium ions is to
Your Answer:
interact with the negative charges on phosphate groups.
Magnesium:phosphate complexes are recognized by the kinases (p. 345).
10.
Level I
The reactions of glycolysis that are regulated are
Your Answer:
reactions that show large free-energy increases.
Correct
(a) and (b)
Answer:
11.
Level II
In both plant and mammalian phosphoglyceromutases, a phosphoenzyme intermediate is formed. The phosphate comes from
Your Answer:
the 2-position of 3-phosphoglycerate in plants, the 3-position of bisphosphoglycerate in mammals.
Correct Answer:
The 3-position of 3-phosphoglycerate in plants, the 3-position of bisphosphoglycerate in mammals.
12.
Level II
Lactose is a disaccharide consisting of glucose and galactose. It can be hydrolyzed into monosaccharides by a galactosidase. Assuming fermentation to lactate, what is the net energy yield per lactose consumed?
Your Answer:
1 ATP generated
Correct Answer:
4 ATP generated
13.
Level II
An organism has a mutant protein kinase A that is overactive. The net result of this is
Your Answer:
a decreased flux of metabolites through glycolysis.
An overactive PKA would phosphorylate both PFK-2 and pyruvate kinase. Phosphorylation of PFK-2 would lead to increased phosphatase activity, and thus to lower concentrations of fructose-2,6-bisphosphate. Since fructose-2,6-bisphosphate increases flux through glycolysis, a lower concentration of this compound would caused reduced flux through glycolysis. Similarly, phosphorylation of pyruvate kinase leads to decreased activity and hence decreased
flux.
14.
Level II
How many ATP molecules are produced from fructose during anaerobic glycolysis?
Your Answer:
0
Correct Answer:
2
15.
Level II
If the enzyme phosphoglycerate kinase were missing from erythrocytes, glycolysis might still take place via the bisphosphoglycerate mutase shunt (BOX 11.2 on p. 338). How many ATP molecules would be produced from each glucose in cells that lacked phosphoglycerate kinase?
Your Answer:
2
Correct Answer:
0
16.
Level II
Why might energy stores be transferred between cells and organs as glucose rather than as glucose-6-phosphate?
Your Answer:
Glucose-6-phosphate cannot readily cross cell membranes.
The charged molecule glucose-6-phosphate cannot passively cross cell membranes. Hexose transporters act on unphosphorylated sugars, and the cell can efficiently maintain intracellular concentrations by quickly phosphorylating any sugars that enter (p. 359).
17.
Level II
Phosphofructokinase (PFK-1) is allosterically regulated. Which of the following is not a regulatory effect?
Your Answer:
AMP activates PFK-1.
Correct Answer:
Protons activate PFK-1.
18.
Level II
The effects of fructose-1,6-bisphosphate on pyruvate knase are an example of
Your Answer:
feed-forward activation.
Fructose-1,6-bisphosphate allosterically activates pyruvate kinase, an enzyme that is "downstream" from it in glycolysis (p. 358).
19.
Level II
How many ATP molecules are produced during anaerobic glycolysis of sucrose?
Your Answer:
4
Two ATP molecules are produced from fructose (problem 5) and two from glucose.
20.
Level III
Fructose is much sweeter than glucose, but less abundant in nature. Therefore, in order to increase the sweetness of foods, glucose is converted to fructose (high-fructose corn syrup) by heating to induce isomerization. Unfortunately, since the reaction goes to equilibrium, the glucose can never be completely converted. What enzymes might you use to completely convert glucose to fructose?
Your Answer:
hexokinase, glucose-6-phosphate isomerase
Correct Answer:
hexokinase, glucose-6-phosphate isomerase, fructose-6-phosphatase
21.
Level III
If glucose-6-phosphate can be broken down into four- and two-carbon units, then so can glucose. The four carbon unit, erythrose, could be further broken down into 2 two-carbon units. The net result of these two successive aldolase-style reactions would be the production of three glycolaldehydes. Assuming that these glycolaldehydes could undergo a reaction similar to glyceraldehyde-3-phosphate, what would the product be, how many ATP molecules could be produced per glucose, and what would the end product of this alternative to glycolysis be?
Your Answer:
1-phosphoglycolaldehyde; 2; glycolate
Correct Answer:
1-phosphoglycolate; 3; glycolate
22.
Level III
An organism has a mannose isomerase rather than a phosphomannose isomerase. What is the product of the mannose isomerase reaction, and how many ATP molecules are produced during its fermentation?
Your Answer:
glucose, 2
Correct Answer:
fructose, 2
23.
Level III
Assuming that the mechanism for bisphosphoglycerate mutase (BOX 11.2 on p. 338) is similar to that of phosphoglycerate mutase, a radiolabeled phosphate at the 1 position of 1,3-bisphosphoglycerate
Your Answer:
would end up at the 3 position of 2,3-bisphosphoglycerate.
Correct Answer:
would end up at the 2 position of 2,3-bisphosphoglycerate.
24.
Level III
In Figure 11.7 (p. 339), what kind of bond is formed during the phosphorylation of histidine?
Your Answer:
phosphoamide (phosphagen)
The phosphoamide (phosphagen) bond, a link between phosphorus and nitrogen, was described in Chapter 10, p. 328.
25.
Level III
If the aldolase reaction proceeded according to Figure 11.5, then the hydrogen extracted from the 4-hydroxyl of fructose 1,6-bisphosphate would eventually end up
Your Answer:
at the 3 position of dihydroxyacetone phosphate.
During the abstraction of the hydrogen from the base on the enzyme, the hydrogen becomes part of the hydroxymethyl moiety on dihydroxyacetone phosphate (note that the numbering in Figure 11-5 reflects the numbering of fructose bisphosphate, not the numbering of glyceraldehyde phosphate or dihydroxyacetone phosphate).
26.
Level III
Glucagon offsets fructose-1,6-bisphosphate activation of pyruvate kinase by
Your Answer:
d) decreasing the synthesis of fructose-2,6-bisphosphate.
Correct
both (b) and (c)
Answer:
27.
Level III
Because of the regulatory interrelationships between metabolites, when glucagon is high, fructose-2,6-bisphosphatase is _______, ____ fructose-2,6-phosphate is produced, and therefore there is ____ flux through glycolysis.
Your Answer:
deactivated, less, less
Correct Answer:
activated, less, less
28.
Level III
ATP with a 32P label at the gamma position might be used to trace phosphate flow during glycolysis. Assuming that you can differentiate "old" radiolabeled ATP from "newly synthesized" radiolabeled ATP (for example, by pulse labeling), where does the radioactive label end up?
Your Answer:
In free phosphate.
Correct Answer:
In the gamma position of ATP.
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Home Chapter 12 Chapter Quiz
Chapter Quiz
This activity contains 21 questions.
Level I
Is glycogen a reducing or nonreducing polysaccharide?
reducing
nonreducing
Level I
The immediate product of the degradation of glycogen by phosphorylase is
fructose-6-phosphate.
glucose-6-phosphate.
glucose.
glucose-1-phosphate.
fructose-1,6-bisphosphate.
Level I
Glucagon activates all of the following enzymes except
protein kinase A.
glycogen synthase.
glycogen phosphorylase.
phosphorylase kinase.
Level I
What are the three enzymatic steps in glycolysis that are bypassed with different enzymes in gluconeogenesis?
hexokinase, phosphofructokinase, phosphoglycerate kinase
hexokinase, phosphoglycerate kinase, pyruvate kinase
phosphofructokinase, phosphoglycerate kinase, pyruvate kinase
hexokinase, phosphofructokinase, pyruvate kinase
Level I
Which of the following pathways in the liver are stimulated by glucagon?
glycogen synthesis
glycolysis
gluconeogenesis
all of the above
None of the above.
Level I
What is the dinucleotide produced by the pentose phosphate pathway?
NADP+
NADH
NAD+
NADPH
Level I
The source of blood glucose during the early stages of starvation (i.e. 8 to 16 hours) is primarily
dietary carbohydrates.
liver glycogen.
fatty acids from triacylglycerols in adipose tissue.
amino acids from the breakdown of protein.
muscle glycogen.
Level I
What is the source of blood glucose during intermediate stages of starvation (1 to 4 days)?
lactate from muscle activity
glycerol from triacylglycerol breakdown in adipose tissue
alanine and other amino acids from protein breakdown
all of the above
None of the above.
Level I
Under conditions of prolonged starvation (weeks), the source of substrates for gluconeogenesis is primarily
glycerol from triacylglycerol breakdown in adipose tissue.
lactate from muscle activity.
alanine and other amino acids from protein breakdown.
all of the above
none of the above
Level II
Although muscle cells synthesize and degrade glycogen, the glucose monomers released cannot be exported to other tissues, because these cells lack
glycogen synthase
phosphoglucomutase.
debranching enzyme.
phosphorylase.
glucose-6-phosphatase
Level II
How does glycogen metabolism in the liver differ from that in skeletal muscle?
Protein phosphatase-1 binds to glycogen phosphorylase a in the liver.
The concentration of glycogen phosphorylase a in the liver is much greater than the concentration of protein phosphatase-a.
Protein phosphatase-1 is inhibited by inhibitor-1 in skeletal muscle.
all of the above
none of the above
Level II
After vigorous excercise, lactate generated in skeletal muscle is
converted to glycerol-3-phosphate.
transported to the liver, where it is converted to glucose in by gluconeogenesis.
imported into muscle mitochondria and further metabolized.
converted back to glucose via gluconeogenesis in skeletal muscle.
Level II
If you purifiy endoplasmic reticulum vesicles from liver cells, and add glucose-6-phosphate, what product would you expect to observe in the medium?
glucose
pyruvate
glucose-1-phosphate
fructose-6-phosphate
none of the above
Level II
If you purify endoplasmic reticulum vesicles from muscle cells and add glucose-6-phosphate, what product would you expect to observe in the medium?
glucose-1-phosphate
glucose
fructose-1-phosphate
glucose-6-phosphate
pyruvate
fructose-6-phosphate
Level II
The 6-phosphogluconate dehydrogenase step in the pentose phosphate pathway is mechanistically similar to
glucose-6-phosphate dehydrogenase.
isocitrate dehydrogenase.
glyceraldehyde-3-phosphate dehydrogenase.
PEP carboxykinase.
Level II
The reaction catalyzed by transketolase is reminiscent of which other enzyme?
phosphoglyceromutase
transaminase
aldolase
malate dehydrogenase
pyruvate dehydrogenase
Level II
Fatty acids are broken down to acetyl CoA (Chapter 16), which is a substrate for the TCA cycle. Oxaloacetate is a component of the TCA cycle and is a substrate for gluconeogenesis. Does this mean that glucose can be synthesized from fatty acids in mammals?
yes
no
Level II
Can glucose be synthesized from fatty acids in plants?
no
yes
Level III
McArdle's disease (a glycogen storage disease) results from a defect or deficiency in one of the enzymes in glycogen breakdown. Patients with this disease suffer from painful muscle cramping after strenuous excercise, yet recover after a brief rest. A deficiency in which enzyme would produce this symptom?
glucose-6-phosphatase
liver glycogen phosphorylase
amylo-1,6-glucosidase activity of the debranching enzyme.
4--glucanotransferase actvity of the debranching enzyme.
muscle glycogen phosphorylase
Level III
What is the energetic cost, in ATP equivalents, for adding 1 molecule of free glucose to glycogen?
0
1
2
3
4
5
Level III
Calculate the number of ATP equivalents required to synthesize 1 molecule of glucose from 2 molecules of lactate via gluconeogenesis.
1
2
3
4
5
6
Answer choices in this exercise appear in a different order each time the page is loaded.
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Your Results for: "Chapter Quiz"Print this page
Site Title:
Principles of Biochemistry, 4th Edition CW
Book Title:
Principles of Biochemistry
Book Author:
Horton
Location on Site:
Chapter 14 > Chapter Quiz
Date/Time Submitted
:
November 6, 2010 at 8:47 AM (EDT)
Summary of Results
30% Correct of 23 Scored items:7 Correct: 30%
16 Incorrect: 70%
More information about scoring
1.Level I
An ‘uncoupler’:
Your Answer:
facilitates the transport of protons from the interior of the mitochondria to the exterior, breaking down the proton gradient.
Correct Answer:
facilitates the transport of protons from the exterior of the mitochondria to the interior, breaking down the proton gradient.
2.Level I
What is the net change in the number of protons across the mitochondrial membrane for each set of two electrons transferred through Complexes I, III, and IV, respectively?
Your Answer:
4; 2; 4
Complex I transfers 4 protons, Complex III transfers 2 protons for
each turn of the Q cycle, and Complex IV transfers 2 protons and eliminates 2 protons on the exterior of the membrane (net change of 4 protons).
3.Level I
If a ketoglutarate/oxaloacetate shuttle was present, could a new shuttle be formed that would replace the malate-aspartate shuttle by eliminating the transamination reactions and the amino acid translocase?
Your Answer:
No. This shuttle would not work, because there would be a net charge change during the antiport of ketoglutarate/oxaloacetate.
Correct Answer:
Yes. Ketoglutarate/oxaloacetate could substitute for glutamate/aspartate.
4.Level I
Assume that the pH of the cytosol is 6 and the pH of the matrix of the mitochondria is 7. The pH of the intermembrane space of the mitochondria:
Your Answer:
It depends heavily on whether oxidative phosphorylation is possible.
Correct Answer:
is 6.
5.Level I
Which of the following enzymes or compounds does not feed into the electron-transport chain at Complex II or III?
Your Answer:
glycerol-3-phosphate
Correct Answer:
glyceraldehyde-3-phosphate
6.Level I
Proton leakage out of the mitochondria:
Your Answer:
may function in heat production, especially in hibernating animals.
7.Level I
In the proposed mechanism for cytochrome c oxidase:
Your Answer:
an oxygen atom is bound, and two electrons and two protons are added to make water.
Correct Answer:
an oxygen molecule is bound, and four electrons and four protons are added to make two waters.
8.Level I
Which of the following compounds can freely pass through the inner mitochondrial membrane?
Your Answer:
H+
Correct Answer:
CO2
9.Level I
Which of the following machines is the F0F1 ATPase most like?
Your Answer:
a pinwheel
Unidirectional flow generates rotary motion.
10Level II
.Based on the values provided in Table 14.1 (p. 423), which of the following compounds might be able to reduce cytochrome a?
Your Answer:
a) cytochrome a3
Correct Answer:
(b) and (c)
11.
Level II
You carry out a ‘crossover analysis’ to determine where an inhibitor of electron transport operates. You look at several different cofactors or proteins and find: FMN = reduced, cytochrome b560 = oxidized, cytochrome a = oxidized. Where did the inhibitor act?
Your Answer:
At complex I, blocking the transfer of electrons to Q.
Since proteins in both complexes III and IV are oxidized, it is possible to transfer electrons out of these complexes. Therefore, the block must be before complexes III and IV, most likely at complex I.
12.
Level II
What is the G for transporting four moles of protons across the membrane of a respiring mitochondrion (from inside to outside)? What is the Gfor electron transport across complex I (from NADH to QH2)?
Your Answer:
19, -35 kJ / mole
Correct Answer:
76, -70 kJ / mole
13.
Level II
For each two electrons transported, there is a net change of 10 protons across the membrane. Given the protonmotive force, how much energy would be yielded by transporting these 10 protons back across the membrane?
Your Answer:
(blank)
14.
Level II
On p. 432, you find: “The stoichiometry of proton entry per ATP synthesized is estimated to be 3 H+ per ATP.” Based on the net change in the number of protons across the membrane per two electrons transported and the estimate that 2.5 ATP molecules are made per NADH (Chapter 12), what might you have expected this number to be?
Your Answer:
3.3 protons / ATP
Correct Answer:
4 protons / ATP
15.
Level II
The difference between the result of problem (5) and the result described in the book (p. 436) is due to:
Your Answer:
the fact that the transport of ADP, ATP, and Pi utilizes and reduces the protonmotive force.
On p. 452 you will find “The combined energy cost of transporting ATP out of the matrix, and ADP and Pi into it, is approximately equivalent to the influx of one proton.” Thus, while four protons are available for ATP synthesis, one is utilized for transport, and the other three actually go toward changing the conformation of the F0F1 ATPase.
16.
Level II
In the malate-aspartate shuttle, glutamate and oxaloacetate serve as amino donors/acceptors for the production/consumption of aspartate. Could alanine and pyruvate substitute for glutamate and oxaloacetate? Assume that the aspartate transaminase can easily use alanine as an amino donor.
Your Answer:
(blank)
17.
Level III
Now consider the contribution of the charge differential across the membrane. For a dicarboxylic acid, the chemical free-energy change for movement across a membrane is -4.5 kJ / mole. If the electrical potential across the membrane is -0.2 V (negative inside), then what is the total free-energy change for the transport of the dicarboxylic acid?
Your Answer:
34.1 kJ / mole
deltaGelec = zFdeltapsi (Equation 14.12, p. 439). z = -2 (dicarboyxlic acid), F = 96.48 kJ / (V mole). The electrical contribution to the free energy is therefore 38.6 kJ / mole. The movement of a negative ion into a more negatively charged environment is energetically unfavorable.
18.
Level III
If the examples described in (1) and (2) are for the same membrane, what would the protonmotive force across this membrane be?
Your Answer:
0.023 V
Correct Answer:
-0.377 V
19.
Level III
For the membrane system described in (1), (2), and (3), what fraction of the protonmotive force is due to the chemical free-energy change?
Your Answer:
47%
-0.177 V / -0.377 V = 0.47.
20.
Level III
The chemical free-energy change for the movement of a compound across a membrane is 34.2 kJ / mole. The electrical potential across the membrane is -0.15 V (negative inside). What would the charge on the compound have to be to ensure energetically favorable movement?
Your Answer:
-1
Correct Answer:
+3
21.
Level III
If we assume that the actual cost of ATP synthesis is roughly 46 kJ/mole (p. 325), rather than the standard free energy of 30 kJ / mole, then what is the efficiency of the synthesis of one ATP/3 protons transported?
Your Answer:
85%
Correct Answer:
80%
22.
Level III
Based on the results of problems (7) and (8), what is the overall efficiency of the conversion of the energy of electron transport into ATP synthesis?
Your Answer:
88%
Correct Answer:
70%
23.
Level III
If the malate-aspartate shuttle were the sole source of electrons (NADH) for establishing a proton concentration gradient, then the concentration gradient would be _____ of its value if NADH were
produced from within the citric-acid cycle.
Your Answer:
95%
Correct Answer:
90%
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Home Chapter 17 Chapter Quiz
Chapter Quiz
This activity contains 26 questions.
Level I
Which of the following amino acids is likely responsible for most transamination reactions in a cell?
glutamate
glutamine
aspartate
alanine
Level I
The amino acid biosynthetic pathways that are missing in humans and thus lead to requirements for 'essential' amino acids:
are for amino acids made from citric-acid-cycle intermediates.
are for amino acids made from glycolytic intermediates.
are for amino acids that are derived in whole or in part from erythrose-4-phosphate.are for amino acids that are derived from a diverse group of metabolic intermediates
Level I
Which of the following compounds is phenylpyruvate?
Level I
One of the amino groups of urea comes from carbamoyl phosphate; the other comes from aspartate. Both are ultimately derived from glutamate. How does the amino group make its way from glutamate to aspartate?
Glutamate dehydrogenase generates a free amine from glutamate, and the free amine reacts with oxaloacetate to aspartate.Glutamate is converted to glutamine by glutamine synthase, and glutamine reacts with oxaloacetate to make aspartate.Glutamine and asparagine freely exchange amino groups, resulting in the production of aspartate.A transamination reaction between glutamate and oxaloacetate results in the production of aspartate.
Level I
Ketoadipate is an intermediate in the degradation of: (a) lysine.
(b) arginine.
(c) tryptophan.
(a) and (b)
(b) and (c)
(a) and (c)
Level I
Maple syrup urine disease is a deficiency of which amino acid degradative pathway?
tyrosine
leucine
tryptophan
arginine
Level I
Blood bicarbonate levels are kept intact during acidosis by the transformation of which amino acid?
glutamate
glutamine
aspartate
asparagine
glycine
Level I
Nitric oxide is synthesized from which amino acid? arginine
lysine
histidine
tryptophan
Level I
Ubiquitination of proteins occurs via: an amide bond with the epsilon amino group of lysine.
an amide bond with the alpha amino group of a protein.
an ester bond with the terminal carboxylate of a protein.
an ester bond with the gamma carboxylate group of glutamate.
glutamine
glutamine, proline, and arginine
proline
proline and arginine
glutamine and arginine
glutamine and proline
Level II
Under conditions of limiting ammonia, for ammonia to be transferred into an amino acid, which cofactors must be utilized?
ATP
ATP, NADH
NADH
NAD+
Level II
Asparagine is sometimes synthesized in a manner analogous to glutamine (fig. 17.20). If aspartate were made in a manner analogous to the reaction catalyzed by glutamate dehydrogenase, then the substrate for its synthesis would be:
pyruvate
glyceraldehyde-3-phosphate
oxaloacetate
fumarate
Level II
Glucose is radiolabeled at its 1 position and fed to an organism. Where does the label end up in serine?
the alpha carbon
the alpha carboxylate
the hydroxymethyl side chain
none of these
Level II
Methionine is radiolabeled at its alpha carboxylate and fed to humans. Where does the label end up in cysteine?
the alpha carboxylate
the alpha carbon
the beta carbon
none of these
Level II
In some organisms methionine is synthesized via methyltetrahydrofolate (Figure 17.23). What is the other substrate that is used in the synthesis of methionine?
homoserine
serine
homocysteine
cysteine
Level II
The three-carbon side chain of chorismate is derived from phosphoenolpyruvate. An organism is fed glucose labeled in the 1 position. Where does the label end up in phenylalanine?
the alpha carboxylate
the alpha carbon
the beta carbon
none of these
is always in the direction of glutamate.
is always in the direction of ketoglutarate.
is in the direction of glutamate when asparate is high.
is in the direction of glutamate when ammonia is high.
Level III
Based on your knowledge of the nitrogen cycle (fig. 17.1), and discounting the energetic contribution of ATP, what is the net equation for the synthesis of nitrate from nitrogen (N2 → 2 NO3
-)? N2 + 6 H2O + 8 e- → 2 NO3
- + 12 H+
N2 + 6 H2O → 2 NO3- + 5 H+ + 4 e- + H2
N2 + 6 H2O → 2 NO3- + 10 H+ + 8 e- + H2
N2 + 6 H2O → 2 NO3- + 5 H+ + 8 e-
Level III
Which of the following is a possible structural intermediate in the synthesis of asparagine from aspartate (section 17.3A)?
Level III
The following compound likely belongs to which biosynthetic pathway?
biosynthesis of aspartate
biosynthesis of serine
biosynthesis of threonine
biosynthesis of glutamate
biosynthesis of histidine
biosynthesis of phenylalanine
Level III
Which of the following is a likely intermediate for the glutamine synthetase reaction?
Level III
Based on BOX 17.3, which property of amino acids is the most ‘expensive’ to synthesize in terms of ATP?
hydrophobicity
negative charge
positive charge
aromaticity
Level III
Glutamate gamma-semialdehyde (Figure 17.13) spontaneously closes to form pyrroline 5-carboxylate. Why doesn’t aspartate beta-semialdehyde (Figure 17.11) undergo a similar reaction?
Aspartate beta-semialdehyde is acetylated to prevent ring closure.
Four-membered rings are much less stable than five-membered rings.
Aspartate beta-semialdehyde branches do form several compounds; therefore, it is probably swept up into other biosynthetic pathways as soon as it is formed.Aspartate beta-semialdehyde remains bound to an enzyme active site in an open configuration.
Level III
Figure 17.40 describes the flux through pathways feeding the urea cycle in the case where either ammonia or aspartate is high. What would the flux through these pathways be like if glutamate was high?
Since glutamate carries ammonia, it would be the same as when ammonia is high.Since glutamate can be readily transaminated to aspartate, it would be the same as when aspartate is high.Since glutamate and ammonia can form glutamine (via glutamine synthase), ammonia would flow away from the urea cycle.Glutamate would flow both into ammonia (via glutamate dehydrogenase) and into aspartate (via aspartate transaminase).
Level III
Why is alanine used to transport ammonia from muscle to liver, rather than aspartate or glutamate?
No pathways connect glucose to aspartate or glutamate.
The transport of aspartate or glutamate would rob the citric-acid cycle of key intermediates necessary for respiration.By transferring alanine, more acid is removed from working muscle than if aspartate or glutamate were transferred.By transferring alanine, the body prevents the conversion of pyruvate to lactate and hence prevents the accumulation of organic acids in muscle.
Level III
A labeled alpha-ketobutyrate is converted to labeled isoleucine, as shown below. Assuming that the synthesis of valine is analogous to the synthesis of isoleucine, pyruvate labeled on its methyl group would produce valine that was labeled:
on the alpha carboxylate.
on the alpha carbon.
on the beta carbon.
on one of the two gamma carbons.
