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Chem 454: Regulatory Mechanisms in Biochemistry
Additional Problems – Chapter 22
1.
The oxidation by microbes of long-chain alkanes, which are found in crude oil, is the
subject of study because of concern about oil spills. In many bacteria, alkane, oxidation
occurs within the outer membrane. A monooxygenase enzyme uses molecular oxygen
and an oxidizable substrate, such as NADH, to convert an alkane to a primary alcohol.
Studies show that three additional reactions are required for the primary alcohol to under
bg oxidation. Propose a pathway for the conversion of a long-chain primary alcohol to a
substrate that can undergo b oxidation. Include cofactors and electron acceptors that
might be required.
2.
Malonyl-CoA, labeled with 14C in the methylene carbon, is used in excess as a substrate
in a system in vivo for the synthesis of palmitoyl-CoA, which is catalyzed by a yeast fatty
acid synthase complex. Acetyl-CoA and other substrates are also present in the system,
but acetyl-CoA carboxlyase is not. Which carbons in palmitoyl-CoA will be labeled?
3.
A deficiency of carnitine acyltransferase I in human muscle causes cellular damage and
recurrent muscle weakness, especially during fasting or exercise. A deficiency of the
enzyme in the liver causes an enlarged and fatty liver, hypoglycemia, and a reduction in
the levels of ketone bodies in blood. Explain the likely causes of these symptoms.
4.
a.
Describe how malonyl-CoA affects the balance between the rates of synthesis and
b oxidation of fatty acids in a liver cell.
b.
Show that failure to regulate theses two processes reciprocally could result in the
wasteful hydrolysis of ATP.
5.
6.
Plant seeds contain triacylglycerols in organelles called spherosomes. During
germination, lipases located in the spherosome membrane convert triacylglycerol to
monoacylglycerols, free fatty acids, and glycerol. Both free fatty acids and
monoacylglycerols enter the glyoxysome (see p 485 in your text), whereas most of the
glycerol is metabolized in the plant cell cytosol. A membrane-bound lipase in the
glyoxysome converts monoacylglycerols to free fatty acids and glycerol.
a.
Describe two possible metabolic fates of glycerol in the cytosol.
b.
What is the fate of fatty acids in the glyoxysome?
c.
When a germinating plant begins to carry out photosynthesis, the number of
glyoxysomes in the germinating plant decreases rapidly. Why?
d.
A difference between plant glyoxysomes and plant mitochondria is that
glyoxysomes cannot oxidize acetyl-CoA, whereas mitochondria can. How is this
observation related to the metabolism of fatty acids in these two organelles.
People concerned about obesity must pay attention not only to triacylglycerol intake but
also to the consumption of starch, glucose, and other carbohydrates. Although
carbohydrates can be converted to glycogen in liver, muscle, and other tissues, only about
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five percent of the energy stored in the body is present as glycogen. What happens to
most carbohydrates that are consumed in excess of caloric need?
7.
Wakil's pioneering studies on fatty acid synthesis included the crucial observation that
bicarbonate is required for the synthesis of palmitoyl-CoA. He was surprised to find that
very low levels of bicarbonate could sustain palmitate synthesis; that is, there was no
correlation between the amount of bicarbonate required and the amount of palmitate
produced. Later he also found that 14C-labeled bicarbonate is not incorporated into
palmitate. Explain these observations.
8.
In tissue culture, cells that are deficient in NADP+-linked malate enzyme can be isolated.
They exhibit a slightly lower rate of fatty acid synthesis when compared with normal
cells. However, cells lacking citrate lyase are very difficult to isolate. Why?
9.
Hydrogenating oils to saturate the double bounds in their fatty acids in order to increase
their melting temperatures causes some of the cis double bonds to convert into the trans
conformation. Predict what would happen if a monoenoic fatty acid with a trans-∆10
bond were produced, ingested, and degraded by the b-oxidation pathway. If another of
the ingested fatty acids contains a cis-∆11 double bond, what would be the out come of
these processes? What effect would the presence of the double bond have on the yield of
ATP obtained by the b oxidation of these fatty acids?
10.
Certain desert mammals can survive long periods of drought by consuming plants and
seeds and then generating water by metabolizing the fuels they provide.
a.
Briefly describe how water is generated through intermediary metabolism.
Include the sources of oxygen and hydrogen, and describe reactions that lead to
the formation of water.
b.
While parts of mature plants are a reliable source of carbohydrates and proteins,
plant seeds contain high quantities of triacylglycerols and free fatty acids. Would
plants or seeds be better for generating metabolic water? Why?
11.
Compare the effects of high levels of intracellular citrate on pathways of fatty acid and
carbohydrate metabolism. Explain how its transport from the mitochondrion to the
cytosol is essential for the action of citrate on both sets of pathways.
12.
In mammals, acetyl-CoA from fatty acid oxidation cannot be used for the net synthesis of
pyruvate or oxaloacetate, which in turn means that net glucose synthesis from acetyl-CoA
is impossible. However, glucose can be radioactively labeled when 14C-labeled acetate is
introduced into human tissue culture cells and converted to acetyl-CoA by acetyl-CoA
synthetase. Radioactive fatty acids can also be used to label glucose. Why? If the
methyl carbon of acetate is labeled, where will glucose be labeled?
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