BIO C HE MIA I I – ĆW ICZ E NI A
L I S T A 1 2
do wykładu dr. hab. inŜ. P. Dobryszyckiego
I n t e g r a c j a m e t a b o l i z m u
1. Consider the following examples of metabolic regulation:
a. Fatty acid oxidation in mitochondria is diminished when fatty acid biosynthesis in the cytosol is active due to the inhibition of carnitine acyltransferase by malonyl CoA.
b. The synthesis of HMG CoA reductase in various cells is inhibited by low-density lipoproteins.
c. Glucose 6-phosphatase is present in the liver and kidneys but not in muscle.
d. Amidophosphoribosyl transferase, the enzyme that catalyzes the committed step in the biosynthesis of purine nudeotides, is inhibited by all purine nucleotides.
e. The enzyme that catalyzes the synthesis and degradation of fructose 2,6-bisphosphate is phosphorylated and dephosphorylated in response to hormonal signals.
Indicate which of these examples apply to each of the following modes of metabolic regulation: i.
Allosteric interactions
ii.
Covalent modifications
iii.
Enzyme levels
iv.
Compartmentation
v.
Metabolic specialization of organs
2. Match each metabolic pathway in the left column with its major role in metabolism from the right column.
a. Glycolysis
(1) Control of glucose levels in blood
b. Gluconeogenesis
(2) Formation of NADH and FADH2
c. Pentose phosphate pathway
(3) Storage of fuel
d. Glycogen synthesis
(4) Synthesis of NADPH and ribose 5-phosphate
e. Fatty acid degradation
(5) Production of ATP and building blocks of
biomolecules
3. The control of phosphofructokinase in the liver and in muscle is different. Both epinephrine and glucagon initiate responses to low glucose levels, yet epinephrine stimulates glycolysis in muscle, whereas glucagon inhibits glycolysis in the liver. Explain this fact.
4. Explain how it is possible to have high NADPH/NADP+ and low NADH/NAD+ ratios in the cytosol of the same cells.
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5. Which of the following statements about the metabolism of the brain are incorrect?
a. It uses fatty acids as fuel in the fasting state.
b. It uses about 60% of the glucose consumed by the whole body in the resting state.
c. It lacks fuel reserves
d. It can use acetoacetate and 3-hydroxybutyrate under starvation conditions
e. It releases lactate during periods of intense activity
6. Adipose cells constantly break down and resynthesize triacylglycerols, but synthesis cannot proceed without an external supply of glucose. Explain why.
7. Select the statements from the right column that best describe the metabolism of each organ, tissue, or cell in the left column.
a. Brain
(1) Releases glycerol and fatty acids into the blood
b. Muscle
during fasting periods
c. Adipose tissue
(2) In a normal nutritional state, utilizes glucose as
d. Liver
the exclusive fuel
e. Red blood cell
(3) Synthesizes ketone bodies when the supply of
acetyl CoA is high
(4) Can release lactate into the blood
(5) Utilizes α-keto acids from amino acid degradation as an
important fuel
(6) Can store glycogen but cannot release glucose into the blood
(7) Can synthesize fatty acids, triacylglycerols, and VLDL when
fuels are abundant
8. Match each hormone in the left column with its properties, major target organ, and metabolic effects from the right column.
a. Insulin
(1) A catecholamine hormone
b. Glucagon
(2) A polypeptide hormone
c. Epinephrine
(3) Secreted by the α cells of the pancreas
(4) Effects are mediated by cAMP
(5) Target organ is the liver
(6) Target organ is muscle
(7) Signals the fed state
(8) Secreted in response to low blood glucose levels
(9) Promotes the storage of fuels
(10) Promotes the breakdown of stored fuels
9. Explain the allosteric effects of glucose on glycogen metabolism.
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10. Match the fuel storage forms in the left column with the most appropriate characteristics from the right column.
a. Glycogen
(1) Largest storage form of calories
b. Triacylglycerols
(2) Most readily available fuel during muscular activity
c. Protein
(3) Major source of precursors for glucose synthesis during
starvation
(4) Depleted most rapidly during starvation
(5) Not normally used as a storage form of fuel
11. Relative to the well-fed state, fuel utilization after three days of starvation shifts in which of the following ways?
a. More glucose is consumed by the brain.
b. Adipose tissue triacylglycerols are degraded to provide fatty acids to most tissues.
c. The brain begins to use ketone bodies as fuels.
d. Proteins are degraded in order to provide three-carbon precursors of glucose.
e. Glycogen is stored as a reserve fuel.
12. Which of the following occur in people with untreated diabetes?
a. Fatty acids become the main fuel for most tissues.
b. Glycolysis is stimulated and gluconeogenesis is inhibited in the liver.
c. Ketone body formation is stimulated.
d. Excess glucose is stored as glycogen.
e. Triacylglycerol breakdown is stimulated.
13. Within a few days after a fast begins, nitrogen excretion accelerates to a relatively high level. After several weeks, the rate of nitrogen excretion falls to a lower level. The excretion of nitrogen then continues at a relatively constant rate until the body is depleted of triacylglycerol stores; then the rate of urea and ammonia excretion again rises to a very high level.
a. What events trigger the initial surge of nitrogen excretion?
b. Why does the nitrogen excretion rate decrease after several weeks of starvation?
c. Explain the increase in nitrogen excretion that occurs when lipid stores are exhausted.
14. For several hours after birth, premature infants are particularly susceptible to hypoglycemia and are also unable to rapidly generate ketone bodies. Describe how each of the characteristics below would contribute to hypoglycemia, low circulating levels of ketone bodies, or both.
a. a large brain-to-body-weight ratio
b. a small store of liver glycogen
c. Low specific activity of cytosolic carnitine long-chain acyl-CoA transferase in liver d. Very low levels of liver phosphoenolpyruvate carboxykinase
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