S/F Fats Flashcards
Why is it more efficient to store energy as lipid rather than as glycogen?
First, the energy yield per gram of lipid (about 38 kJ/g) is more than twice that for carbohydrate (about 17 kJ/g).
Second, lipid is stored as anhydrous lipid droplets, but carbohydrates such as glycogen and starch are stored hydrated, and the water of hydration roughly triples the effective weight of the carbohydrate, reducing the energy yield to about 6 kJ/g.
In the first step of fatty acid oxidation, the fatty acid (R—COOH) is converted to its coenzyme A derivative in the following reaction:
R–COOH + ATP + CoA–SH → R–CO–S–CoA + AMP + PPi
The standard free-energy change (∆G’°) for this reaction is –15 kJ/mol
What will tend to make the reaction more favorable when it takes place within a cell?
The hydrolysis of PPi by inorganic pyrophosphatase, for which ∆G’° is –19 kJ/mol, makes the
overall ∆G’° more negative.
The oxidation of acetyl-CoA added to isolated, intact mitochondria is stimulated strongly by carnitine. Why?
Carnitine is essential in the transport of fatty acyl groups into the mitochondrial matrix, where fatty acid oxidation occurs
The β oxidation of fatty acids begins with this activation reaction:
R–CH2–CH2–CH2–COOH + ATP + CoA–SH→
R–CH2–CH2–CH2–CO–S–CoA + AMP + PPi
What are the next two steps (after transport into the mitochondria)? Show structures and indicate where any cofactors participate.
The reactions are those catalyzed by fatty acyl–CoA dehydrogenase and enoyl hydratase.
One of the steps in fatty acid oxidation in mitochondria involves the addition of water across a double bond. What is the next step in the process? Show structures and indicate where any cofactor(s) participate(s).
The reaction is that catalyzed by β-hydroxyacyl-CoA dehydrogenase, for which NAD+ is cofactor
.For each two-carbon increase in the length of a saturated fatty acid chain, how many additional moles of ATP can be formed upon complete oxidation of one mole of the fatty acid to CO2 and H2O?
Each —CH2—CH2— unit yields 14 extra ATP molecules. The two oxidations of the β- oxidation pathway produce 1 FADH2 and 1 NADH, which yield 1.5 and 2.5 ATP, respectively, by
oxidative phosphorylation. The extra acetyl-CoA, when oxidized via the citric acid cycle, yields another 10 ATP equivalents: 3 NADH, 1 FADH2, and 1 ATP or GTP.
If you received a laboratory report showing the presence of a high concentration of ketone bodies in the urine of a patient, what disease would you suspect? Why do ketone bodies accumulate in such patients?
The patient is probably an untreated diabetic, but the condition might also result from fasting.
In either case, the unavailability of glucose from the blood stimulates gluconeogenesis in the liver.
As the substrate for glucose formation, oxaloacetate is withdrawn from the citric acid cycle, bringing that cycle to a near halt.
The fatty acids being oxidized in the liver yield acetyl-CoA, which now cannot be oxidized via the citric acid cycle.
Reversal of the thiolase reaction produces acetoacetyl- CoA, which is then converted into ketone bodies and exported from the liver.
Draw the structure of one ketone body, and describe circumstances under which you would expect to find high concentrations of this compound in the urine of a human.
The ketone bodies, acetoacetate, β-hydroxybutyrate, and acetone (p. 615), are overproduced in untreated diabetes mellitus and during prolonged fasting, when fatty acids become the principle energy source.
What are ketone bodies and why do they form during fasting?
The ketone bodies, acetoacetate, β-hydroxybutyrate, and acetone, are overproduced during fasting, when fatty acids from stored triacylglycerols become the principle oxidizable fuel.
Accumulation of acetyl-CoA and its precursor acetoacetyl-CoA favors ketone body formation.
Because oxaloacetate is used for gluconeogenesis, it is withdrawn from the citric acid cycle, bringing that cycle to a near halt.
The acetyl-CoA that is produced by β oxidation can no longer be oxidized via the citric acid cycle so it accumulates.
