Chapter 27 Practice Questions

The hormone __________ induces lipolysis, whereas the hormone __________ inhibits the process. A) epinephrine; adrenocorticotropic hormone B) glucagon; insulin C) insulin; norepinephrine D) glucagon; epinephrine E) epinephrine; glucagon

B

Three rounds of fatty acid oxidation result in A) two acetyl CoA molecules. B) three acetyl CoA molecules. C) the synthesis of palmitate. D) the formation of enoyl CoA. E) the complete oxidation of palmitate B.

B

Enzymes that digest the triacylglycerols into free fatty acids and monoacylglycerol are called A) hydrases. B) glyases. C) lipases. D) All of the above. E) None of the above.

C

Every biochemistry student knows that most naturally occurring unsaturated fatty acids are "cis" fatty acids. Why then do so many fat-containing foods come with the notation "Contains no trans fats"? A) Vegetable oils contain polyunsaturated fatty acids prone to oxidation, which then convert them to trans fats. B) Olive oils, normally healthy oils, are converted to trans fat when cooked at high temperatures. Thus, olive oils are used only at low temperatures. C) Vegetable oils are often hydrogenated to form trans unsaturated fatty acids to preserve shelf life. D) Oils that contain an odd number of carbon atoms are particularly susceptible to oxidation to trans fats. Thus, no odd numbered carbon fatty acids are used in these foods. E) Oils that contain an odd number of carbon atoms are particularly susceptible to reduction to trans fats. Thus, no odd number carbon fatty acids are used in these foods.

C

How many rounds of β oxidation would be required for a 16-carbon fatty acyl chain to be degraded to acetyl CoA? A) 16 B) 8 C) 7 D) 15 E) None of the above.

C

The enzyme acyl CoA synthetase catalyzes A) lipolysis to release free glyceraldehyde. B) ATP-dependent reduction prior to activation. C) ATP-dependent activation of fatty acids using CoA. D) All of the above. E) None of the above.

C

The β oxidation of myristyl-CoA (14:0) yields A) 7 acetyl CoA + 7 FADH₂ + 7 NADH + 7 H⁺ B) 6 acetyl CoA + 7 FADH₂+ 7 NADH + 7 H⁺ C) 7 acetyl CoA + 6 FADH₂ + 6 NADH + 6 H⁺ D) 7 acetyl CoA + 7 FAD + 7 NAD⁺ E) 14 acetyl CoA + 12 FADH₂ + 12 NADH + 12 H⁺

C

What does muscle weakness and cramping affecting skeletal muscle, heart, and kidney likely indicate? A) carnitine excess in the diet B) loss of hormone-sensitive lipase activity C) malfunctioning or deficient carnitine translocase D) ketone-body production E) All of the above.

C

Why is the anhydrous nature of triacylglycerols important in energy efficiency?

Carbohydrates need to be hydrated to be stored properly. Thus, storage of carbohydrates takes up more space because of the water of hydration. Triacylglycerols can be stored in "concentrated" form; so by net weight, triacylglycerols store six times as much energy as carbohydrates, gram for gram.

Coenzyme(s) involved in the degradation of saturated fatty acyl CoA include(s) A) FAD. B) NAD+ C) TPP. D) A and B. E) A, B, and C.

D

Exercising first thing in the morning without eating mobilizes lipid stores for fuel. All of the below are involved in this mobilization, EXCEPT A) glucagon binds 7TM receptors that activate adenylate cyclase. B) perilipin is phosphorylated. C) cAMP stimulates protein kinase A. D) hormone-sensitive lipase completes the mobilization of fatty acids with the production of a free fatty acid and glycerol. E) hormone-sensitive lipase is phosphorylated.

D

Fatty acids are the building blocks of A) cholesterol. B) phospholipids. C) glycolipids. D) B and C. E) A, B, and C.

D

How does the formation of acetyl CoA by β oxidation not only provide energy for cellular respiration, but also spare glucose? A) As acetyl CoA levels increase during lipid metabolism, D-3-hydroxybutyrate is reduced to pyruvate for gluconeogenesis. B) Acetyl CoA provides carbon for the formation of oxaloacetate, which in turn can be decarboxylated to form pyruvate. C) Acetyl can be converted to acetoacetate and from there to lactate for anaerobic metabolism. D) Acetyl CoA derived from fatty acids inhibits pyruvate dehydrogenase, the enzyme that converts pyruvate into acetyl CoA. E) Acetyl CoA derived from fatty acids activates pyruvate carboxylase stimulating gluconeogenesis.

D

Triacylglycerol stored in adipose tissue is used by the liver and other tissues for A) glycerol for pyruvate and glucose in liver. B) ATP production via the citric acid cycle in muscle. C) conversion to acetyl CoA and ketone bodies during starvation for the brain. D) All of the above. E) None of the above.

D

Which organs or tissues prefer to use ketone bodies such as acetoacetate as a fuel energy source instead of glucose? A) the heart muscle B) the renal cortex C) the brain D) A and B E) A, B, and C

D

What is the role of acetoacetate in lipid metabolism in addition to providing energy when blood glucose is low? A) Acetoacetate is used to synthesize glucose when glucose stores are low. B) Acetoacetate can be readily converted to oxaloacetate, a precursor for the amino acid aspartate. C) Acetoacetate has a pKa near 7.4 and so it can act as a blood buffer when glucose is low. D) Acetoacetate can be converted to lactate and so it can provide energy in the same way that is done under anaerobic glycoliysis. E) The liver lacks CoA transferase and so it releases acetoacetate into the blood for us by other tissues.

E

The first step of β oxidation is a(n) _____-linked oxidation of the fatty acyl chain.

FAD

How and where are fatty acids stored?

Fatty acids are stored as triacylglycerol in adipocytes of adipose tissue. This fuel-rich tissue can be found under the skin (subcutaneous) or surrounding internal organs (visceral).

Briefly outline the steps in fatty acid degradation.

First, fatty acids must be activated by reacting with coenzyme A to form acyl CoA. This requires energy from the hydrolysis of ATP to AMP, a cost of two high-energy phosphoryl bonds. The activated acyl group is oxidized by FAD, hydrated, oxidized by NAD⁺ to form a second carbonyl, and then cleaved by thiolysis. This releases one molecule of acetyl CoA.

_____ from fatty acid oxidation may enter glycolysis after phosphorylation and oxidation.

Glycerol

What pathological conditions can arise from a large increase in blood levels of ketone bodies in diabetics?

In insulin-resistant diabetes mellitus, the lack of insulin means the liver cannot absorb glucose, no oxaloacetate is made, and the fatty acid mobilization is slowed. The large amounts of ketone bodies actually cause acidosis. This pH change causes impairment of tissue function.

How is pyrophosphate formation an important theme in biochemistry?

The hydrolysis of inorganic pyrophosphate is a high-energy reaction that is not easily reversed. The hydrolysis of PPi drives a reversible reaction to completion. Essentially, two phosphoanhydride bonds are used to drive an energetically unfavorable reaction. It is a common strategy in several paths.

Calculate the amount of ATP generated from the total oxidation of an activated fatty acid with an acyl chain of 16 carbons.

The pathway would generate 7 NADH and 7 FADH₂ and 8 acetyl CoA molecules, which would be further metabolized in the citric acid cycle. Thus, the net gain in ATP would be 17.5 ATP (NADH) + 10.5 ATP (FADH₂) + 108 ATP (acetyl CoA). Two are used for activation. The net amount generated is therefore 106 ATP.

What are the repeated steps of fatty acid degradation?

The repeated steps in fatty acid degradation are oxidation, hydration, another oxidation and finally, thiolysis. These are detailed in steps 3-6 in Table 27.1.

Explain why animals cannot convert fatty acids to glucose.

The synthesis of glucose via gluconeogenesis requires pyruvate or oxaloacetate. In animals, the only metabolic route to convert acetyl CoA to oxaloacetate is through the citric acid cycle, which involves the loss of two CO₂ molecules. So animals cannot bring about the net conversion acetyl CoA into pyruvate or oxaloacetate. Plants and bacteria possess the glyoxylate cycle, which can convert two acetyl groups into succinate.

What three stages are required to utilize the energy stores in adipocytes?

Three stages of processing are required. The lipids are mobilized by degradation of the triacylglycerols to fatty acids and monoacylglycerols, and transported to the appropriate tissues. Then, the fatty acids are activated to fatty acyl CoAs and transported into the mitochondria. Finally, the fatty acids are degraded stepwise into acetyl CoA, which is processed in the citric acid cycle.

Acetoacetate spontaneously decarboxylates to form _____.

acetone

Triacylglycerols are stored in ____________ cells in animals.

adipose

Glucagon and _____ stimulate the breakdown of triacylglycerols by activation of a hormone-sensitive lipase.

epinephrine

The molecule ____________stimulates lipolysis when energy reserves are low.

glucagon

The enzyme ____________ is activated by hormones and is responsible for the first hydrolysis of triacylglycerol.

hormone-sensitive lipase

The brain uses _____ instead of glucose for its source of ATP production during long-term fasting.

ketone bodies

Untreated, chronic ketone body production will _____ blood pH levels.

lower

In eukaryotes, the degradation of fatty acids occurs in the _____.

mitochondrial matrix

The availability of the metabolite ____________ determines whether acetyl CoA, made from fatty acids, can enter the citric acid cycle.

oxaloacetate

β oxidation of odd-numbered fatty acids produces acetyl CoA and _____.

propionyl CoA

The transfer of an acyl group to coenzyme A is driven by the hydrolysis of ____________.

pyrophosphate

Increases in acetyl CoA during starvation inhibit the enzyme ____________.

pyruvate dehydrogenase

The final step of each round of oxidation of fatty acids is the _____ cleavage to yield acetyl CoA.

thyolytic

Acetoacetate and _____are referred to as ketone bodies and are a major fuel source for muscle tissue.

β hydroxybutyrate

The pathway by which an acyl CoA is degraded to acetyl CoA is ____________.

β oxidation

What organelle contains the enzymes for β oxidation and how does the fatty acid enter this organelle?

β oxidation occurs in the mitochondrial matrix; however, free fatty acids cannot cross the inner membrane. The entry of fatty acids is mediated by a translocase that carries acyl carnitine into the matrix while transporting carnitine back out.

The compound ____________ is considered one of the "ketone bodies."

β-hydroxybutyrate

The approximate energy yield from complete oxidation of palmitate is ____________.

106 ATP