The carnitine shuttle and fatty acid metabolism
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The carnitine shuttle and fatty acid metabolism
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[post_date] => 2024-12-23 10:44:14
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[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Whereas short-chain and medium-chain fatty acids can diffuse directly across the mitochondrial membranes, the transport long-chain fatty acids requires the activity of carnitine palmitoyltransferase (CPT) enzymes. CPT I, located on the outer mitochondrial membrane, catalyzes the transfer of the fatty acyl group from fatty acyl-CoA to carnitine, forming fatty acylcarnitine. Fatty acylcarnitine is then able to pass through the mitochondrial inner membrane using a transport protein called the carnitine-acylcarnitine translocase (CACT or CACT1). Once inside the mitochondrial matrix, fatty acylcarnitine is converted back into fatty acyl-CoA by another enzyme called CPT II, which is located on the inner mitochondrial membrane. Now in the form of fatty acyl-CoA, the fatty acids can undergo beta-oxidation within the mitochondria.
Figure 1 Carnitine shuttle
CPT II deficiency has an autosomal recessive inheritance pattern. Individuals with CPT II deficiency experience hypoglycemia and muscle weakness, particularly during times of increased energy demand. In some patients, accumulated fatty acid derivatives leads to mitochondrial dysfunction, muscle damage, and rhabdomyolysis, a condition characterized by the breakdown of muscle tissue and the release of toxic substances into the bloodstream. Managing CPT II deficiency often involves dietary modifications and strategies to these complications.
[post_title] => The carnitine shuttle and fatty acid metabolism
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[question] => All of the following dietary recommendations would be beneficial for individuals with a CPTII deficiency EXCEPT:
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
According to the passage CPTII is required for the transport of long-chain fatty acids.
Why is fat important? Recall it is a major storage fuel inside our body to sustain ATP levels during times of fasting.
These individuals cannot break down fats effectively during fasts, so it is advisable to avoid long periods of fasting.
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[answers] => Array
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[each_answer] => A. Eat large meals spaced out by long periods of fasting.
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[each_answer] => B. Consume small-chain fatty acids.
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[each_answer] => C. Consume medium-chain fatty acids.
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[each_answer] => D. Eat frequently.
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Levels of carnitine in individuals with a CPTII deficiency compared to a normal individual will be:
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
What is the role of CPTII?
CPTII converts acyl-carnitine into a free carnitine molecule + acyl-CoA.
Less CPTII activity will decrease levels of free carnitine in the cell.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. the same.
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[each_answer] => B. Increased.
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[each_answer] => C. decreased.
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[each_answer] => D. increased in the matrix only.
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[quiz_unique_key] => 1403770772
[question] => Why do some patients with CPTII deficiency suffer from hypoglycemia?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
How does the body respond to a state of starvation?
The body relies on its glycogen stores and then reverts to gluconeogenesis. However, gluconeogenesis is an anabolic process and cannot proceed without ATP.
Fatty-acid oxidation fuels gluconeogenesis. Being unable to effectively oxidize fatty acids will compromise gluconeogenesis, which maintains blood glucose levels in a fast.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Fatty-acid oxidation yields free glucose.
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[each_answer] => B. Fatty-acid oxidation fuels gluconeogenesis.
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[each_answer] => C. Fatty-acid oxidation promotes insulin secretion.
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[each_answer] => D. Fatty-acid oxidation yields acetyl CoA.
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[quiz_unique_key] => 1403770772
[question] => A deficiency in what other enzyme would most closely mimic the clinical manifestations of a CPTII enzyme deficiency?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
CPT II deficiency impairs the transport of long-chain fatty acids into the mitochondria for oxidation. As a result, individuals with CPT II deficiency often experience symptoms such as muscle weakness, fatigue, and muscle pain, especially during periods of increased energy demand.
Anything that affects the oxidation of fatty acids is likely to have similar manifestations to a CPTII deficiency.
Pyruvate carboxylase and glucose-6-phosphatase are enzymes involved in gluconeogenesis. Deficiency of these may cause hypoglycemia, but this is not the best answer choice as additional symptoms will be present.
Hormone-sensitive lipase (HSL) is an enzyme involved in the breakdown of triglycerides (a type of fat storage molecule) in adipose tissue to release fatty acids into the bloodstream. If HSL is deficient or dysfunctional, it would also lead to impaired and utilization release of fatty acids from adipose tissue, resulting in elevated levels of circulating triglycerides and reduced availability of fatty acids for energy production in various tissues, including muscle.
Note that a deficiency in fatty acid synthase (FAS), which produces fatty acids, would potentially leading to issues like reduced fat storage and therefore utilization, but it would be fundamentally different from CPT II deficiency, which impairs the breakdown of fatty acids at times of high energy demand.
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[each_answer] => A. Glucose-6-phosphatase
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[each_answer] => B. Hormone-sensitive lipase
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[each_answer] => C. Pyruvate carboxylase
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[each_answer] => D. Fatty acid synthase
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[question] => Administration of glucagon to a symptomatic individual with a CPTII deficiency is predicted to:
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
What does glucagon do? Recall that it stimulates gluconeogenesis and fatty acid oxidation.
In CPTII deficiency, there is a build-up of unused fatty acids, which presumably causes the muscle weakness and pain in some individuals. Moreover, the inability to effectively oxidize fatty acids will impair gluconeogenesis, which is reliant on the ATP produce in fatty acid oxidation.
Increased levels of glucagon will stimulate further fatty acid release from adipose cells and maintain the overall “catabolic” state of the body. Because individuals with CPTII deficiency cannot effectively oxidize fatty acids, increased release of fatty acids will cause an increased build-up of fatty acids and continued impairment of gluconeogenesis, which will likely worsen the patient’s symptoms.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. worsen a patient’s symptoms.
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[1] => Array
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[each_answer] => B. improve a patient’s symptoms.
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[each_answer] => C. not change a patient’s symptoms.
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[each_answer] => D. completely reverse a patient’s symptoms.
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[quiz_unique_key] => 2103402213
[question] => Which structure represents a molecule that has been transported by the carnitine shuttle and is prepared for beta oxidation?
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Fatty acid oxidation occurs in the mitochondrial matrix.
CPT II converts fatty acids from the carnitine shuttle back to fatty acyl CoA molecules.
Only long chain fatty acid derivatives are transferred by the carnitine shuttle.
Long chain fatty acids generally have more than 12 carbons. Only Choice C shows a fatty acyl CoA derivative of a long chain fatty acid that has been transported by the carnitine shuttle and prepared for beta oxidation.
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[each_answer] => A. 
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[each_answer] => B.
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[each_answer] => C.
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[each_answer] => D.
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[553993|4] => B
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Figure 1
