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Impaired beta oxidation of fatty acids
An infant presents with lethargy, sweating, and irritability. He is admitted to the Pediatrics unit, where the attending pediatrician notices that his symptoms are pronounced when the feeding is delayed. After a series of tests, the child has been diagnosed with an enzyme deficiency that catalyzes the first step in the β-oxidation spiral, which is,
A. Fatty acid synthase
B. Acyl co A dehydrogenase
C. Enoyl Co A hydratase
D. β- Hydroxyacyl co A dehydrogenase
The correct answer is- Acyl co A dehydrogenase.
Acyl co A dehydrogenase is the first enzyme of β-oxidation spiral.
A saturated acyl Co A is degraded by a recurring sequence of four reactions (Figure)
1) Oxidation by flavin adenine dinucleotide (FAD)
3) Oxidation by NAD+, and
4) Thiolysis by Co A
The reaction catalyzed by acyl-CoA dehydrogenase involves the removal of two hydrogen atoms from the 2( α)- and 3( β)-carbon atoms. The enzyme requires FAD as a coenzyme. (Figure).
Fatty acid oxidation is an important source of energy. Impaired activity of Acyl co A dehydrogenase or deficiency of any of the enzymes of beta oxidation leads to an energy deficit.
In the given case study, the infants presents with symptoms of hypoglycemia, due to an imbalance between demand and supply of glucose. In the conditions of impaired beta oxidation of fatty acids, glucose becomes the major source of energy, since glycogen stores are not well-developed and the substrates of gluconeogenesis are also not adequately available, the diet remains the only source of glucose. Any delay in feeding results in precipitation of symptoms of hypoglycemia.
Carnitine deficiency also produces hypoglycemia, due to similar reason of non- availability of energy from fatty acid oxidation (Carnitine acts as a transporter of fatty acids from cytoplasm to mitochondrial matrix since fatty acids due to long hydrophobic chain cannot pass through inner mitochondrial membrane).
Figure- Steps of beta oxidation of fatty acids. The fatty acyl chain is shortened by two carbon atoms as a result of these reactions, and FADH2, NADH, and acetyl Co A are generated. Because oxidation is on the β carbon and the chain is broken between the α (2) – and β (3)-carbon atoms—hence the name – β oxidation.
As regards other options
Fatty acid synthase is a multienzyme complex involved in fatty acid synthesis.
Enoyl Co A hydratase- is an enzyme of beta oxidation (Figure); it catalyzes the second step of beta oxidation (hydration of fatty acids)
β- Hydroxyacyl co A dehydrogenase and Thiolase are also enzymes of beta oxidation spiral, but they catalyze the third and fourth steps (oxidation by NAD+ and thiolytic cleavage) respectively.
Thus the enzyme that catalyzes the first step in the β-oxidation spiral is Acyl co A dehydrogenase.
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