0.4. A 1-week-old infant, who was born at home in a rural, medicallyunderserved area, has undetected classic phenylketonuria. Which statement about this baby and/or her treatment is correct?
A. A diet devoid of phenylalanine should be initiated immediately.
B. Dietary treatment will be discontinued in adulthood.
C. Supplementation with vitamin B6 is required.
D. Tyrosine is an essential amino acid.
Correct answer = D. In patients with phenylketonuria, tyrosine cannot be synthesized from phenylalanine and, hence, becomes essential and must be supplied in the diet. Phenylalanine in the diet must be controlled but cannot be eliminated entirely because it is an essential amino acid. Dietary treatment must begin during the first 7–10 days of life to prevent intellectual disability, and lifelong restriction of phenylalanine is recommended to prevent cognitive decline. Additionally, elevated levels of phenylalanine are teratogenic to a developing fetus.
0.5. Which one of the following statements concerning amino acids is correct?
A. Alanine is ketogenic.
B. Amino acids that are catabolized to acetyl coenzyme A are glucogenic.
C. Branched-chain amino acids are catabolized primarily in the liver.
D. Cysteine is essential for individuals consuming a diet severely limited in methionine.
Correct answer = D. Methionine is the precursor of cysteine, which becomes essential if methionine is severely restricted. Alanine is a key glucogenic amino acid. Acetyl coenzyme A (CoA) cannot be used for the net synthesis of glucose. Amino acids catabolized to acetyl CoA are ketogenic. Branched-chain amino acids are catabolized primarily in skeletal muscle.
0.6. In an individual with the dihydrolipoyl dehydrogenase (E3)-deficient form of maple syrup urine disease, why would lactic acidosis be an expected finding?
The three α-keto acid dehydrogenase complexes (pyruvate dehydrogenase [PDH], α-ketoglutarate dehydrogenase, and branched-chain α-keto acid dehydrogenase [BCKD]) have dihydrolipoyl dehydrogenase (Enzyme 3, or E3) in common. In E3-deficient maple syrup urine disease, in addition to the branched-chain amino acids and their α-keto acid derivatives accumulating as a result of decreased activity of BCKD, lactate will also be increased because of decreased activity of PDH.
0.7. In contrast to the vitamin B6–derived pyridoxal phosphate required in most enzymic reactions involving amino acids, what coenzyme is required by the aromatic amino acid hydroxylases?
Tetrahydrobiopterin, made from guanosine triphosphate, is the required coenzyme.
Amino Acids: Conversion to Specialized Products 21
For additional ancillary materials related to this chapter, please visit thePoint.
I. OVERVIEW
In addition to serving as building blocks for proteins, amino acids are precursors of many nitrogen-containing compounds that have important physiologic functions (Fig. 21.1). These molecules include porphyrins, neurotransmitters, hormones, purines, and pyrimidines. [Note: See p. 151 for the synthesis of nitric oxide from arginine.]
II. PORPHYRIN METABOLISM