Disorders of Protein Metabolism and MCQs for GPAT, NEET, CSIR NET, UPSC, SSC

1. Metabolic disorders Associated with Urea Cycle:-

The five disorders of urea cycle are associated with the 5 enzymes of urea cycle. In these disorders accumulation of compounds takes place. All of them are described in below table

Symptoms of disorders of urea cycle include:

• Ammonia intoxication
• Protein inducing vomiting
• Blurred vision
• Irritability
• Mental retardation
• Slurred speech

Causes:-  It is a genetic disorder caused due to the deficiency of enzyme involved in urea cycle

Treatment:- Low protein diet, amino acid supplements, ammonia controlling medicines.

2. Phenylketonuria (PKU):-

It is an acquired disease of phenylalanine metabolism. Due to this defect, it leads to inability to convert phenylalanine to tyrosine. In PKU, there is accumulation of phenylalanine in tissues and blood and results in increased excretion of phenylalanine in urine.

PKU is classified into three groups. These 3 are described in below table

Causes:– A genetic disorder due to defective copy of PAH (phenyl alanine hydroxylase) gene transferred by both parents.

Treatment:- It can be treated by having low phenylalanine diet by adding proteins that have low content of phenylalanine. A phenylalanine diet must be started very soon to prevent the irreversible brain damage.

Hartnup’s disease:-

It is an inherited disorder of tryptophan metabolism. It is due to the defect in the intestinal and renal transport of tryptophan and thus leads to its deficiency.

• Tryptophan deficiency leads to decreased synthesis of vitamin niacin.
• Deficiency of niacin leads to pellagra.
• Tryptophan deficiency leads to decreased synthesis of serotonin.
• Deficiency of serotonin leads to neurological symptoms.
• Hartnup’s disease also leads to amino aciduria, due to the failure to transport amino acid from the kidney.
• Any unabsorbed tryptophan in the intestine is metabolized by the intestinal bacteria to indolacetic acid and indolpyruvic acid which are then excreted through urine.

Causes:-  Caused by mutation of gene that controls process of absorption and reabsorption of amino acid.

Treatment:- Oral supply of nicotinic acid will supply adequate amount of NAD+  in the patient which will thus correct the pellagra symptoms.

Histidinemia

It is a hereditary disorder of histidine catabolism. Histidenemia is characterized by increased blood-histidine and urine-histidine concentration. The defective enzyme is histidase, because of which it results in impaired conversion of histidine to urocanate.

Periodic hyperlysinemia:-

It is a disorder of lysine metabolism. It is increased blood-ammonia level as well as elevated plasma lysine. Increased liver lysine levels inhibits liver arginase, which causes hyperammonemia and show symptoms of ammonia intoxication.

Cause: The cause is uncertain

Treatment:- Proper regular reading of serum-lysine, arginine and ammonia level

Multiple choice questions (MCQs)

1. Citrullinemia is the disorder of which pathway?

A. Transamination

B. Deamination

C. Urea cycle

D. None of the above

2. Which compound is accumulated during the defective Ornithine trans-carboxylase enzyme?
A. Ornithine

B. Ammonia

C. Citrulline

D. All of the above

3. Which of the following is the symptom of argininemia?
A. Ammonia intoxication

B. Intestinal damage

C. Renal failure

D. All of the above

4. Match the following disorder and its treatment-
a. PKU 1. amino acid supplements
b. Hyperlysinemia 2. oral supply of nicotinic acid
c. Hartnup’s disease 3. regular reading of serum-arginine level
d. Urea cycle disorder 4. low phenylalanine diet

5. Hartnup’s disease is a disorder of which metabolism?
A. Phenylalanine

B. Glutamic acid

C. Tryptophan

D. Lysine

6. PKU is the disorder of which metabolism?
A. Phenylalanine

B. Glutamic acid

C. Tryptophan

D. Aspartic acid

7. In Classic PKU, which enzyme is defective?
A. Phenylalanine hydrolase

B. Dihydrobiopterin reductase

C. Dihydrobiopterin synthesis

D. None of the above

8. When is pellagra caused?
A. Deficiency of serotonin

B. Deficiency of niacin

C. Deficiency of histidine

D. Deficiency of arginine

9. What causes mental retardation?
A. Citrullemia

B. Arginosuccinic-aciduria

C. Argininemia

D. All of the above

10. Which of the following statement NOT true?
A. In citrullinemia, citrulline is accumulated
B. PKU can be treated by low phenylalanine diet
C. Classical hyperlysinemia is a tryptophan metabolism disorder
D. Histidinemia means increase in blood histidine level

11. What happens during histidinemia?
A. Catabolism of histidine

B. Synthesis of histidine

C. Both

D. None

12. What happens during PKU?
A. Accumulation of phenylalanine
B. Accumulation of tyrosine
C. Accumulation of tryptophan
D. All of the above

13. Periodic hyperlysinemia is a disorder of which metabolism?
A. Phenylalanine

B. Glutamic acid

C. Tryptophan

D. Lysine

14. Which bacteria metabolize the unabsorbed tryptophan?
A. Mouth bacteria

B. Intestinal bacteria

C. Both

D. none

15. In Hyperphenylalaninemia type-4 and 5, which enzyme is defected?
A. Phenylalanine hydrolase

B. Dihydrobiopterin reductase

C. Dihydrobiopterin synthesis

D. None of the above

ANSWERS:- 

1. Urea cycle
2. Ammonia
3. Ammonia intoxication
4. a – 4 b – 3 c – 2 d – 1
5. Tryptophan
6. Phenylalanine
7. Phenylalanine hydrolase
8. Deficiency of niacin
9. All of the above
10. Classical hyperlysinemia is a tryptophan metabolism disorder
11. Catabolism of histidine
12. Accumulation of phenylalanine
13. Lysine
14. Intestinal bacteria
15. Dihydrobiopterin synthesis

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 222, 223, 224, 236, 255