It takes place in almost all the cells but largely in bone marrow and liver. This happens because in liver and bone marrow, the requirements for the incorporation of heme into hemoglobin and cytochromes are high.

Biosynthesis of heme is divided in 3 stages:-

• Biosynthesis of delta-aminolevulinate (ALA) from glycine and succinyl-CoA.
• Formation of porphobillinogen (PBG) from two molecules of delta-aminolevulinate (ALA)
• Conversion of PBG to the cyclic tetra-pyrrol porphyrin ring and heme.

The first and the last step of the biosynthesis reaction is catalyzed in mitochondria while the intermediate steps are catalyzed in cytosol.

Reactions of biosynthesis of heme

1. The first step in the biosynthesis is the condensation of glycine and succinyl-CoA to form ALA. Other than ALA synthase enzyme, PLP (pyridoxal phosphate) is also necessary. This is the rate controlling step.
2. Two molecules of ALA condense to form 2 molecules of water and 1 PBG molecule; catalyzed by PBG synthase, where this is zinc requiring enzyme.
3. 4 PBG molecule condense to form ,linear tetra-pyrrol. Further, linear tetra-pyrrol forms a cyclic structure known as uroporphyrinogen-3. Here the basic ring structure is formed
4. Uroporphyrinogen-3 is further converted to coproporphyrinogen-3 by decarboxylation of acetate side chains to methyl groups.
5. Corpoporphyrinogen-3 is then converted into protoporphyrinogen-9 by corpoporphyrinogen oxidase. This enzyme is able to act only on type-3 corpoporphyrinogen; that’s why type-1 protoporphyrin do not occur in nature.
6. The oxidation of protoporphyrinogen-9 to protoporphyrin is catalyzed by protoporphyrinogen oxidase.
7. Finally, incorporation of ferrous ion in protoporphyrin in a reaction forms heme by heme synthase enzyme.

This picture is taken only for educational purpose from th.bing.com

Several factors regulate the synthesis of heme which are discussed below-

• ALA synthase enzyme which catalyzes the rate controlling step is feedback inhibited by excess product (heme)
• Excess level of heme inhibits the synthesis of delta-aminolevulinic acid synthase and decreased level of heme stimulates the synthesis of delta-aminolevulinic acid synthase
• Two iron atom activates heme synthesis

Several substances like steroid hormones, ethanol, barbiturates, metabolites etc stimulates the synthesis of ALA synthase enzyme.

Multiple Choice Questions (MCQs)

1. Biosynthesis of heme mainly takes place in which organ?
A. Liver

B. Muscle

C. Bone marrow

D. Both A and C

2. What is the location for initial and final steps of the biosynthesis of heme?
A. Cytosol

B. Nucleus

C. Mitochondria

D. Ribosomes

3. What is the location for intermediate steps of the biosynthesis of heme?
A. Cytosol

B. Nucleus

C. Mitochondria

D. Ribosomes

4. biosynthesis of heme is divided in how many stages?
A. 2

B. 3

C. 4

D. 5

5. Which of the following enzyme catalyzes the rate determining step of biosynthesis of heme?
A. PBG synthase

B. ALA synthase

C. Heme synthase

D. both A and C

6. Which compound begins the process for the biosynthesis of heme?
A. Vinyl groups

B. Acetyl-CoA

C. Glycine

D. All of the above

7. How many molecules of ALA is required for its condensation to form PBG?
A. 2

B. 3

C. 4

D. 5

8. Match the following enzymes and its substrates-

A. ALA synthase 1. Coproporphyrinogen-3

B. PBG synthase 2. protoporphyrin

C. Heme synthase 3. ALA

D. Coproporphyrinogen oxidase 4. Succinyl-CoA

9. Which of the following enzyme requires zinc?

A. PBG synthase

B. ALA synthase

C. Heme synthase

D. both A and B

10. Why do type-1 protoporphyrin do not generally occur in nature?

A. coproporphyrinogen oxidase only act on type-1 coproporphyrinogen

B. coproporphyrinogen oxidase only act on type-2 coproporphyrinogen

C. coproporphyrinogen oxidase only act on type-3 coproporphyrinogen

D. coproporphyrinogen oxidase only act on type-4 coproporphyrinogen

11. Which of the following inhibits ALA synthase enzyme?

A. Excess heme

B. Steroid hormones

C. Ethanol

D. All of the above

12. Which of the following statement is NOT true?

A. 2 molecules of ALA condenses to form only 1 water molecule

B. In 3rd step, basic ring structure is formed

C. Iron atom activates heme synthesis

D. Initial steps occur in mitochondria

13. Which of the following compound is also known as hydroxyl-methylbilane?

A. Succinyl-CoA

b. Uroporphyrinogen-3

C. Linear tetra-pyrrol

D. None of the above

14. decarboxylation of which side chain compound to methyl groups converts uroporphyrinogen-3 to corpoporphyrinogen-3?

A. Vinyl group

B. Acetate

C. Proprionate

D. All of the above

15. Which ion activates the biosynthesis of heme?

A. Calcium

B. Potassium

C. Sodium

D. None of the above

ANSWERS-
1. Both A and C
2. Mitochondria
3. Cytosol
4. 3
5. ALA synthase
6. Glycine
7. 2
8. a – 4 b – 3 c – 2 d – 1
9. PBG synthase
10. Coproporphyrinogen oxidase only act on type-3 coproporphyrinogen
11. All of the above
12. 2 molecules of ALA condenses to form only 1 water molecule
13. Linear tetra-pyrrol
14. Acetate
15. None of the above

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REFERENCE:- 1. Pankaja Naik- Biochemistry; 4^th edition; page no:- 313-315

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Fate of Globin

The globin may be reutilized for the formation of hemoglobin or degraded to the individual amino acids. The amino acids than undergo their own metabolism, and also participates in fresh globin synthesis.

Degradation of Heme

The catabolism of heme is carried out in small fractions of cells by a complex enzyme system known as heme oxygenase. It occurs in the presence of NADPH and oxygen. Ferric ion, CO and a green pigment known as biliverdin are released. Biliverdin is then converted to red orange pigment known as bilirubin. Bilirubin and its derivatives are collectively known as bile pigments.

The daily bilirubin formation in the body is 250-350 mg. this is yielded not only from hemoglobin but from ineffective erythropoiesis and also from various other heme proteins like cytochrome P₄₅₀.

Multiple Choice Questions (MCQs)

1. What is the average life span of RBCs?
A. 1 week
B. 1 month
C. 2 years
D. 4 months

2. Where do RE system takes place?
A. Spleen
B. Liver
C. Kidney
D. Both A and B

3. What happens to the globin part of hemoglobin after its dissociation?
A. Excreted through urine
B. Stored in liver
C. Degraded to its amino acid
D. None of the above

4. What is the complex enzyme system for the catabolism of heme called?
A. Heme dehydrogenase
B. Heme transferase
C. Heme oxygenase
D. Heme synthetase

5. What is the product released after catabolism of heme?
A. Biliverdin
B. Carbon mono-oxide
C. Ferric ion
D. All of the above

6. What is the color of bilirubin?
A. Green
B. Brown
C. Red-orange
d. Yellow-orange

7. What is the approximate formation of bilirubin in adults?
A. 150-220 mg
B. 50-70mg
C. 250-350 mg
D. 500-700 mg

8. Match the following substrate and its product-
A. Globin 1. Bilirubin
B. Heme 2. Biliverdin
C. Biliverdin 3. Amino acid

9. 1gm of hemoglobin yields how much of bilirubin?
A. 20mg
B. 25mg
C. 30mg
D. 35mg

10. What is the color of biliverdin?
A. Green
B. Brown
C. Red-orange
d. Yellow-orange

11. Complex enzyme system occurs aerobically or anaerobically?
A. Aerobically
B. Anaerobically
C. Both
D. None

12. Which of the following statement is NOT true?

A. Bilirubin is lipophilic in nature

B. Biliverdin reductase is a ATP dependent soluble enzyme

C. Albumin has 2 binding sites for bilirubin

D. Heme oxygenase enzyme produces biliverdin, ferrous ion and CO.
13. Which form of energy is required for the working of complex enzyme system?
A. ATP
B. ADP
C. NAD
D. None of the above

14. What are the sources that yield bilirubin other than hemoglobin?
A. Ineffective erythropoiesis
B. Cytochrome P450
C. Both
D. None

15. Fill in the blank- bilirubin is the most abundant ——-?
A. Lubricant
B. Astringent
C. Anti-oxidant
D. Antiseptic

ANSWERS-

1. 4 months
2. Both A and B
3. Degraded to its amino acid
4. Heme oxygenase
5. All of the above
6. Red orange
7. 250-350mg
8. A – 3 B – 2 C – 1
9. 35mg
10. Green
11. Aerobically
12. Biliverdin reductase is a ATP dependent soluble enzyme
13. None of the above
14. Both
15. Anti-oxidant

For More Standard and Quality Question Bank you can Join Our Test Series Programme for GPAT, NIPER JEE, Pharmacist Recruitment Exam, Drug Inspector Recruitment Exams, PhD Entrance Exam for Pharmacy

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REFERENCE:- 1. Pankaja Naik- Biochemistry; 4^th edition; page no:- 316-317.

2. U. Satyanarayana, U. Chakrapani; Biochemistry; Elsevier; 4th edition; page no- 214-216.

The post Catabolism of Hemoglobin and MCQs For Pharmacist, GPAT, NEET, GATE and CSIR NET Exams appeared first on Gpatindia: Pharmacy Jobs, Admissions, Scholarships, Conference,Grants, Exam Alerts.