(G-6-PD)

G-6-PD catalyzes the first step of pentose phosphate pathway which produces NADPH. Due to the defect, all the functions performed by the NADPH are altered. Genetics defects of G-6-PD is common in people of Mediterranean and Afro-Caribbean origin. The gene is mainly on X-chromosome, so males are mostly affected by this disorder. The defect is manifested as hemolytic anemia.

Causes:- Infections, certain drugs like anti-malarial drugs, food items like feva beans, henna, high dose of intravenous Vitamin C

Treatment:- Aspirin (high doses), Nonsteroidal anti-inflammatory drugs (NSAIDs), Sulfa drugs., Antibiotics such as quinolones

2. Glycogen storage disease or Glycogenosis

This is a group of genetic disease that results from the defect in the in an enzyme which is essential for both glycogen synthesis as well as degradation; and characterized by deposition of normal or abnormal deposition of glycogen in specific tissues.

Causes:- Enzyme deficiency which stops both glycogen synthesis and degradation, hereditary, acquired

Treatment:- Surgical care, consultations to the doctor, diet, long term monitoring etc.

3. Diabetes Mellitus

It is a syndrome of disorder of carbohydrate, protein, and fat metabolism. It is caused either due to lack of insulin secretion, or decreased sensitivity of the tissues to the insulin. Diabetes mellitus is broadly divided in two groups:-

• Type-1 diabetes mellitus or insulin dependent  diabetes mellitus(IDDM):- It is also known as juvenile onset diabetes. Its symptoms include frequent urination, excessive thirst, excessive hunger ketoacidosis etc.

Causes:- It is caused due to lack of insulin secretion because of destruction of pancreatic beta-cells they cause destruction of the beta cells due to: viral infection, autoimmune disorder or hereditary of beta-cell degeneration.

Treatment:- Carbohydrate, protein fat count, frequent blood sugar monitoring, taking insulin.

• Type-2 diabetes mellitus or non-insulin dependent diabetes mellitus(NIDDM). Its symptoms include obesity, excessive hunger, excessive thirst but not ketoacidosis.

Causes:-  Caused by decreased sensitivity of target tissues to insulin. This happens due to inadequate insulin receptors on the cell surface of the target tissues.

Treatment:- It can be treated

• At early stage by diet control, exercise, weight reduction and no exogenous insulin administration is required.
• Drugs that increases insulin sensitivity or drugs that causes more release of insulin by pancreas.
• In last stages, insulin administration is required.

4. Pompe’s Disease

It is a kind of glycogen storage diseases. In this disease, glycogen accumulates on the lysosomes in almost all the tissues especially heart, enlarged liver. The defective enzyme in this disease is lysosomal alpha-1, 4 glucosidase (acid maltase). This disease can also cause death at an early age due to heart failure.

Causes:- This is a genetic disorder. The defective gene is located on an autosome. It is caused due to deficiency of alpha glucosidase which breaks the complex sugars. 

Treatment:- These are treated according to their symptoms. Physical and occupational therapy is helpful. It can be diagnosed by chest X ray, electrocardiogram and echocardiography.

5. Wernicke-Korsakoff syndrome

It is a HMP-shunt disorder. It is a genetic disorder in which alteration in the activity of transketolase enzyme occurs. This alteration reduces its affinity with thiamine pyro-phosphate. The symptoms include:

• Mental disorder
• Memory loss
• Partial paralysis

Causes:- Caused due to thiamine (Vitamin B1 deficiency), chronic alcoholics, liver and stomach damage etc.

Treatment:-  Immediate administration of thiamine. Dose 500mg of thiamine is prescribed to the patients. Other than this,  banana bag,  a bag of intravenous fluids containing vitamins is also helpful. 

6. Cancer and Glycolysis

The cancer cells shows increased uptake of glucose. As the tumors grow, the blood vessels are unable to supply adequate oxygen and thus causes hypoxia. Due to this  anerobic glycolysis occurs mostly and supply oxygen. Hypoxia-inducible transcription factor (HIF) increases the synthesis of glycolytic enzymes and glycolytic transporters.

Causes:- It is caused due to acid- producing breakdown of glucose during glycolysis.

Treatment:- Since cancer cells cannot survive and grow without vascularization. So for the treatment drugs that inhibit the vascularization will be helpful. 

Multiple choice questions (MCQs)

1. Deficiency of glucose-6-phosphate dehydrogenase is the disorder of which pathway?
A. HMP shunt

B. Glycolysis

C. Gluconeogenesis

D. Glycogen metabolism

2. What is the product of glucose-6-phosphate dehydrogenase?
A. ATP

B. NADH

C. GTP

D. FAD

3. glycogenosis is the disorder of which pathway?
A. Gluconeogenesis

B. Glycogen metabolism

C. Glycolysis

D. HMP-shunt

4. What type disease is glycogenosis?
A. Genetic

B. Physically acquired

C. Chemically acquired

D. All of the above

5. Match the following treatment and its disorder-
A. Deficiency of G-6-PD 1. Nonsteroidal anti-inflammatory drugs
b. Glycogenosis 2. Long term monitoring
c. Diabetes mellitus 3. Insulin injection

6. What causes deficiency of G-6-PD?
A. Anti-malarial drugs

B. Feva beans

C. Glucagon deficiency

D. Both A and B

7. What are the causes of diabetes mellitus?
A. Lack of insulin receptor in tissue cells
B. Lack of glucagon receptor in tissue cells
C. Both
D. None

8. Diabetes is the disorder of which metabolism?
A. Carbohydrate metabolism

B. Phospholipid metabolism

C. Fructose metabolism

D. Cholesterol metabolism

9. Diabetes mellitus is caused due to the deficiency of which hormone?
A. Insulin

B. Glucagon
C. Epinephrine

D. None of the above

10. How many types of diabetes mellitus?
A. 2

B. 3

C. 4

D. 5

11. Which of the following statement is NOT true?
A. Deficiency of G-6-PD mainly occur in afro-caribbean people
B. Deficiency of G-6-PD can cause hemolytic anemia
C. Deficiency of G-6-PD is common in females
D. Deficiency of G-6-PD is caused due to high IV of vitamin C

12. What is the cause of glycogenosis?
A. Enzyme deficiency which stop the processes
B. Due to lack of co-enzymes
C. Both
D. None

13. What is the treatment of diabetes mellitus?
A. Diet

B. Long term monitoring

C. Insulin injection

D. All of the above

14. Type 2 diabetes mellitus is also known as?
A. NIDDM

B. IDDM

C. IIDM

D. IIMD

15. Type 1 diabetes mellitus is also known as?
A. NIDDM

B. IDDM

C. IIDM

D. IIMD

ANSWERS:- 
1. HMP-SHUNT
2. NADH
3. Glycogen metabolism
4. Genetic
5. a – 1 b – 2 c – 3
6. Both A and B
7. Lack of insulin receptor in tissue cells
8. Carbohydrate metabolism
9. None of the above
10. 2
11. Deficiency of G-6-PD is common in females
12. Enzyme deficiency which stop the processes
13. All of the above
14. IDDM
15. NIDDM

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REFERENCE:- 1. Pankaja Naik- Biochemistry; 4^th edition; page no:- 187, 190, 197 and 198

2. U Satyanarayana, U Chakrapani- Biochemistry; 4th edition; page no- 249, 250, 269, 275. 

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The concentration of liver glycogen is greater than in muscle tissues; because muscle tissue comprises of large amount of mass, so its storage capacity is three to four times that of liver.

The formation and utilization of of glycogen in the body with the help of enzymatic system is called glycogen metabolism.

In this article we are going to study about 2nd part of glycogen metabolism which is known as glycogenolysis.

Glycogenolysis

Glycogenolysis is the degradation of glycogen to glucose-6-phosphate in muscle and glucose in liver. Glycogenolysis is not the reverse of glycogenesis but it is the separate pathway.

Glycogenolysis occurs in the cytosol of muscle and liver cells

Pathway for Glycogenolysis

The pathway for both glycogenesis and glycogenolysis is described in the below picture

Reactions of glycogenolysis

1. It occurs via the breakage of 1,4-linkage by utilizing the phosphate group so that’s why it is called phosphorolytic cleavage.

2. Phosphorysis continue up to it reaches the branching point; where the other enzyme called debranching enzyme comes into action. Debranching enzyme has two functions

• Glucan transfer: i transfer the 3-4 unit to other branch
• 1,6- glycosidase- breaks the 1,6- linkage.

3. The combine action of both glycogen phosphorylase and debranching enzyme results in complete breakdown of glycogen into glucose-1-phosphate and free glucose.

4. Glucose1-1-phosphate is converted to glucose-6- phosphate by phosphoglucomutase enzyme. It is a reversible reaction and is utilized in both glycogenesis and glycogenolysis.

5. Only in liver, glucose-6-phosphate is converted to glucose because the glucose-6-phosphatase enzyme is found in liver. The glucose is then transferred from hepatic cell to the blood tp maintain the blood glucose level in humans.

Regulation of Glycogenolysis

The principal enzyme which controls the glycogenolysis is glycogen phosphorylase. The control of this enzyme in muscle and liver differs. In muscle, the role of glycogen is to provide glucose-6-phosphate in response to the ATP for the muscle contractions. Whereas in liver, glycogen provides free glucose for maintaining the blood glucose level.

1. Phosphorylase enzyme enzyme is stimulated by cAMP. cAMP is in turn stimulated by epinephrine in liver and muscle and glucagon in liver. Whereas cAMP is inhibited by high concentration of insulin which in turn inhibits the glycogenolysis

2. Also the glycogen phosphorylase enzyme is inhibited by the high concentration of glucose-6-phosphate and ATP present in muscle.

3. Phosphorylase enzyme is also stimulated by high concentration of Ca ions and AMP.

Significance of glycogenolysis

1. In liver, in between the meals, the blood glucose level are maintained within the normal range by release of glucose.

2. In muscle, glycogen must be readily available all the time as the source of glucose for muscle contraction.

Multiple choice questions(MCQs)

1. Why do glycogen metabolism takes place?
A. carbohydrate metabolism B. fat metabolism
C. lipid metabolism D. amino acid metabolism

2. Which enzyme is considered as the principal enzyme for the regulation of glycogenolysis?
A. phosphoglucomutase B. glycogen phosphorylase
C. glucose-6-phosphatase D. glucan transferase

3. Where glycogenolysis takes place?
A. cytosol B. mitochondria
C. ribosomes D. endoplasmic reticulum

4. Which of the following statement is NOT true?
A. glycogen metabolism is divided in 4 parts
B. glycogenolysis is degradation of glycogen
C. glycogen metabolism occurs in muscle and liver
D. the concentration of liver glycogen is greater than muscle tissue

5. What is the main function of debranching enzyme?
A. forms free glucose B. forms ATP molecules
C. transfers the glycosidic units D. both A and C

6. Which enzyme catalyzes the reaction which is utilized In both glycogenesis and glycogenolysis?
A. phosphoglucomutase B. glycogen phosphorylase
C. glucose-6-phosphatase D. glucan transferase

7. Match the following-
a. epinephrine             1. Stimulates cAMP
b. insulin                       2. Inhibits glycogen phosphorylase
c. glucose-6-phosphate  3. Inhibits cAMP
d. Ca ions                           4. Stimulates glycogen phosphorylase

8. What is the need of glycogenolysis in muscle?
A. for muscle relaxation B. muscle contraction
C. maintain blood glucose level D. all of the above

9. Which enzyme catalyzes the only reversible reaction of whole glycogen metabolism
A. phosphoglucomutase B. glycogen phosphorylase
C. glucose-6-phosphatase D. glucan transferase

10. Why muscle cannot release glucose into blood and is used exclusively by itself?
A. muscles cannot do extra work B. all glucose is used for muscle contraction
C. absence of glucose-6-phosphatase D. both A and B

11. Which of the following inhibits the glycogenolysis?
A. epinephrine B. glucagon
C. Ca ions D. none of the above

12. Which form of energy inhibits the glycogenolysis?
A. FAD B. AMP
C. ATP D. NADH

13. Which type of bond links the glycosyl units of glycogen compounds?
A. hydrogen bonds B. covalent bonds
C. glycosidic bonds D. sometimes A and sometimes C

14. Which enzyme catalyzes the reaction of glycogenolysis?
A. phosphoglucomutase B. glycogen phosphorylase
C. glucose-6-phosphatase D. all of the above

15. Where is glucose-6-phosphate mainly used?
A. glycolysis B. TCA cycle
C. HMP shunt D. both A and C

ANSWERS:-

1. carbohydrate metabolism
2. glycogen phosphorylase
3. cytosol
4. glycogen metabolism is divided in four parts
5. both A and C
6. phosphoglucomutase
7. a – 1 b – 3 c – 2 d – 4
8. muscle contraction
9. phosphoglucomutase
10. absence of glucose-6-phosphatase
11. none of the above
12. ATP
13. glycosidic bonds
14. all of the above
15. both A and C

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 183-187.

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The enzymes for HMP shunt are present in the cytosol of the cell. This pathway takes place in all the cells

Characteristics of HMP shunt

1. It is a multi-cyclic process in which 3 molecules of glucose-6-phosphate give rise to the three molecules of CO2 and 3 molecules of ribulose-5-phosphate(5 carbon sugar).

2. the three molecules of ribulose-5-phosphate are arranged to form 2 molecules of fructose-6-phosphate and 1 molecule of glyceraldehyde-3-phosphate.

3. It does not generate ATP.

Pentose Phosphate Pathway

Reactions involved:- The reactions of the pathway are divided in two phases

• 1st phase- oxidative irreversible phase
• 2nd phase- non-oxidative reversible phase

1st phase

1. dehydrogenation of glucose-6-phosphate to 6-phosphogluconolactone is catalyzed by glucose-6-phosphate dehydrogenase using NADP.

2. 6-phosphogluconolactone is hydrolyzed to 6-phosphogluconate.

3. oxidative decarboxylation of 6-phosphogluconate is catalyzed by 6-phosphogluconate dehydrogenase; this produces ribulose-5-phosphate, CO2 molecules and second molecule of NADPH.

2nd phase

4. Ribulose-5-phosphate serves as a substrate for two enzymes:

• ribulose-5-phosphate epimerase which catalyzes the epimerization of ribulose-5-phosphate to xylulose-5-phosphate.
• ribulose-5-phosphate isomerase catalyzes the isomerization of ribulose-5-phosphate to ribose-5-phosphate.

5. Transketolase enzyme catalyzes the transfer of 2 carbon units from xylulose-5-phosphate to ribose-5-phosphate; hence forming sedoheptulose-7-phosphate and glyceraldehyde 3 phosphate. This reaction requires a co-enzyme thiamine pyrophosphate(TPP) and Mg ions.

6. Transaldolase catalyzes the transfer of 3 carbon units from sedoheptulose-7-phosphate to glyceraldehyde 3 phosphate; hence forming fructose-6-phosphate and erythrose-4-phosphate.

7. Again, the transketolase catalyzes the transfer of 2 carbon units from xylulose-5-phosphate to erythroe-4-phosphate; hence produces glyceraldehyde 3 phosphate and fructose-6-phosphate.

8. Further, fructose-6-phosphate and glyceraldehyde 3 phosphate are catabolized through the glycolysis and TCA cycle.

Regulation of Pentose Phosphate Pathway

Glucose-6-phosphate dehydrogenase (G-6-PD) catalyzes the 1st step of the pathway and is the rate limiting step.  This enzyme is regulated by concentration of NADPH. high concentration of NADPH inhibits the enzyme, which in turn inhibits the pathway.

• under well-fed condition, the concentration of NADPH decreases, hence the HMP shunt is stimulated.
• under starvation and diabetes, the concentration of NADPH is high, hence the pathway is inhibited.

Insulin also regulates the pathway. High concentration of insulin stimulates the pathway by stimulating G-6-PD and 6-phosphogluconolactone dehydrogenase.

Significance of HMP shunt

1. Ribose-5-phosphate required for the biosynthesis of DNA and RNA are provided by this pathway

2. It provides the route for the interconversion of pentoses to hexoses.

3. It generates NADPH which is required for the biosynthesis of fatty acids, cholesterol, steroid hormones and neurotransmitters.

4. NADPH also keeps the iron of hemoglobin in ferrous state and prevents the formation of methemoglobin.

5. NADPH is important for phagocytosis carried out by WBCs

Multiple choice questions(MCQs)

1. HMP shunt is required for which kind of metabolism?
A. carbohydrate metabolism B. fat metabolism
C. lipid metabolism D. amino acid metabolism

2. What is the product generated after the completion of 1st phase?
A. ribulose-5-phosphate B. CO2
C. NADPH D. all of the above

3. What is the location of pentose phosphate pathway to take place?
A. cell membrane B. cytosol
C. ribosomes D. mitochondria

4. Match the following-
a. insulin 1. Oxidative irreversible
b. high concentration of NADPH 2. Stimulates HMP shunt
c. 1st phase 3. Non-oxidative reversible
d. 2nd phase 4. Inhibits HMP shunt

5. What type of reactions occurs in 2nd phase?
A. oxidative reversible B. non-oxidative reversible
C. non-oxidative irreversible D. oxidative irreversible

6. What is the role of NADPH in RBCs?
A. transports O2 B. forms Hb
C. keeps iron of Hb in ferrous form D. none of the above

7. transaldolase enzyme catalyzes the transfer of how many carbon units from sedoheptulose-7-phosphate?
A. 2 units C. 3 units
C. 4 units D. 5 units

8. Which of the following statement is NOT true?
A. HMP shunt stands for hexose monophosphate shunt
B. HMP shunt does not generate CO2
C. HMP does not generate ATP
D. pentose phosphate pathway takes place in cytosol

9. What stimulates the pentose phosphate pathway?
A. high concentration of insulin B. low level of NADPH
C. high level of NADPH d. both A and B

10. Which enzyme requires thiamine pyrophosphate as the co-enzyme?
A. transketolase B. ribulose-5- phosphate isomerase
C. transaldolase D. glucose-6-phosphate dehydrogenase

11. what is the product generated after the transketolase catalyzes the transfer of 2 carbon units from xylulose-5-phosphate to erythrose-4-phosphate?
A. fructose-4-phosphate B. glyceraldehyde 3 phosphate
C. fructose-6-phosphate D. both B and C

12. Which enzyme belongs to the 2nd phase of HMP shunt?
A. glucose-6-phosphate dehydrogenase B. 6-phosphogluconate
C. 6-phosphogluconate dehydrogenase D. none of the above

13. What type of reactions occurs in the 1st phase of pentose phosphate pathway?
A. oxidative reversible B. non-oxidative reversible
C. non-oxidative irreversible D. oxidative irreversible

14. What is the function of ribulose-5-phosphate epimerase?
A. epimerization of glucose-6-phosphate
B. epimerization of ribulose-5-phosphate
C. epimerization of xylulose-5-phosphate
D. epimerization of fructose -4-phosphate

15. Which form of energy is used by the glucose-6-phosphate dehydrogenase enzyme?
A. ATP B. AMP
C. GTP D. NADPH

ANSWERS:-

1. carbohydrate metabolism
2. all of the above
3. cytosol
4. a – 2 b – 4 c – 1 d – 3
5. non oxidative reversible
6. keeps the iron of Hb in ferrous form
7. 3 units
8. HMP shunt does not generate CO2
9. Both A and B
10. transketolase
11. both B and C
12. none of the above
13. oxidative irreversible
14. ribulose-5-phosphate
15. NADPH

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 187-190.

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