Question

26. Which one of the following is NOT characteristic of the hexose monophosphate pathway?

1. It produces CO₂
2. It uses NADP⁺ as a cofactor
3. It produces ATP
4. It produces ribose-5-phosphate
5. It produces glyceraldehydes-3-phosphate

27. A person with less than 10% of the normal amount of glucose-6-phosphate dehydrogenase would likely :

1. Be at risk for hemolytic anemia and persistent chronic infections.
2. Be unable to produce sufficient  levels of pentoses for nucleic acid synthesis
3. Have a glycogen storage disease.
4. Have a fatty liver
5. Have a reduced ability to produce NADP

28. When the PLC second messenger system is activated, which of the following causes an increase in intracellular Ca²⁺?

1. Phosphatidyl inositol (PI)
2. Phosphatidylnositol-4-phosphate(PIP)
3. Phosphatidylinositol 4,5- biphosphate(PIP₂)
4. Inositol triphosphate(IP₃)
5. Diacylglycerol

Answer 1

1) Hexose monophosphate pathway (HMP) is also called 'Pentose phosphate pathway'. as in this process hexose sugars undergo oxidative degradation through 5-carbon sugar intermediates. It has oxidative and non-oxidative phases. During the oxidative phase, glucose-6-phosphate is oxidized with the generation of 2 molecules of NADPH and one molecule of pentose phosphate with the liberation of one molecule of CO2. The NADP+ act as a cofactor in this pathway. During the non-oxidative phase, the pentose phosphates are converted to intermediates of glycolysis. The HMP produces Ribose-5-phosphate in the non-oxidative phase where the ribulose-5-phosphate formed in the oxidative phase is isomerized to ribose-5-phosphate or epimerized to xylulose-5-phosphate. This pathway produces Glyceraldehyde-3-phosphate via transketolase reaction. But in this pathway, ATP is neither utilized nor produced. Cells do not use the shunt pathway for energy production. So option (C) is the correct answer.

2) A person with less than 10% of the normal amount of glucose-6-phosphate dehydrogenase (GPD) may develop serious complications. It is the most common enzyme deficiency seen in clinical practice. The defect is transmitted as an X-linked recessive trait. It will lead to drug-induced hemolytic anemia. The deficiency is manifested only when exposed to certain drugs or toxins, e.g intake of antimalarial drugs like primaquine. This stimulates peroxide formation inside RBC. In GPD deficient cells, the level of NADPH is low. hence further production of peroxides will lead to cell lysis. GPD deficient persons also show increased met-haemoglobin in circulation which cannot carry oxygen to tissues and cells. So option (A) is the correct answer.

3) The signal transduction via Phospholipase C takes place through PIP2 cascade. Phospholipase C hydrolyses phosphatidyl inositol (PI) in membrane lipids to 1,4,5-Inosito triphosphate (IP3) and Diacyl glycerol (DAG) that act as second messengers. The phospholipase C may be activated either by G-proteins or calcium ions. DAG can also be generated by the action of phospholipase D that produces phosphatidic acid which is hydrolyzed to DAG. The binding of hormones like serotonin to cell surface receptor triggers the activation of the enzyme phospholipase C which hydrolyzes the phosphatidyl inositol to diacylglycerol. IP3 stimulates the release of Ca2+ from intracellular stores, such as from endoplasmic reticulum and from sarcoplasmic reticulum. The elevated intracellular calcium then triggers processes like smooth muscle contraction, glycogen breakdown and exocytosis. So option (D) is the correct answer.