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Hexose Mono Phosphate (HMP) Pathway- Subjective Questions (Solved) Set-2
How is the processing of this important metabolite partitioned between these two metabolic routes?
Write a balanced chemical reaction equation for the pentose pathway for each of the following modes: a) where ribose-5-phosphate synthesis is maximized b) where NADPH production is maximized, by conversion of sugar phosphate products to glucose-6-phosphate for repeated operation of the pathway c) where the fructose-6-phosphate and glyceraldehyde-3-phosphate generated in each passage through the pathway are catabolized via glycolysis and the citric acid cycle.
Answer- The Flow of Glucose 6-phosphate depends on the need for NADPH, Ribose 5-phosphate, and ATP. Glucose 6-phosphate can be metabolized in four different metabolic situations
Mode 1- Much more ribose 5-phosphate than NADPH is required For example, rapidly dividing cells need ribose 5-phosphate for the synthesis of nucleotide precursors of DNA. Most of the glucose 6-phosphate is converted into fructose 6-phosphate and glyceraldehyde 3-phosphate by the glycolytic pathway. Transaldolase and transketolase then convert two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate into three molecules of ribose 5- phosphate by a reversal of the reactions as shown below
The net reaction can be represented as follows-
Little or no ribose circulates in the bloodstream, so tissues have to synthesize the ribose they require for nucleotide and nucleic acid synthesis using the pentose phosphate pathway. It is not necessary to have a completely functioning pentose phosphate pathway for a tissue to synthesize ribose 5-phosphate. Muscle has only low activity of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, but, like most other tissues, it is capable of synthesizing ribose 5-phosphate by reversal of the nonoxidative phase of the pentose phosphate pathway utilizing fructose 6-phosphate.Thus Glycolytic and HMP pathway are linked together by Transaldolase and Transketolase. (Figure -1)
Mode 2- The needs for NADPH and ribose 5-phosphate are balanced. The predominant reaction under these conditions is the formation of two molecules of NADPH and one molecule of ribose 5-phosphate from one molecule of glucose 6- phosphate in the oxidative phase of the pentose phosphate pathway. The stoichiometry of mode 2 is-
Mode 3- Much more NADPH than ribose 5-phosphate is required For example, adipose tissue requires a high level of NADPH for the synthesis of fatty acids, In this case, glucose 6-phosphate is completely oxidized to CO2.
Three groups of reactions are active in this situation. First, the oxidative phase of the pentose phosphate pathway forms two molecules of NADPH and one molecule of ribose 5-phosphate. Presuming that 6 molecules of glucose 6- phosphate enter the pathway, the net reaction would be-
Then, ribose 5-phosphate is converted into fructose 6-phosphate and glyceraldehyde 3-phosphate by transketolase and transaldolase. (See the reaction above for mode I)
Finally, glucose 6-phosphate is resynthesized from fructose 6-phosphate and glyceraldehyde 3-phosphate by the gluconeogenic pathway. The stoichiometry of these three sets of reactions is-
Glucose-6-P thus generated can re enter the pathway to produce more of NADPH.
Hence three pathways, HMP, Glycolysis and Gluconeogenesis interact to meet the excess needs of NADPH.
The sum of the mode 3 reactions is-
Thus, the equivalent of glucose 6-phosphate can be completely oxidized to CO2 with the concomitant generation of NADPH. (Figure 1)
Mode 4- Both NADPH and ATP are required. Alternatively, ribose 5-phosphate formed by the oxidative phase of the pentose phosphate pathway can be converted into pyruvate. Fructose 6-phosphate and glyceraldehyde 3-phosphate derived from ribose 5-phosphate enter the glycolytic pathway rather than reverting to glucose 6-phosphate. In this mode, ATP and NADPH are concomitantly generated, and five of the six carbons of glucose 6-phosphate emerge in pyruvate.
Pyruvate formed by these reactions can be oxidized to generate more ATP or it can be used as a building block in a variety of biosynthesis.(Figure -1)
Figure -1- showing the four modes of pentose phosphate pathway
Q.6 – Discuss the metabolic significance of HMP pathway.
Answer-The pentose phosphate pathway is primarily an anabolic pathway that utilizes 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents. However, this pathway does oxidize glucose and under certain conditions can completely oxidize glucose to CO2 and water. The primary functions of this pathway are:
1. To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells.
The examples of reactions requiring NADPH are as follows-
i) De novo fatty acid synthesis
ii) Synthesis of cholesterol
iii) Synthesis of steroids
iv) Synthesis of sphingolipids
v) Synthesis of neurotransmitters
vi) Microsomal desaturation of fatty acids
vii) Conversion of phenyl Alanine to tyrosine
viii) Drug detoxification
ix) Reduction of glutathione
x) Reduction of folate
xi) Reduction of Met Hb to normal Hb
xii) The conversion of ribonucleotides to deoxy ribonucleotides (through the action of ribonucleotide reductase) requires NADPH as the electron source; therefore, any rapidly proliferating cell needs large quantities of NADPH.
xiii) Macrophageal functions
2. To provide the cell with ribose-5-phosphate (R5P) for the synthesis of the nucleotides, nucleic acids, ATP and coenzymes.
3. Although not a significant function of the PPP, it can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary pentoses into glycolytic/gluconeogenic intermediates. Glyceraldehyde-3-P and fructose-6-P formed from 5‐C sugar phosphates may enter Glycolysis for ATP synthesis. The Pentose Phosphate Pathway thus serves as an entry into Glycolysis for both 5‐carbon & 6‐carbon sugars.
4. CO2 produced from this pathway can be utilized for CO2 fixation reactions.
Q. 7- What are the predominant pathways of Glucose utilization in Erythrocytes? Give the significance of each.
Answer- The predominant pathways of carbohydrate metabolism in the red blood cells (RBC) are glycolysis, the PPP and 2,3-bisphosphoglycerate pathway. Glycolysis provides ATP for membrane ion pumps and NADH for re-oxidation of methemoglobin. 2,3-bisphosphoglycerate is required for unloading of O2 to the peripheral tissues.
The PPP supplies the RBC with NADPH to maintain the reduced state of Glutathione (Figure-2)
Figure-2 showing the reduction of glutathione through NADPH formed in HMP pathway
The inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H2O2, that in turn results in a weakening of the cell membrane and concomitant hemolysis. Accumulation of H2O2 also leads to increased rates of oxidation of hemoglobin to methemoglobin that also weakens the cell wall. Glutathione removes peroxides via the action of glutathione peroxidase. The PPP in erythrocytes is essentially the only pathway for these cells to produce NADPH. Any defect in the production of NADPH could, therefore, have profound effects on erythrocyte survival.
Deficiency in the level of activity of glucose-6-phosphate dehydrogenase (G6PDH) is the basis of favism, primaquine (an anti-malarial drug) sensitivity and some other drug-sensitive hemolytic anemias, anemia and jaundice in the newborn, and chronic nonspherocytic hemolytic anemia. In addition, G6PDH deficiencies are associated with resistance to the malarial parasite, Plasmodium falciparum, among individuals of Mediterranean and African descent.The basis for this resistance is the weakening of the red cell membrane (the erythrocyte is the host cell for the parasite) such that it cannot sustain the parasitic life cycle long enough for productive growth.
Q.8- Is it correct to say that excessive carbohydrate ingestion leads to obesity?
Answer- Yes, it is true. Excessive carbohydrate ingestion promotes triglyceride synthesis through following mechanisms-
1) Glycolysis yields pyruvate and hence Acetyl coA which is a precursor for fatty acid biosynthesis.
2) Glycolysis provides glycerol-3-p through dihydroxyacetone phosphate
3) HMP pathway provides NADPH which can be used for reductive biosynthesis.
By all these mechanisms, fatty acids are synthesized and esterified with glycerol to produce triglycerides. The adipose mass increases and the person gets obese.
Q.9- What is the reason that individuals with reduced ability to produce NADPH are at increased risk for specific recurrent infections?
Answer- The highest levels of PPP enzymes (in particular glucose 6-phosphate dehydrogenase) are found in neutrophils and macrophages. These leukocytes are the phagocytic cells of the immune system and they utilize NADPH to generate superoxide radicals from molecular oxygen in a reaction catalyzed by NADPH oxidase. Superoxide anion, in turn, serves to generate other reactive oxygen species (ROS) the kill the phagocytized microorganisms. Following exposure to bacteria and other foreign substances there is a dramatic increase in O2 consumption by phagocytes. This phenomenon is referred to as the oxygen burst.
Because of the need for NADPH in phagocytic cells, by the NADPH oxidase system, any defect in enzymes in this process can result in impaired killing of infectious organisms. Chronic granulomatous disease, CGD is a syndrome that results in individuals harboring defects in the NADPH oxidase system. There are several forms of CGD involving defects in various components of the NADPH oxidase system. Individuals with CGD are at increased risk for specific recurrent infections. The most common are pneumonia, abscesses of the skin, tissues, and organs, suppurative arthritis (invasion of the joints by infectious agent leading to generation of pus), and Osteomyelitis (infection of the bone). The majority of patients with CGD harbor mutations in an X-chromosome gene that encodes a component of the NADPH oxidase system. Given the role of NADPH in the process of phagocytic killing it is clear that individuals with reduced ability to produce NADPH (such as those with G6PDH deficiencies) may also manifest with symptoms of CGD.
Q 10.- What two products of the linear portion of the Pentose Phosphate Pathway have essential roles in anabolic metabolism? What are these roles?
Answer- The linear portion of the pathway carries out oxidation and decarboxylation of glucose-6-phosphate, producing the 5-C sugar ribulose-5-phosphate. NADP+ serves as electron acceptor. Ribulose-5-phosphate is isomerized to Ribose-5-phosphate, that can be used for the synthesis of nucleotides and coenzymes. NADPH the product of linear portion of the pathway is used for reductive biosynthesis like fatty acid synthesis, cholesterol or steroid biosynthesis etc. The reversible or non oxidative phase of HMP pathway is also called cyclic portion of HMP pathway.
Q.11- If a patient has glucose-6-phosphate dehydrogenase deficiency, why are red blood cells lysed while other cells of the body remain intact? What is the biochemical basis for hemolysis? Why doesn’t this disease show up earlier in life? Give a brief account of the drug induced hemolytic anemia in G6 PD deficiency.
A 34- year-old African – American man was seen with fever and shortness of breath. Shortly afterwards he developed pancreatitis and was treated with an antibiotic, clindamycin and primaquine. After four days in to this therapy the onset of hematuria was noted. The patient’s Hb fell from 11.0g/dl to 7.4g/dl, his total bilirubin increased from 1.2 mg/dl to 4.3 mg/dl.
What is the probable diagnosis?
What is the relationship of Primaquin and hemolytic anemia?
Answer- The patient is most probably suffering from Glucose -6- phosphate dehydrogenase deficiency. The hemolysis is primaquine induced which is an oxidant drug. The rise in bilirubin is due to hemolytic jaundice
Reactive oxygen species (ROS) generated in oxidative metabolism inflict damage on all classes of macromolecules and can ultimately lead to cell death. Indeed, ROS are implicated in a number of human diseases. Reduced glutathione (GSH), a tripeptide with a free sulfhydryl group, is required to combat oxidative stress and maintain the normal reduced state in the cell. Oxidized glutathione (GSSG) is reduced by NADPH generated by glucose 6-phosphate dehydrogenase in the pentose phosphate pathway (Figure-2). Indeed, cells with reduced levels of glucose 6-phosphate dehydrogenase are especially sensitive to oxidative stress. This stress is most acute in red blood cells because, lacking mitochondria; they have no alternative means of generating reducing power.
Figure 2- showing the formation of NADPH Through oxidative phase of HMP pathway
Glucose- 6- phosphate dehydrogenase deficiency
Glucose-6-phosphatase dehydrogenase (G6PD) deficiency is the most common disease-producing enzymopathy in humans. Inherited as an X-linked disorder, glucose-6-phosphatase dehydrogenase (G6PD) deficiency affects 400 million people worldwide.
G6PD deficiency is a prime example of a hemolytic anemia due to interaction between an intracorpuscular and an extracorpuscular cause, because in the majority of cases hemolysis is triggered by an exogenous agent. People deficient in glucose-6-phosphatase dehydrogenase (G6PD) are not prescribed oxidative drugs, because their red blood cells undergo rapid hemolysis under this stress. Although in G6PD-deficient subjects there is a decrease in G6PD activity in most tissues, this is less marked than in red cells, and it does not seem to produce symptoms.
1) The vast majority of people with G6PD deficiency remain clinically asymptomatic throughout their lifetime.
2) However, there is an increased risk of developing neonatal jaundice (NNJ) and a risk of developing acute HA when challenged by a number of oxidative agents.
3) The onset can be extremely abrupt, especially with favism in children. The anemia is moderate to extremely severe, usually normocytic normochromic, and due partly to intravascular hemolysis; hence, it is associated with haemogobinemia and hemoglobinuria,
Acute HA can develop as a result of three types of triggers: (1) fava beans, (2) infections, and (3) drugs like- Antimalarials, antibiotics, antipyretics/ analgesics, sulfonamides etc
The presence of pamaquine, a purine glycoside of fava beans, or other nonenzymatic oxidative agents leads to the generation of peroxides, reactive oxygen species that can damage membranes as well as other biomolecules. Peroxides are normally eliminated by glutathione peroxidase with the use of glutathione as a reducing agent.
Moreover, in the absence of the enzyme, the hemoglobin sulfhydryl groups can no longer be maintained in the reduced form and hemoglobin molecules then cross-link with one another to form aggregates called Heinz bodies on cell membranes. A membrane damaged by the Heinz bodies and reactive oxygen species become deformed and the cell is likely to undergo lysis. In the absence of oxidative stress, however, the deficiency is quite benign.
Identification and discontinuation of the precipitating agent is critical in cases of glucose-6-phosphatase dehydrogenase (G6PD) deficiency. Affected individuals are treated with oxygen and bed rest, which may afford symptomatic relief. Prevention of drug-induced hemolysis is possible in most cases by choosing alternative drugs.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!