a)-Carboxylated to phosphoenol pyruvate for entry in to gluconeogenesis
b)- Oxidatively decarboxylated to acetyl co A for entry in to ketogenesis
c)- Reduced to lactate for entry in to gluconeogenesis
d)- Oxidatively decarboxylated to acetyl co A for oxidation in TCA cycle
e)- Carboxylated to oxaloacetate for entry in to gluconeogenesis
Answer- The right answer is- e) – Mitochondrial pyruvate is carboxylated to oxaloacetate for entry in to gluconeogenesis. This is the first step of gluconeogenesis. Gluconeogenesis is the process of converting noncarbohydrate precursors to glucose or glycogen. The major substrates are the glucogenic amino acids, and lactate, glycerol, and propionate.
Fatty acid oxidation takes place during conditions of prolonged fasting /starvation or in diabetes mellitus. Under these conditions Insulin to glucagon ratio is reversed with more circulating concentration of Glucagon than insulin. As a result, a state of catabolism is there with more lipolysis, glycogenolysis and protein catabolism. After 12–18 hours of fasting, liver glycogen is almost totally depleted. Gluconeogenesis meets the needs of the body for glucose when sufficient carbohydrate is not available from the diet or glycogen reserves. A supply of glucose is necessary especially for the nervous system and erythrocytes.
The option a) – Carboxylation of pyruvate to phosphoenol pyruvate for entry to gluconeogenesis is not correct. Pyruvate is first carboxylated to form oxaloacetate and then it is decarboxylated and phosphorylated to form phosphoenol pyruvate that gets converted to 2-phosphoglycetare to gain entry in to the pathway of gluconeogenesis.
Pyruvate can be oxidatively decarboxylated to form acetyl co A but under conditions of starvation or diabetes mellitus, the equilibrium is more towards formation of glucose. PDH complex remains inhibited
in insulin deficiency and by acetyl co A, that is the product of fatty acid oxidation. TCA is also in a state of suppression due to utilization of oxaloacetate for glucose production.
Pyruvate can be reduced to lactate, but for entry in to the pathway of gluconeogenesis, lactate is not the direct substrate, it has to be converted to pyruvate that is channeled to the pathway of gluconeogenesis.
Thus out of all the most suited option is e) – Mitochondrial pyruvate is carboxylated to oxaloacetate for entry in to gluconeogenesis.
Q.2- An 8-year old boy, a known diabetic (type 1 DM), has been brought to emergency room in a state of coma. His breathing is rapid and deep, and his breath has a fruity odor. His blood glucose is 480 mg/dL. The attending physician has administered IV fluids, insulin, and potassium chloride. A rapid effect of insulin in this situation is to stimulate
a) gluconeogenesis in liver
b) fatty acid release from adipose
c) glucose transport in muscle
d) ketone utilization in the brain
e) glycogenolysis in the liver
Answer- The right answer is- c) Glucose transport in muscle.
The child is suffering from diabetic ketoacidosis, the commonest complication of Type 1 diabetes mellitus. Fruity odor of breath is due to the presence of acetone, one of the ketone bodies (the other two are acetoacetate and beta hydroxy butyrate). Acetone is excreted through lungs. High blood glucose is due to non utilization or extra synthesis of glucose in the presence of reversed insulin to glucagon ratio.
In the conditions of non utilization of glucose, fats are alternatively oxidized to provide energy. The extra Acetyl co A produced by fatty acid oxidation is diverted to the pathway of ketogenesis.
As regards other options-
Insulin does not promote gluconeogenesis, rather it inhibits it.
Similarly fatty acid release from adipose tissue (adipolysis) is an action of glucagon and catecholamines, insulin inhibits this action also.
Ketone utilization in brain is also not the correct option. By promoting glucose utilization, insulin inhibits ketosis; in fact ketosis occurs only when glucose is not available for utilization as in starvation, low carbohydrate/high fat diet, or diabetes mellitus.
Glycogenolysis is also not the correct answer. Insulin promotes glycogenesis, it is an anabolic hormone, and it prevents all the catabolic processes including glycogenolysis.
In diabetic ketoacidosis, Insulin promotes glucose uptake through GLUT4 transporters in skeletal, cardiac muscle and adipose tissue. It also promotes glucose utilization by stimulating the enzymes of pathways of glucose utilization.
IV fluids are given to treat dehydration as DKA is mostly associated with polyuria. Potassium chloride is given to maintain potassium balance.
For details of DKA follow the link
Q.3-A 10-month-old child is being evaluated for the underlying cause of a hemolytic anemia. The oxygen dissociation curve for hemoglobin in his erythrocytes is compared with the curve obtained with normal cells. The results are shown in the image. A deficiency of which of the following enzymes is most likely to account for the hemolytic anemia in this patient?
b) Glucose-6-P dehydrogenase
c) Pyruvate carboxylase
d) Glutathione reductase
e) Pyruvate Kinase
Answer- The right answer is -e) – Pyruvate kinase.
There is right ward shift of oxy Hb dissociation curve, showing more unloading of oxygen that might be due to more concentration of 2 3 BPG. Out of the given options, it is pyruvate kinase deficiency that can cause more formation of 2, 3 BPG.
Glucokinase deficiency can cause hemolytic anemia, but there is decreased formation of 2, 3 BPG also, there cannot be right ward shit of oxy Hb dissociation curve. Glucokinase deficiency or impaired activity is found in type 2 DM.
Glucose-6-P dehydrogenase deficiency causes hemolytic anemia but that is due to met Hb formation as a result of impaired activity of glutathione reductase caused because of reduced availability of NADPH.
Pyruvate carboxylase deficiency causes impaired formation of oxaloacetate, it can cause hypoglycemia. This enzyme is mitochondrial, hence is absent in red blood cells. Thus this is not the right option.
Impaired activity of glutathione reductase is associated with hemolytic anemia, but 2, 3 BPG concentration is not affected.
Hence out of all the options, Pyruvate kinase deficiency is the most suited option. Pyruvate kinase lies at the end of the glycolytic pathway in RBCs followed only by lactate dehydrogenase. Pyruvate kinase activity is critical for the pathway and therefore critical for energy production. If ATP is not produced in amounts sufficient to meet the energy demand, then those functions are compromised. Energy is required to maintain the Na+/K+ balance within the RBC and to maintain the flexible discoid shape of the cell. In the absence of sufficient pyruvate kinase activity and therefore ATP, the ionic balance fails, and the membrane becomes misshapen. Cells reflecting pyruvate kinase insufficiency rather than a change in membrane composition are removed from the circulation by the macrophages of the spleen. This results in an increased number of circulating reticulocytes and possibly bone marrow hyperplasia, which is a biological response to lowered RBC count as a result of hemolysis of erythrocytes. Important intermediates proximal to the PK defect influence erythrocyte function. Two- to 3-fold increases of 2, 3-bisphosphoglycerate levels result in a significant rightward shift in the hemoglobin-oxygen dissociation curve. Physiologically, the hemoglobin of affected individuals has an increased capacity to release oxygen into the tissues, thereby enhancing oxygen delivery.
Q.4- A 54- year-old man with Type 1 diabetes is referred to an ophthalmologist for evaluation of developing cataract. Blood Biochemistry results are shown below-
Fasting blood glucose 198 mg/dl
Hemoglobin A 15 gm/dl
Hemoglobin A 1c 10% of total Hb
Urine ketones Positive
Urine glucose Positive
Which of the following enzymes is most strongly associated with cataract formation on this patient ?
b) Aldose reductase
d) Galactose-1-P uridyl transferase
Answer- The right answer is -b) Aldolase reductase. The patient is suffering from uncontrolled diabetes mellitus. Cataract, ketosis and glycosuria, the complications of diabetes mellitus are evident from the history and blood biochemistry results.
Aldose reductase is responsible for reduction of glucose to sorbitol. Sorbitol accumulation is responsible for most of the diabetic complications including cataract,
Diabetic cataract is due to sorbitol accumulation as well as due to non enzymatic glycation of lens proteins.
Galactokinase is not the right option. Galactokinase is the enzyme for phosphorylation of galactose. Galactokinase deficiency in galactosemia can lead to accumulation of excess galactose that can also get converted to galacitol by aldolase reductase. Galacitol accumulation is responsible for premature cataract in galactosemia.
Glucokinase is the enzyme for phosphorylation of glucose (1st step of glycolysis); it is not associated with cataract in diabetes mellitus.
Galactose-1-P uridyl transferase is the enzyme for conversion of galactose-1-P to UDP galactose. Deficiency is responsible for classical galactosemia. It has no role in diabetic cataract.
Aldolase enzyme has two isoenzymes. Aldolase A is responsible for cleavage of Fr-1, 6 Bisphosphate to Glyceraldehyde-3-P and Dihydroxy acetone- phosphate, whereas Aldolase B is responsible for cleavage of Fructose-1-P to form glyceraldehyde and dihydroxy acetone phosphate in fructose metabolism.
Both of these two enzymes are not responsible for Diabetic cataract.
Q.5- A physician is examining a patient who exhibits fasting hypoglycemia and needs to decide between a carnitine deficiency and a carnitine acyl transferase 2 deficiency as the possible cause. A blood test for confirmation of diagnosis has been ordered. The levels of which one of the following would help in confirmation of diagnosis?
b) Ketone bodies
Answer- The right answer is d) – Acyl carnitine.
Activated long-chain fatty acids are transported across the mitochondrial membrane by conjugating them to carnitine, a zwitterionic alcohol, ß-hydroxy-Υ-trimethyl ammonium butyrate- (CH3)3N+—CH2—CH(OH)—CH2—COO–,
The acyl group is transferred to the hydroxyl group of carnitine to form acyl carnitine. This reaction is catalyzed by carnitine acyl transferase I. Acyl carnitine is then shuttled across the inner mitochondrial membrane by a translocase. The acyl group is transferred back to CoA on the matrix side of the membrane. This reaction, is catalyzed by carnitine acyl transferase II. Finally, the translocase returns carnitine to the cytosolic side in exchange for an incoming acyl carnitine
With a carnitine deficiency, fatty acids cannot be added to carnitine, and acyl-carnitine would not be synthesized. Hence the Acyl carnitine level would be low.
With a carnitine acyl-transferase II deficiency, the fatty acids are added to carnitine, but the acyl-carnitine cannot release the acyl group within the mitochondria. This will lead to an accumulation of acyl carnitine, which will lead to an accumulation in the circulation.
Hence acyl carnitine level would help to differentiate between carnitine deficiency and Carnitine acyl transferase II deficiency.
The end result of either deficiency is a lack of fatty acid oxidation, such that ketone body levels would be minimal under both conditions, and blood glucose levels would also be similar in either condition.
Hypo glycemia and low ketone body level are characteristic of carnitine as well as associated enzyme deficiencies but their levels cannot discriminate between either of these two conditions.
Insulin release is not affected by either deficiency, and carnitine levels, normally low, would not be significantly modified in either deficiency.
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