Metabolism MCQ - IIT JAM MCQ

The first irreversible reaction unique to the glycolytic pathway, the committed step, is the phosphorylation of fructose 6-phosphate to fructose 1, 6-bisphosphate. Thus, it is highly appropriate for phosphofructokinase to be the primary control site in glycolysis. In general, the enzyme catalyzing the committed step in a metabolic sequence is the most important control element in the pathway. A committed step means, it has high free energy and remains essentially irreversible.

The two shuttles operating between mitochondria and cytosol are malate shuttle and glycerol 3 phosphate shuttle. Among these two, malate shuttle generate more energy per glycolytic cycle. Glycolate and glutamate shuttles do not exist. In glycerol 3 phosphate shuttle, Dihydroxyacetone phosphate (DHAP) is converted to glycerol 3 phosphate by oxidizing NADH to NAD+ in cytosol. However, when glycerol 3 phosphate is converted back to DHAP by mitochondrial glycerol 3 phosphate dehydrogenase, FAD is reduced to FADH instead of NADH and we know, that one molecule of NADH gives 2.5 ATP and 1 molecule of FADH gives 1.5 ATP. Therefore, via glycerol 3 phosphate shuttle, 1 molecule of NADH produced from glycolysis in cytoplasm is transported to mitochondria in the form of FADH which gives 1.5 ATP unlike NADH which gives 2.5 ATP. Therefore via glycerol 3 phosphate shuttle, less number of ATP are produced from glycolysis compared to malate aspartate shuttle. In malate aspartate shuttle, NADH produced in cytoplasm from glycolysis is transported in the mitochondria for oxidation in the form of NADH only.

Histidine is semi-essential as it is required in children and arginine as it is required due to most of the consumption in urea cycle.

Amino acids which can be converted to glucose are called glucogenic amino acids and amino acids which cannot be converted to glucose are called ketogenic amino acids. Aspartate on deamination can produce oxaloacetic acid, while glutamate on deamination can produce alpha-ketogluterate, both of which are TCA cycle intermediates and can be converted to glucose. Lysine is ketogenic in nature. Lysine and leucine are the only amino acids which are exclusively ketogenic in nature.

Alanine transaminase and aspartate transaminase (sGPT and sGOT) are commonly used serum biomarkers for hepatic health.
SGPT (Alanine amino transferase): There is high concentration of SGPT in the liver. This enzyme is present in the cytosolic and mitochondrial forms in the liver. SGPT enzyme is specific for the liver cell necrosis. In case of liver cell necrosis, first cytosolic SGPT is released and then mitochondrial SGPT is released into circulation. There is always a low level of SGPT in the blood under normal conditions. However, the levels of SGPT increase in case of liver disease.      
SGOT (Asparate amino transferase) : This enzyme is found in high concentration in liver, heart and skeletal muscles. SGOT is released into circulation in liver diseases. The amount of SGOT is directly related to the number of cells injured.

Glycine, glutamine and aspartate provide one or more atoms to the nucleotide backbone (especially purines), however, histidine does not contribute to nucleotide biosynthesis.

One molecule of glucose gives 2 molecules of pyruvate and 2 molecules of pyruvate are further converted into 2 molecules of acetyl CoA. For every molecule of acetyl CoA that enters TCA cycle, 10 ATP are produced because each TCA cycle produces 3NADH, 1FADH and 1GTP [1 NADH = 2.5 ATP and 1FADH = 1.5 ATP]. So 2 molecules of acetyl CoA [produced from one molecule of glucose] will give 10 ATP × 2 = 20 ATP through citric acid cycle.    

Statement a, b, d are incorrect.
Option (a), is incorrect as in eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion. In prokaryotic cells, such as bacteria, which lack mitochondria, the citric acid cycle reaction sequence is performed in the cytosol.
Option (b) is incorrect because 10 ATP’s are produced from one TCA cycle.
Option (d) is incorrect as TCA involves conversion of malate to OAA.

The number of ATP produced from one molecule of fatty acid depends upon the number of carbon atoms present in the fatty acids.
Palmitate is a 16 carbon fatty acid. One molecule of palmitate gives 8 molecules of acetyl CoA and 8 molecules of acetyl CoA are produced from 7 β -oxidation cycles. In each β -oxidation cycle, one molecule
each of NADH and FADH is produced. Therefore 4 ATP are produced per β -oxidation cycle as 1 NADH gives 2.5 ATP and 1 FADH gives 1.5 ATP.
One molecule of acetyl CoA produces 10 ATP per TCA cycle. Therefore 8 molecules of acetyl CoA will produces = 10 x 8 = 80 ATP.

In peroxisomes, beta oxidation do not produce any ATP due to the fact that FADH is not coupled to ATP generation.

All carboxylases use biotin as coenzyme. All dehydrogenases use NAD, FAD or NADP as coenzyme.
For example: Malate dehydrogenase uses NAD as coenzyme, whereas succinate dehydrogenase uses FAD as coenzyme.
Transaminases use PLP as coenzyme and Transketolases use TPP as coenzyme.    

Carbamoyl phosphate is the first compound that is formed from the reaction of bicarbonate and ammonia in mitochondria by the enzyme carbamoyl phosphate synthase 1. Another isoform of this enzyme is present in the cytosol that forms the same compound which is involved in biosynthesis of nucleotides (pyrimidine biosynthesis).

Carbon 1 or the glucose is retained in 50% of the pyruvate molecules. While C3 and C4 make carbon dioxide. Therefore only 50% of the pyruvate will have radioactivity and it will also be observed in acetyl CoA, but not in carbon dioxide.

Glycine is needed on in the synthesis of pyrimidine, while aspartate and glutamate is needed for both.

It occur in cytoplasm and generate pentose sugar like Xylose-5-phosphate and Ribose-5-phospliate.It convert as shown in figure from hexose to those, tetrose. pentose, hexose and heptose sugar are generated as intermediate products in pentose phosphate pathway.

Urea cycle occurs in both mitochondria and cytosol. The urea cycle is closely linked to the citric acid cycle as it derives one of its nitrogens through transamination of oxaloacetate to form aspartate and returns fumarate to the citric acid cycle.

Statement (a) is correct.
Acetyl CoA is converted into malonyl CoA by acetyl CoA carboxylase enzyme during fatty acid biosynthesis. Malonyl CoA so formed inhibits carnitine acyl transferase I (CAT-I), thereby preventing transport of fatty acids into mitochondria, hence inhibiting oxidation of fatty acids.
Statement (b) is correct. Net gain of two ATP occurs if glucose is converted into ethanol because NADH formed during the process of glycolysis is used in converting pyruvate to ethanol.
Statement (c) is incorrect. Pyruvate is not converted directly into phosphoenol pyruvate. Pyruvate is first converted into oxaloacetate by pyruvate carboxylase. Oxaloacetate is then converted into phosphoenolpyruvate [PEP] by PEP carboxykinase.
Statement (d) is incorrect
Gluconeogenesis occurs both in liver and kidney. Glucose 6 phosphatase enzyme of gluconeogenesis is found on the lumenal side of endoplasmic reticulum of both hepatocytes and renal / kidney cells.    

For one molecule of glucose, there is requirement of two ATP molecules in preparatory phase, therefore for 12 molecules of glucose, 24 ATP will be required.

C20 fatty acid will produce 10 molecules of acetyl CoA after undergoing 9 oxidation cycle.    
No. of acetyl CoA = 20/2 = 10
10 ATP are produced for every acetyl CoA that enters citric acid cycle. Therefore, 10 acetyl CoA will produce 10×10 = 100 ATP
No. of  oxidation cycles = 9
1  oxidation cycle produces 1 NADH and 1 FADH each, therefore, the number of NADH and FADH produced from 9 oxidation cycles are as follows:-      
NADH = 9
FADH = 9
Net ATP = 10 x10 (10 ATP per Acetyl CoA) + 9 x 2.5 (2.5 ATP per NADH) + 9 x 1.5 (1.5 ATP per FADH molecule) = 136 ATP

Each mole of acetyl CoA undergoes a cycle of citric acid pathway and during each citric acid pathway, ten ATPs are generated, so the number of ATP molecules per mole of acetyl CoA will be 10  and for 2 moles of acetyl CoA, the number of moles of ATP would be 20

If NADH is not utilized, only 4 ATPs will be produced per F6P molecule but 1 ATP will be utilized (unlike glucose where 2 are utilized) in preparatory phase. So net ATP production will be 4–1 = 3.