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An individu with chronic hypoglycemia was suspected of having a defect in one of the enzymes unique to gluconeogenesis. To identify the defective enzyme, tissue samples from a normal liver were compared to samples from the patient's liver biopsy, using a biochemical assay that measures glucose production from glycerol or malate. It was found that incubation with glycerol produced normal amounts of glucose in both the control and biopsied liver samples; however, incubation with malate did not lead to glucose production in the liver biopsy, even though it did lead to glucose production in the control liver sample. Based on these observations, which of the 4 unique gluconeogenesis enzymes is most likely defective in the individual? Consider each enzyme and explain your choice, including why you ruled out enzymes that you did not choose. To answer this question, review how glycerol enters gluconeogenesis (figure 9.48) and how citrate cycle metabolites are transported in and out of the mitochondria (figrue 10.39). One Paragraph

An individu with chronic hypoglycemia was suspected of having a defect in one of the enzymes unique to gluconeogenesis. To identify the defective enzyme, tissue samples from a normal liver were compared to samples from the patient's liver biopsy, using a biochemical assay that measures glucose production from glycerol or malate. It was found that incubation with glycerol produced normal amounts of glucose in both the control and biopsied liver samples; however, incubation with malate did not lead to glucose production in the liver biopsy, even though it did lead to glucose production in the control liver sample. Based on these observations, which of the 4 unique gluconeogenesis enzymes is most likely defective in the individual? Consider each enzyme and explain your choice, including why you ruled out enzymes that you did not choose. To answer this question, review how glycerol enters gluconeogenesis (figure 9.48) and how citrate cycle metabolites are transported in and out of the mitochondria (figrue 10.39). One Paragraph

Biochemistry
Biochemistry
9th Edition
ISBN:9781319114671
Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.
Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.
Chapter1: Biochemistry: An Evolving Science
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An individu with chronic hypoglycemia was suspected of having a defect in one of the enzymes unique to gluconeogenesis. To identify the defective enzyme, tissue samples from a normal liver were compared to samples from the patient's liver biopsy, using a biochemical assay that measures glucose production from glycerol or malate. It was found that incubation with glycerol produced normal amounts of glucose in both the control and biopsied liver samples; however, incubation with malate did not lead to glucose production in the liver biopsy, even though it did lead to glucose production in the control liver sample. Based on these observations, which of the 4 unique gluconeogenesis enzymes is most likely defective in the individual? Consider each enzyme and explain your choice, including why you ruled out enzymes that you did not choose. To answer this question, review how glycerol enters gluconeogenesis (figure 9.48) and how citrate cycle metabolites are transported in and out of the mitochondria (figrue 10.39). One Paragraph Figure 9.48: Sucrose Sucrase Fructose ATP Fructokinase (liver) ADP 4 Fructose-1-P ATP Hexokinase (muscle) ADP Fructose-1-P (aldolase B) Dihydroxyacetone-P Glyceraldehyde ATP ADP Maltose ✓ Glucose Hexokinase Maltase Glucose-6-P PFK-1 Fructose-6-P ATP ADP 2 ADP 2 ATP+ Fructose-1,6-BP ATP ADP Glyceraldehyde-3-P+ I 1,3-Bisphosphoglycerate Pyruvate 2 ADP →2 ATP Acetyl-CoA CO₂ Dihydroxyacetone-P Lactose ATP ADP Galactose. Lactase Galactokinase Galactose-1-P UDP-glucose + UDP-galactose + Glucose-1-P Glycerol-3-P dehydrogenase T -Glycerol-3-P FADH₂ FAD ADP Glycerol kinase ATP Glycerol -Fatty acid -Fatty acid+ -Fatty acid Lipase 3 Lipase 2 Lipase 1 Triglycerides Figure 9.48 Metabolism of fructose, galactose, and glycerol involves feeder pathways t t generate the glycolytic intermediates glucose-6-P, fructose-6-P, dihydroxyacetone-P, and glyceraldehyde-3-P. Increased substrate availability from alternative carbohydrate sources stimulates metabolic flux through the pathway. Red arrows indicate positions where metabolites feed into the glycolytic pathway, which is indicated by blue arrows. Glycolytic pathway metabolites produced from other pathways are boxed in blue. Dietary sources of carbohydrates and triglycerides are boxed in yellow green, which can be broken down into simpler molecules (boxed in green), which can eventually feed into the glycolytic pathway. Note that the 2 net ATP shown in the ATP earnings stage from glyceraldehyde-3-P to pyruvate refers to the metabolism of hexose sugars, whereas only 1 net ATP can be produced from the C3 metabolite glycerol. Glycerol-3-P = glycerol-3-phosphate; glycerol-3-P dehydrogenase = glycerol-3-phosphate dehydrogenase. figure 10.39: Cytosol Malate Amino acids Mitochondrial matrix Mitochondrial inner membrane Malate Glucose Malate Oxaloacetate Acetyl-CoA Oxaloacetate Citrate Cycle Fatty acids Acetyl-CoA Citrate Succinyl-CoA a-ketoglutarate Heme biosynthesis Cholesterol Citrate Citrate ww Amino acids
Transcribed Image Text:An individu with chronic hypoglycemia was suspected of having a defect in one of the enzymes unique to gluconeogenesis. To identify the defective enzyme, tissue samples from a normal liver were compared to samples from the patient's liver biopsy, using a biochemical assay that measures glucose production from glycerol or malate. It was found that incubation with glycerol produced normal amounts of glucose in both the control and biopsied liver samples; however, incubation with malate did not lead to glucose production in the liver biopsy, even though it did lead to glucose production in the control liver sample. Based on these observations, which of the 4 unique gluconeogenesis enzymes is most likely defective in the individual? Consider each enzyme and explain your choice, including why you ruled out enzymes that you did not choose. To answer this question, review how glycerol enters gluconeogenesis (figure 9.48) and how citrate cycle metabolites are transported in and out of the mitochondria (figrue 10.39). One Paragraph Figure 9.48: Sucrose Sucrase Fructose ATP Fructokinase (liver) ADP 4 Fructose-1-P ATP Hexokinase (muscle) ADP Fructose-1-P (aldolase B) Dihydroxyacetone-P Glyceraldehyde ATP ADP Maltose ✓ Glucose Hexokinase Maltase Glucose-6-P PFK-1 Fructose-6-P ATP ADP 2 ADP 2 ATP+ Fructose-1,6-BP ATP ADP Glyceraldehyde-3-P+ I 1,3-Bisphosphoglycerate Pyruvate 2 ADP →2 ATP Acetyl-CoA CO₂ Dihydroxyacetone-P Lactose ATP ADP Galactose. Lactase Galactokinase Galactose-1-P UDP-glucose + UDP-galactose + Glucose-1-P Glycerol-3-P dehydrogenase T -Glycerol-3-P FADH₂ FAD ADP Glycerol kinase ATP Glycerol -Fatty acid -Fatty acid+ -Fatty acid Lipase 3 Lipase 2 Lipase 1 Triglycerides Figure 9.48 Metabolism of fructose, galactose, and glycerol involves feeder pathways t t generate the glycolytic intermediates glucose-6-P, fructose-6-P, dihydroxyacetone-P, and glyceraldehyde-3-P. Increased substrate availability from alternative carbohydrate sources stimulates metabolic flux through the pathway. Red arrows indicate positions where metabolites feed into the glycolytic pathway, which is indicated by blue arrows. Glycolytic pathway metabolites produced from other pathways are boxed in blue. Dietary sources of carbohydrates and triglycerides are boxed in yellow green, which can be broken down into simpler molecules (boxed in green), which can eventually feed into the glycolytic pathway. Note that the 2 net ATP shown in the ATP earnings stage from glyceraldehyde-3-P to pyruvate refers to the metabolism of hexose sugars, whereas only 1 net ATP can be produced from the C3 metabolite glycerol. Glycerol-3-P = glycerol-3-phosphate; glycerol-3-P dehydrogenase = glycerol-3-phosphate dehydrogenase. figure 10.39: Cytosol Malate Amino acids Mitochondrial matrix Mitochondrial inner membrane Malate Glucose Malate Oxaloacetate Acetyl-CoA Oxaloacetate Citrate Cycle Fatty acids Acetyl-CoA Citrate Succinyl-CoA a-ketoglutarate Heme biosynthesis Cholesterol Citrate Citrate ww Amino acids
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