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Passage 11

In human neonates, the transition from intrauterine to extrauterine environment involves a series of important biochemical adaptations. For instance, the sudden disruption of glucose supply from maternal blood is immediately followed by the activation of metabolic pathways involved in maintenance of glucose homeostasis. This adjustment is especially important because of the central role that glucose plays in brain metabolism.

At birth, the serum glucose concentration of a normal infant (approximately 4.5 mM) is only enough to support normal metabolism for 70 minutes. After the depletion of the blood glucose pool, the initial source for energy replacement is from hepatic glycogen, which is sufficient to support basic metabolism for the first 12 hours. Mobilization of these stores is dependent on a rapid increase in the level of glucagon and activation of its 66 kDa G protein-coupled receptors in the liver. Because infants normally do not receive an adequate supply of glucose from feeding for the first days of life, hepatic glycogen stores are soon exhausted, and the glucose pool must be replenished from non-carbohydrate sources including fatty acids and glucogenic amino acids.

Infants born to mothers with poorly controlled diabetes receive an oversupply of glucose during their fetal development and as a result have problems maintaining neonatal glucose homeostasis. These infants continue to have higher than normal serum insulin concentrations even after depletion of their blood glucose. This particular hormonal environment inhibits the rate of glucose release from the liver, and unless steps are taken to supply exogenous glucose or to alter the endocrine environment so endogenous stores can be mobilized, the infant of a diabetic mother is at great risk for brain injury.

Questions

80. Based on the passage, which metabolic pathways are upregulated during the transition from intrauterine to extrauterine environment?

  1. Glycogenesis followed by glycogenolysis
  2. Glycogenolysis followed by gluconeogenesis
  3. Gluconeogenesis followed by glycogenolysis
  4. Gluconeogenesis followed by glycogenesis

81. After the depletion of hepatic glycogen in newborns, which compounds can be used as precursors to sustain the blood glucose level?

  1. Acetyl-CoA
  2. Lactate
  3. Oxaloacetate
  4. α-Ketoglutarate
  1. I and II only
  2. II and III only
  3. I, II,and IV only
  4. II, III, and IV only

82. Which event is NOT a likely outcome of glucagon binding to its receptor? Increase in:

  1. GDP binding to Gα subunit of the G protein
  2. adenylate cyclase activity
  3. protein kinase A activity
  4. cAMP generation

83. Infusion of which peptide hormone will most likely prevent brain injury in newborn infants exposed to high glucose levels during their fetal development?

  1. Insulin
  2. Epinephrine
  3. Glucagon
  4. Cortisol

84. Activation of which enzyme would support the metabolism of newborn infants during the first 12 hours?

  1. Glycogen phosphatase
  2. Glycogen phosphorylase
  3. Glycogen hydrolase
  4. Glycogen kinase

85. The initial source of energy replacement in the liver of newborn infants is formed by glycosidic bonds between glucose molecules through:

  1. α(1→4) linkage linearly and β(1→6) linkage at branch points.
  2. β(1→6) linkage linearly and α(1→4) linkage at branch points.
  3. α(1→4) linkage linearly and α(1→6) linkage at branch points.
  4. α(1→6) linkage linearly and α(1→4) linkage at branch points.

86. Which of the following compounds is NOT a gluconeogenic precursor or substrate?

  1. Lactate
  2. Glycerol
  3. Oxaloacetate
  4. Phosphogluconate

87. Which type of enzyme removes the chemical groups that are added to proteins by kinases?

  1. Phosphorylase
  2. Cyclase
  3. Phosphatase
  4. Acetylase

88. Under physiological conditions, increased activity of succinyl–CoA synthetase will most likely result in:

  1. increased levels of succinyl–CoA.
  2. increased levels of succinate.
  3. increased levels of GTP.
  1. I only
  2. II only
  3. I and III only
  4. II and III only

89. The pentose phosphate pathway results in the generation of:

  1. NADPH, which is used as a reductive agent in cellular respiratory processes.
  2. NADPH, which is used as an oxidative agent in cellular respiratory processes.
  3. NADH, which is used as a reductive agent in cellular respiratory processes.
  4. NADH, which is used as an oxidative agent in cellular respiratory processes.

90. The Vmax of an enzymatic reaction is altered by which types of inhibitors?

  1. Competitive
  2. Noncompetitive
  3. Uncompetitive
  4. Mixed
  1. I and II only
  2. II and IV only
  3. I, II, and III only
  4. II, III, and IV only

Answers

Question 80 Solution: The correct answer is B. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is B because the passage states that the initial source for energy replacement is from hepatic glycogen indicating upregulation of glycogen breakdown (glycogenolysis). The passage also states that after hepatic glycogen stores are exhausted the glucose pool must be replenished from non-carbohydrate sources implying upregulation of the gluconeogenic pathway. It is a Knowledge of Scientific Concepts and Principles question because you must relate the functions of metabolic pathways with their scientific definitions.
Question 81 Solution: The correct answer is D. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is D because among the listed options, only lactate, oxaloacetate, and α-ketoglutarate are used as starting materials in gluconeogenesis. It is a Knowledge of Scientific Concepts and Principles question because you must recall the compounds that are used in gluconeogenesis.
Question 82 Solution: The correct answer is A. This is correct. This Biochemistry question falls under the content category “Structure and functions of the nervous and endocrine systems and ways in which these systems coordinate the organ systems.” The answer to this question is A because following glucagon binding to its receptor and activation of its coupled G protein, activities of the adenylate cyclase and the protein kinase A, and level of cAMP are all increased. In contrast activation of the G protein promotes the dissociation of bound GDP and its exchange for GTP on the α subunit. It is a Knowledge of Scientific Concepts and Principles question because you must recall the intermediates and various steps in the G-protein signaling.
Question 83 Solution: The correct answer is C. This is correct. This Biology question falls under the content category “Structure and functions of the nervous and endocrine systems and ways in which these systems coordinate the organ systems.” The answer to this question is C because exposure to high glucose levels results in elevated levels of circulating insulin which prevents mobilization of endogenous glucose storage. Infusion of glucagon will result in mobilization of endogenous glucose storage thereby preventing hypoglycemia and brain injury. It is a Scientific Reasoning and Problem Solving question because you must work with scientific models of the endocrine system to answer this question.
Question 84 Solution: The correct answer is B. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is B because the passage states that hepatic glycogen supports the basic metabolism of newborn during the first 12 hours. Glycogen phosphorylase is the enzyme that catalyzes the rate-limiting step in glycogen breakdown (glycogenolysis). It is a Knowledge of Scientific Concepts and Principles question because you must recall the enzymes that participate in metabolic reactions.
Question 85 Solution: The correct answer is C. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is C because the glucose polymer in liver (glycogen) is formed by glycosidic bonds between glucose molecules through α(1→4) linkage linearly and α(1→6) linkage at branch point. It is a Knowledge of Scientific Concepts and Principles question because you must recall the configuration of glycosidic bonds and the carbon atoms involved in formation of glycogen.
Question 86 Solution: The correct answer is D. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is D because while lactate, oxaloacetate and glycerol are gluconeogenic precursors, phosphogluconate is involved in the pentose phosphate pathway and is not a precursor or substrate in gluconeogenesis. It is a Knowledge of Scientific Concepts and Principles question because you must recognize the enzymes that participate in metabolic pathway.
Question 87 Solution: The correct answer is C. This is correct. This Biochemistry question falls under the content category “Structure and function of proteins and their constituent amino acids.” The answer to this question is C because while kinase catalyzes the addition of a phosphate group to a substrate, the removal of the phosphate groups are catalyzed by a phosphatase. It is a Knowledge of Scientific Concepts and Principles question because you must recall enzymes that catalyze opposite reactions.
Question 88 Solution: The correct answer is D. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is D because increased activity of succinyl–CoA synthetase will result in greater levels of the reaction products, succinate and GTP. Succinyl–CoA is the substrate of the reaction and its levels will likely decrease with increased succinyl–CoA function. It is a Knowledge of Scientific Concepts and Principles question because you must recall the substrates and products of the citric acid cycle.
Question 89 Solution: The correct answer is A. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is A because the pentose phosphate pathway results in the generation of NADPH, which is utilized in reductive reactions. It is a Knowledge of Scientific Concepts and Principles question because you must recall the products of the pentose phosphate pathway and the function of NADPH as an reductive agent in cellular respiratory processes.
Question 90 Solution: The correct answer is D. This is correct. This Biochemistry question falls under the content category “Structure and function of proteins and their constituent amino acids.” The answer to this question is D because as observed in 1/V0 versus 1/[S] plot (Lineweaver–Burk plot), competitive inhibitors are the only inhibitors that do not alter the intercept on the y-axis of the plot and therefore the Vmax of the enzymatic reaction. It is a Scientific Reasoning and Problem Solving question because you must work with scientific models of enzyme inhibitions to answer the question.