SB-BB

Passage 9

Leptin is a 16 kDa peptide hormone that regulates body weight by both suppressing appetite and promoting energy expenditure. Leptin is encoded by the ob gene and is primarily expressed in adipocytes in response to feeding. It circulates at a level of 5 to 15 ng/mL in the plasma of lean subjects and exerts its anorexigenic actions by binding to the leptin receptor (LEPR) located on the surface of hypothalamic neurons.

The LEPR gene produces multiple leptin receptor isoforms (a, b, c, d, e, and f). All isoforms have an extracellular leptin-binding domain, but only the longest form, LEPRb, contains a full-length intracellular domain required for cell signaling. LEPRe is the secreted form that binds circulating leptin and regulates the concentration of free leptin.

Ligand binding causes LEPRb to undergo dimerization and to bind to Janus-activated kinase 2 (JAK2), resulting in JAK2 autophosphorylation at Y570 and its activation. JAK2 also phosphorylates Y1138 within a YXXQ amino acid motif in the cytoplasmic domain of LEPRb. The YXXQ motif is a consensus-binding site for a nuclear factor known as STAT3. Upon leptin-induced phosphorylation of the Y1138, STAT3 is recruited to the LEPRb/JAK2 complex, allowing JAK2 to phosphorylate STAT3 at Y705.

Phosphorylated STAT3 molecules form homodimers, which activate transcription of several target genes, including the gene for a member of the cytokine signaling family (SOCS3). SOCS3 binds to Y985 within the LEPRb, thereby blocking recruitment of STAT3 to the LEPRb/JAK2 complex.

Questions

63. Based on the information in the passage, which protein domain of STAT3 is NOT predicted to play a role in its signaling?

  1. Nuclear localization domain
  2. Signal sequence domain
  3. DNA binding domain
  4. Protein binding domain

64. Which amino acid substitution will most likely result in upregulation of leptin signaling?

  1. Y570F within JAK2
  2. Y705F within STAT3
  3. Y985F within LEPRb
  4. Y1138F within LEPRb

65. Which mechanism restricts the expression of leptin to adipocytes? Only adipocytes contain:

  1. the ob gene.
  2. a promoter for the expression of the ob gene.
  3. enhancers for the expression of the ob gene.
  4. nuclear factors for the expression of the ob gene.

66. Which amino acid substitution within the consensus-binding site for STAT3 is LEAST likely to interfere with STAT3 binding?

  1. Gln to Glu
  2. Gln to Gly
  3. Gln to Asn
  4. Gln to Ala

67. Which amino acids are most likely present at the dimerization interface of STAT3 proteins?

  1. Polar amino acids
  2. Hydrophobic amino acids
  3. Positively charged amino acids
  4. Negatively charged amino acids

68. The six different isoforms of LEPR are produced by using different:

  1. exons of the LEPR gene.
  2. isoforms of the LEPR gene.
  3. promoters of the LEPR gene.
  4. protein cleavage sites of LEPRb.

69. Researchers purified the wild-type and a variant form of a 140 kDa protein (Protein X). The researchers performed a native gel and an SDS gel electrophoresis under non-reducing condition and observed the following electrophoretic patterns.

Which conclusion is NOT valid?

  1. Gel B is a native gel.
  2. Protein X is a tetramer composed of 2 identical heterodimers.
  3. The variant form contains covalently linked subunits.
  4. The largest subunit of Protein X is approximately composed of 120 amino acids.

70. Which methods separate proteins based on their charge?

  1. SDS-PAGE
  2. Isoelectric focusing
  3. Ion-exchange chromatography
  4. Affinity chromatography
  1. I and II only
  2. II and III only
  3. I, II, and III only
  4. II, III, and IV only

71. How many molecules of reduced electron carrier are generated during conversion of α-ketoglutarate to oxaloacetate in the citric acid cycle?

  1. One
  2. Two
  3. Three
  4. Four

72. Which enzyme is used both in gluconeogenesis and glycogenolysis?

  1. Phosphoglucomutase
  2. Glucose 6-phosphatase
  3. Hexokinase
  4. Glucokinase

73. The kinetic parameters of several isoenzymes are shown.

Isoenzyme KM (M) Vmax (mol•min)-1
I 10-6 20
II 10-5 50
III 10-4 10
IV 10-3 60

Assuming the same isoenzyme concentration, which isoenzyme has the highest catalytic efficiency?

  1. I
  2. II
  3. III
  4. IV

Answers

Question 63 Solution: The correct answer is B. 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 B because based on the passage, STAT3 is a nuclear protein, which means that it requires a nuclear localization domain for nuclear translocation and a DNA binding domain for binding to regulatory regions of targeted genes. The passage also states that in addition to forming a homodimer, STAT3 associates with LEPRb/JAK2 complex which infers the presence of a protein binding domain within the STAT3 sequence. In contrast, signal sequence domains are protein domains required for proteins that are directed toward secretory pathways. It is a Scientific Reasoning and Problem Solving question because you must reach a conclusion based on the presence of different protein domains within STAT3 in context of its signaling.
Question 64 Solution: The correct answer is C. 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 C because a Y985F substitution within LEPRb sequence eliminates the binding site for SOCS3, a protein that functions as negative regulator of leptin signaling as it blocks STAT3 recruitment to the LEPRb/JAK2 complex. It is a Scientific Reasoning and Problem Solving question because you must identify the amino acid binding site for the protein that functions as the negative regulator of leptin signaling.
Question 65 Solution: The correct answer is D. This is correct. This Biology question falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is D because among the listed options, nuclear factors are the only elements that vary in different cells and therefore can confer both temporal and spatial regulation of their target genes. It is a Knowledge of Scientific Concepts and Principles question because you must recall that while different cells within an organism contain the same DNA sequence, the cells regulatory trans-acting elements (nuclear factors) are different and vary based on cell type.
Question 66 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 a glutamine to asparagine substitution within the consensus-binding site for STAT3 (YXXQ) is the most conservative as both glutamine and asparagine are polar amino acids. It is a Knowledge of Scientific Concepts and Principles question because you must recall the properties of amino acids side chains.
Question 67 Solution: The correct answer is B. 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 B because the polar and charged amino acids most likely interact with water molecules in cytosol and would not be involved in protein-protein interactions. In contrast the side chains of hydrophobic amino acids are free and most likely participate in dimerization of STAT3. It is a Knowledge of Scientific Concepts and Principles question because you must recall the contribution hydrophobic amino acids in formation of protein oligomers.
Question 68 Solution: The correct answer is A. This is correct. This Biology question falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is A because different isoforms of proteins are expressed from single genes through alternative splicing of exons of the primary transcript. It is a Knowledge of Scientific Concepts and Principles question because you must recall that different isoforms are produced through alternative splicing of exons of the primary transcript.
Question 69 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 the average molecular weight of an amino acid is 110 Da; therefore a protein with a mass of 50 kDa(the large subunit of Protein X, as shown in gel A) is approximately composed of 455 amino acid residues. Gel B is a native gel because Protein X is not denatured into its subunits. Protein X is a tetramer composed of 2 identical heterodimers because while in gel B (native gel) Protein X runs as a single band of approximately 140 kDa, in gel A (non-reducing SDS-PAGE) it is denatured into 2 large subunits of 50 kDa each and 2 small subunits of 20 kDa each. The two subunits of the heterodimers in the variant form are covalently linked because gel A (non-reducing SDS-PAGE) shows the two subunits run as a single band at approximately 70 kDa. It is a Data-based and Statistical Reasoning question because you must evaluate the patterns of the generated bands for each protein in each gel and determine which statement is valid.
Question 70 Solution: The correct answer is B. 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 B because isoelectric focusing separates proteins based on their isoelectric point (the pH at which the net charge of the protein is zero) and ion exchange chromatography separates proteins based on their net charge. In contrast SDS-PAGE separates proteins based on their mass and affinity chromatography separates proteins based on their interactions with specific ligands. It is a Reasoning about the Design and Execution of Research question because you must determine the appropriate methods to separates proteins based on their charge.
Question 71 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 during conversion of α-ketoglutarate to oxaloacetate in the citric acid cycle 2 molecules of NADH and one molecule of FADH2 are generated. It is a Knowledge of Scientific Concepts and Principles question because you must recall steps in the citric acid cycle where reducing equivalents are generated.
Question 72 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 glucose 6-phosphatase catalyzes the final step of both gluconeogenesis and glycogenolysis. It is a Knowledge of Scientific Concepts and Principles question because you must recall the enzymes that participate in each pathway.
Question 73 Solution: The correct answer is A. 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 A because the efficiency of an enzyme is measured by kcat/KM ratio. Since Vmax = kcat[E], at constant concentration, the isoenzyme I has the highest kcat/KM ratio. It is a Data-based and Statistical Reasoning question because you must evaluate the presented data and compare the efficiency of each isoenzyme.