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Free STEP1 Exam Braindumps (page: 67)

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PDF with 845 Questions

There is but a single enzyme-catalyzed reaction in the human body known to generate carbon monoxide (CO) as one of its products. Which of the following enzymes represents the one that catalyzes this CO- producing reaction?

  1. biliverdin reductase
  2. coproporphyrinogen oxidase
  3. heme oxygenase
  4. protoporphyrinogen oxidase
  5. uroporphyrinogen decarboxylase

Answer(s): C

Explanation:

Heme is oxidized, with the heme ring being opened by the endoplasmic reticulum enzyme, heme oxygenase (see below figure). The oxidation step requires heme as a substrate, and any hemin is reduced to heme prior to oxidation by heme oxygenase. The oxidation occurs on a specific carbon producing the linear tetrapyrrole biliverdin, ferric iron , and CO. This is the only
reaction in the body that is known to produce CO. Most of the CO is excreted through the lungs, with the result that the CO content of expired air is a direct measure of the activity of heme oxidase in an individual.
Biliverdin reductase (choice A) catalyzes the conversion of biliverdin to bilirubin as shown in figure.
Coproporphyrinogen oxidase (choice B), protoporphyrinogen oxidase (choice D), and uroporphyrinogen decarboxylase (choice E) are all involved in the synthesis of heme and do not produce CO in the course of catalyzing their reactions.



An infant admitted to the emergency room has been found to be suffering from ammonia intoxication, which was verified by measurement of an elevation of N + in the serum. Treatment of this infant with arginine results in a reduction serum N + and a lessening of the effects of the ammonia toxicity. The ability of arginine to render this effect stems from its role in the synthesis of an allosteric activator of the the urea cycle enzyme, carbamoylphosphate synthetase-I (CPS-I). Which of the following represents this potent allosteric effector of CPS-I?

  1. argininosuccinate
  2. bicarbonate ion
  3. fumarate
  4. N-acetylcysteine
  5. N-acetylglutamate

Answer(s): E

Explanation:

CPS-I is absolutely dependent on the allosteric effector N-acetylglutamate for its activity. This allosteric effector is synthsized by the enzymeN-acetylglutamate synthetase, which is activated by the urea cycle amino acid arginine. None of the other compunds (choices A, B, C, and D) have any effect on CPS-I activity.



Which of the following peptide hormones is released in response to stimulation of pituitary gonadotropes?

  1. ACTH
  2. follicle-stimulating hormone
  3. growth hormone
  4. prolactin
  5. thyroid-stimulating hormone

Answer(s): B

Explanation:

The pituitary gonadotrophs are cells that secrete the gonadotrophic peptide hormones, follicle- stimulating hormone (FSH), and leutinizing hormone (LH). FSH acts on Sertoli cells of the testis inducing androgen- binding protein synthesis, which maintains a high concentration of testosterone in tubules increasing spermatogenesis. In the ovary, FSH stimulates aromatase activity in the granulose cells stimulating ovum maturation and estradiol production. Pituitary orticotrophs are the ACTH- secreting cells (choice A), somatotrophs secrete growth hormone (choice C), lactotrophs secrete prolactin (choice D), and thyrotrophs secrete thyroid-stimulating hormone (choice E).



The rate-limiting step in glycolysis occurs at the step catalyzed by which of the following enzymes?

  1. glyceraldehyde-3-phosphate dehydrogenase
  2. 6-phosphofructo-1 kinase, PFK-1
  3. 6-PFK-2
  4. phosphoglycerate kinase
  5. pyruvate kinase

Answer(s): B

Explanation:

There are three reactions of glycolysis that are thermodynamically irreversible. These are the hexokinase (glucokinase), PFK-1, and PKcatalyzed reactions. Reactions that are essentially irreversible in most metabolic pathways are subject to complex regulatory controls and represent rate-limiting steps in the pathway. The primary site of regulation of glycolysis occurs at the level of the PFK-1-catalyzed step. Hence, this reaction is the rate-limiting step in glycolysis. PFK-1 is subject to allosteric control by numerous compounds. Citrate and ATP inhibit the activity of PFK-1, while AMP and fructose 2, 6- bisphosphate (F2, 6- BP) activate the enzyme. The principal control of PFK-1 activity is exerted by alterations in the level of F2, 6- BP. This compound is synthesized from fructose-6-phosphate by the bifunctional enzyme, PFK-2/fructose- 2, 6-bisphosphatase (PFK-2/F2, 6-BPase). PFK-2 (choice C) contains two catalytic domains, one a kinase and the other a phosphatase, the activities of which are affected by the state of phosphorylation. The phosphatase domain is active when the enzyme is phosphorylated and converts F2, 6-BP back to F6-P, thereby reducing the levels of this powerful activator of PFK-1. Thus, although the activity of PFK-2 will determine the rate of activity of PFK-1, it is itself not the rate-limiting enzyme in glycolysis. Glyceraldehyde- 3-phosphate dehydrogenase (choice A) and PGK (choice D) are not regulated enzymes of glycolysis. PK (choice E) is regulated during glycolysis, but does not constitute a rate-limiting step.






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