Answered: Which enzyme do you think is defective,…

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

Section: Chapter Questions

Problem 1P

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Question

Your patient is a 2-year-old male named Justin. He is suffering from hypotonia (decreased muscle tone), weakness and growth failure, and is unable to walk. His mother has just brought him into the emergency room from the family beach house, where they have been spending the summer, because he has had a seizure. X-rays indicate that the toddler is suffering from rickets, which is a result of a nutritional deficiency of Vitamin D. But his mother insists that her son’s diet is not Vitamin D-deficient. He drinks three glasses of milk a day, and his diet also includes meat and eggs. A simplified scheme of Vitamin D metabolism is shown to the left. The chemical name of active Vitamin D is 1α, 25-dihydroxycholecalciferol, and it is synthesized via the pathway shown. Catalysts, both in the form of enzymes and ultraviolet light, are required for Vitamin D synthesis. The two main sources of active Vitamin D are diet and sunlight. Food supplemented with “Vitamin D” usually contains cholecalciferol (Vitamin D 3 ). In the liver, dietary cholecalciferol is converted to 25-dihydroxycholecalciferol. Next, in the kidney, that chemical is converted to active Vitamin D. Sunlight is also responsible for producing Vitamin D. The skin contains a precursor, 7-dehydrocholesterol. In the presence of ultraviolet light, which acts as a catalyst, a ring-opening reaction occurs, which is followed by the spontaneous conversion of this intermediate to Vitamin D 3 , and this follows the dietary Vitamin D 3 pathway to become active Vitamin D. Active Vitamin D is a steroid-like compound that acts in combination with other hormones to increase the concentration of Ca 2+ via a variety of mechanisms, one of which includes increasing the intestinal absorption of dietary calcium (intestinal absorption of dietary phosphate, a calcium counter-ion, also increases as a result). Calcium ions are required to form hydroxyapatite Ca5 (PO 4 ) 3 OH, the main mineral constituent of bone. You decide to carry out further analysis and take a sample of Justin’s blood. The laboratory results are below:

	patient	Normal Range
Serum Calcium, mg/dL	5.1	8.7-10.1
Serum Phosphorous, mg/dL	4.2	2.4-4.3
Serum 1alpha, 25-dihydroxycholecalciferol, pg/mL	13	20-76
Serum 25-hydroxycholecalciferol, ng/mL	48	10-55

After obtaining the results from the laboratory, you suspect that your patient might have a defective enzyme in the Vitamin D synthetic pathway. Which enzyme do you think is defective, and why?