Which enzyme do you think is defective, and why?
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
Related questions
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?

Transcribed Image Text:Introduction: 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.
HO
HO
HO
CH
uv radiation
(skin)
CH
spontaneous
CH 3
CH 3
Ca²+
(aq)
(intestine)
CH 3
CH3
CH 3
CH 3
7-dehydrocholesterol
(diet)
CH 3
CH₂
CH 3
-CH 3
cholecalciferol
(Vitamin D3)
•CH 3
25-hydroxylase
(liver)
0₂
Ca²+
(82)
(blood)
HO
1a-hydroxylase
(kidney)
HO
CH
CH3
CH₂
CH3
CH 3
CH₂
OH
Cas (PO )3OH)
(bone)
-CH 3
CH 3 OH
25-hydroxy-
cholecalcife rol
- CH 3
CH 3 OH
1a,25-dihydroxy-
chole calcife rol
(Active Vitamin D)
A simplified scheme of
Vitamin D metabolism is
shown to the left. The
chemical name of active
Vitamin D is la, 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 D3). 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

Transcribed Image Text: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 D3, and this
follows the dietary Vitamin D3 pathway to become active Vitamin D.
2+
Active Vitamin D is a steroid-like compound that acts in combination with other
hormones to increase the concentration of Ca²+ 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(PO4)3OH, 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
Serum Calcium, mg/dL
5.1
Serum Phosphorous, mg/dL
4.2
Serum 1a, 25-dihydroxycholecalciferol, pg/mL 13
Serum 25-hydroxycholecalciferol, ng/mL
48
Normal Range
8.7-10.1
2.4-4.3
20-76
10-55
1. 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?
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