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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{Rim}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|MainCategory=Biochemistry
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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=A pregnant woman known to have classic galactokinase deficiency is worried she will not be able to breastfeed her baby because she has been on a strict galactose free diet since her birth. The physician comforts her and tells her that she will be having normal breast milk for lactation. The ability of the mammary glands to synthesize milk in the absence of dietary galactose in this patient is due to the activity of which of the following enzymes?
|Prompt=A pregnant woman known to have classic galactokinase deficiency is worried she will not be able to breastfeed her baby because she has been on a strict galactose free diet since her birth. The physician comforts her and tells her that she will be having normal breast milk for lactation. The ability of the mammary glands to synthesize milk in the absence of dietary galactose in this patient is due to the activity of which of the following enzymes?
|Explanation=The patient suffers from galactokinase deficiency, a genetic disease of the galactose metabolism. Patients with galactokinase deficiency are required to have a galactose free diet to avoid the accumulation of galacticol.  Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs in the body. Galactose is produced in the form of UDP galactose through the activity of the epimerase enzyme on UDP glucose.   The synthesis of lactose (milk) in the mammary glands results from the activity of lactose synthase on UDP galactose and glucose.
|Explanation=The patient suffers from galactokinase deficiency, a genetic disease of galactose metabolism. Patients with galactokinase deficiency are required to have a galactose free diet to avoid the accumulation of galacticol in tissues most commonly manifested as early onset cataracts.  Even in the absence of dietary galactose, the endogenous synthesis of galactose can occur. Galactose is produced as UDP-galactose by the activity of the epimerase enzyme on UDP-glucose. The synthesis of the disaccharide lactose (the primary milk sugar) in the mammary glands occurs via lactose synthase with UDP-galactose and UDP-glucose as substrates.
 
 
Learning objective:
Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs in the body through the activity of the epimerase enzyme on UDP glucose.


Reference: Shcwarz V. Disorders of galactose metabolism.  Journal of clinical pathology (1969).  
[[File:Galactose_metabolism_disorders_with_lactose.png|700px]]
|AnswerA=Aldose reductase
|AnswerA=Aldose reductase
 
|AnswerAExp=Aldose reductase catalyzes the conversion of galactose into galacticol and glucose to sorbitol.
 
 
|AnswerAExp=Aldose reductase catalyzes the conversion of galactose to galacticol and glucose to sorbitol.  It is not involved in the synthesis of galactose or lactose.
|AnswerB=Galactokinase
|AnswerB=Galactokinase
|AnswerBExp=Galactokinase catalyzes the conversion of galactose to galactokinase 1 phosphate. The enzyme is already deficient in this patient and it is not directly involved in the synthesis of lactose.
|AnswerBExp=Galactokinase catalyzes the conversion of galactose to galactose-1-phosphate. The enzyme is deficient in this patient, but it is not directly involved in the synthesis of lactose.
|AnswerC=Galactose 1 phosphate uridyl transferase
|AnswerC=Galactose-1-phosphate uridyltransferase
|AnswerCExp=Galactose 1 phosphate uridyl transferase (GALT) catalyzes the reversible conversion of galactose 1 phosphate to UDP galactose. This enzyme is deficient in patients with classic galactosemia.
|AnswerCExp=Galactose-1-phosphate uridyltransferase (GALT) catalyzes the reversible conversion of galactose-1-phosphate into UDP-galactose. This enzyme is deficient in patients with classic galactosemia.
|AnswerD=UDP galactose epimerase
|AnswerD=UDP-galactose-4-epimerase
|AnswerDExp=UDP galactose 4-epimerase (GALE), also known as UDP glucose 4 epimerase, catalyzing the reversible conversion of UDP-galactose to UDP-glucose. GALE tightly binds nicotinamide adenine dinucleotide (NAD+), a co-factor required for catalytic activity.  In the absence of an exogenous source of galactose, GALE catalyzes the formation of UDP galactose necessary for the synthesis of lactose.
|AnswerDExp=UDP-galactose-4-epimerase (GALE), also known as UDP-glucose-4-epimerase, catalyzes the reversible conversion of UDP-galactose to UDP-glucose. In the absence of an exogenous source of galactose, GALE catalyzes the formation of UDP-galactose necessary for the synthesis of lactose.
|AnswerE=UDP glucose pyrophosphate
|AnswerE=UDP-glucose-pyrophosphorylase
|AnswerEExp=UDP glucose pyrophosphate catalyzes the conversion of UDP glucose to glucose 1 phosphate. The enzyme is not directly involved in the synthesis of lactose.
|AnswerEExp=UDP-glucose-pyrophosphorylase catalyzes the conversion of UDP-glucose to glucose-1-phosphate. The enzyme is not directly involved in the synthesis of lactose.
|EducationalObjectives=Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs via the epimerase enzyme using UDP-glucose.
|References=Shcwarz V. Disorders of galactose metabolism.  Journal of clinical pathology (1969).
|RightAnswer=D
|RightAnswer=D
|Approved=No
|WBRKeyword=Galactose, Lactose, Glucose, Galactokinase deficiency, Breastfeeding, Epimerase,
|Approved=Yes
}}
}}

Latest revision as of 02:02, 28 October 2020
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Serge Korjian)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::A pregnant woman known to have classic galactokinase deficiency is worried she will not be able to breastfeed her baby because she has been on a strict galactose free diet since her birth. The physician comforts her and tells her that she will be having normal breast milk for lactation. The ability of the mammary glands to synthesize milk in the absence of dietary galactose in this patient is due to the activity of which of the following enzymes?]]
Answer A AnswerA::Aldose reductase
Answer A Explanation AnswerAExp::Aldose reductase catalyzes the conversion of galactose into galacticol and glucose to sorbitol.
Answer B AnswerB::Galactokinase
Answer B Explanation AnswerBExp::Galactokinase catalyzes the conversion of galactose to galactose-1-phosphate. The enzyme is deficient in this patient, but it is not directly involved in the synthesis of lactose.
Answer C AnswerC::Galactose-1-phosphate uridyltransferase
Answer C Explanation AnswerCExp::Galactose-1-phosphate uridyltransferase (GALT) catalyzes the reversible conversion of galactose-1-phosphate into UDP-galactose. This enzyme is deficient in patients with classic galactosemia.
Answer D AnswerD::UDP-galactose-4-epimerase
Answer D Explanation [[AnswerDExp::UDP-galactose-4-epimerase (GALE), also known as UDP-glucose-4-epimerase, catalyzes the reversible conversion of UDP-galactose to UDP-glucose. In the absence of an exogenous source of galactose, GALE catalyzes the formation of UDP-galactose necessary for the synthesis of lactose.]]
Answer E AnswerE::UDP-glucose-pyrophosphorylase
Answer E Explanation AnswerEExp::UDP-glucose-pyrophosphorylase catalyzes the conversion of UDP-glucose to glucose-1-phosphate. The enzyme is not directly involved in the synthesis of lactose.
Right Answer RightAnswer::D
Explanation [[Explanation::The patient suffers from galactokinase deficiency, a genetic disease of galactose metabolism. Patients with galactokinase deficiency are required to have a galactose free diet to avoid the accumulation of galacticol in tissues most commonly manifested as early onset cataracts. Even in the absence of dietary galactose, the endogenous synthesis of galactose can occur. Galactose is produced as UDP-galactose by the activity of the epimerase enzyme on UDP-glucose. The synthesis of the disaccharide lactose (the primary milk sugar) in the mammary glands occurs via lactose synthase with UDP-galactose and UDP-glucose as substrates.


Educational Objective: Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs via the epimerase enzyme using UDP-glucose.
References: Shcwarz V. Disorders of galactose metabolism. Journal of clinical pathology (1969).]]

Approved Approved::Yes
Keyword WBRKeyword::Galactose, WBRKeyword::Lactose, WBRKeyword::Glucose, WBRKeyword::Galactokinase deficiency, WBRKeyword::Breastfeeding, WBRKeyword::Epimerase
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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