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Galactose Metabolism


MCAT Biochemistry

Galactose metabolism involves the conversion of galactose into a usable form by the body, ultimately entering the pathways of glycolysis and glycogenesis. Galactose is commonly known as "milk sugar" as it makes up half of lactose, found in dairy products. The metabolism of galactose involves three main enzymes: galactokinase, galactose-1-phosphate uridyltransferase (GALT), and galactose 4-epimerase (GALE). Galactokinase phosphorylates galactose to form galactose-1-phosphate, GALT transfers a phosphate for a UDP from UDP-glucose, and GALE recycles UDP-galactose back to UDP-glucose.

Galactosemia is a disorder resulting from a deficiency in any of the three main enzymes involved in galactose metabolism. The three forms of galactosemia include classic galactosemia (most severe, caused by GALT deficiency), galactokinase deficiency (less severe), and GALE deficiency (least severe, often asymptomatic). Classic galactosemia presents with various symptoms such as poor feeding, liver or renal dysfunction, and cataracts due to galactitol deposition in the lens. Treatment involves the elimination of lactose and galactose from the diet.

Lesson Outline

<ul> <li>Introduction to Galactose Metabolism</li> <ul> <li>Galactose as half of lactose, the other half being glucose</li> <li>Galactose can undergo glycolysis and be used to make glycogen</li> </ul> <li>Galactose Metabolism Pathway</li> <ul> <li>First step: Galactokinase transfers a phosphate group from ATP to galactose, producing galactose-1-phosphate</li> <li>Second step: Galactose-1-phosphate uridyltransferase (GALT) turns galactose-1-phosphate and UDP-glucose to glucose-1-phosphate and UDP-galactose</li> <li>Galactose 4-epimerase (GALE) transforms UDP-galactose back into UDP-glucose</li> </ul> <li>Glucose-1-phosphate (G1P) can be used to make glycogen or be turned into glucose-6-phosphate (G6P) for glycolysis</li> <li>Galactitol</li> <ul> <li>Lactitol is present in sweeteners and sugar-free foods</li> <li>Can cause bloating, flatus, and osmotic diarrhea</li> <li>Derived from galactose, reduced to galactitol by aldose reductase</li> </ul> <li>Disorders of Galactose Metabolism</li> <ul> <li>Galactosemia: Disorder caused by deficiency of galactokinase, GALT, or GALE</li> <ul> <li>Classic galactosemia: Most common and severe, caused by complete absence of GALT</li> <li>Galactokinase deficiency: Less severe than classic, still causes bilateral cataracts</li> <li>GALE deficiency: Least severe, often asymptomatic</li> </ul> </ul> <li>Treatment for all forms of galactosemia: Eliminating galactose and lactose from diet</li> </ul> </ul>

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What is the role of galactose metabolism in the breakdown of lactose?

Galactose metabolism plays a crucial role in the breakdown of lactose, a disaccharide sugar found in milk and dairy products. Lactose is composed of glucose and galactose, and its metabolism involves the conversion of galactose to glucose. This conversion makes it possible for the body to utilize the energy provided by lactose, as glucose can be readily used in glycolysis or stored as glycogen for future energy needs.

How does galactokinase relate to the process of galactose metabolism?

Galactokinase is a key enzyme involved in the first step of galactose metabolism. It catalyzes the phosphorylation of galactose into galactose-1-phosphate. This conversion is essential for the continuation of the metabolic pathway, as galactose-1-phosphate serves as a substrate for the subsequent enzymatic reactions that convert galactose into glucose.

What is the role of GALT and GALE in galactose metabolism?

GALT (galactose-1-phosphate uridylyltransferase) and GALE (UDP-galactose-4-epimerase) are two critical enzymes involved in the latter stages of galactose metabolism. GALT converts galactose-1-phosphate and UDP-glucose into glucose-1-phosphate and UDP-galactose. GALE then catalyzes the conversion of UDP-galactose into UDP-glucose, which can be used for glycogen synthesis or further processed in the glycolytic pathway, ultimately yielding energy for cellular functions.

What are the main consequences of galactosemia in affected individuals?

Galactosemia is a rare genetic disorder that affects the body's ability to metabolize galactose due to the deficiency or absence of the GALT enzyme. As a result, galactose and its metabolite, galactose-1-phosphate, may accumulate in the body, causing toxicity and damage to various organs, particularly the liver, brain, and eyes. Common symptoms of galactosemia include poor feeding, vomiting, diarrhea, lethargy, liver dysfunction, cataracts, and developmental delays. If left untreated, galactosemia can lead to life-threatening complications, such as liver failure, sepsis, and cerebral edema.

How is galactosemia diagnosed and managed?

Galactosemia is typically diagnosed through newborn screening tests, which involve measuring the levels of galactose and galactose-1-phosphate in the blood, as well as assessing GALT enzyme activity. Genetic testing can be performed to confirm the diagnosis and determine the specific type of galactosemia. The primary treatment for galactosemia is strict dietary restriction of lactose and galactose, which involves avoiding milk and dairy products, as well as other foods containing galactose. Regular monitoring of dietary adherence, liver function, and growth is essential to ensure optimal management of the condition.