Interconnection of Fructose, Galactose, and Glucose Metabolism PDF

## Interconnection of Fructose, Galactose and Glucose Metabolism ### Fructose - **Integration of fructose and galactose:** These sugars are found in abundance in the diet. - **Fructose absorption and production:** Fructose is primarily used for the production of insulin. - **Origin of fructose:*...

## Interconnection of Fructose, Galactose and Glucose Metabolism ### Fructose - **Integration of fructose and galactose:** These sugars are found in abundance in the diet. - **Fructose absorption and production:** Fructose is primarily used for the production of insulin. - **Origin of fructose:** Primarily from sucrose. - **Cleavage of sucrose:** Sucrose is cleaved by sucrase into glucose and fructose. - **Fructose in the body:** Found in its free form in the blood. - **Fructose entry into glycolysis:** Enters through pathways like GLUT2 and GLUT5 - **Fructose entry into muscle and adipose tissue:** Occurs via a non-insulin dependent pathway. - **Fructose phosphorylation:** Phosphorylated by hexokinase to F6P in tissues outside of the liver. ### Fructose Metabolism in the Liver - **Fructose metabolism in the liver:** Occurs via a non-insulin dependent pathway. - **Fructoses phosphorylation in the liver:** Fructose is phosphorylated to form fructose-1-phosphate. - **Fructose-1-phosphate cleavage:** Cleaved by aldolase B enzyme into glyceraldehyde and dihydroxyacetone phosphate (DHAP). - **Metabolized by triose phosphates:** DHAP and glyceraldehyde-3-phosphate are further metabolized. - **Fructose 1-phosphate metabolism:** The primary pathway for fructose metabolism. - **Fast pathway of PFK1:** Fructose-1-phosphate bypasses the rate-limiting step, PFK1, in glycolysis. - **Phosphorylation by fructokinase:** Fructose is phosphorylated by fructokinase into fructose-1-phosphate. ### Galactose - **Galactose absorption and production:** Galactose is primarily absorbed from the digestion of lactose. - **Galactose origin:** Primarily from lactose, a disaccharide in dairy products. - **Galactose entry into glycolysis:** Enters through pathways like GLUT1, GLUT2 and GLUT5. - **Galactose phosphorylation:** Phosphorylated to galactose 1-phosphate by galactokinase. - **Galactose 1-phosphate is converted to UDP - galactose:** This reaction is catalyzed by UDP-galactose 4-epimerase. - **UDP-galactose is then converted to UDP-glucose:** This conversion is catalyzed by UDP-galactose 4-epimerase and uses NAD+ as a cofactor. - **UDP - glucose is converted to glucose 1-phosphate:** This step is catalyzed by phosphoglucomutase. - **Galactose metabolism and glycogenesis:** Depending on energy demand, galactose can enter the glycolytic pathway or be used to synthesize glycogen. ### Deficiency in Fructokinase - **Deficiency in fructokinase:** This is rare and usually affects the liver. - **Fructose accumulation:** Fructose accumulates in the blood and urine. - **Benign fructosemia:** This is often asymptomatic. ### Deficiency in Aldolase B - **Deficiency in aldolase B:** This is a rarer and more severe disorder that primarily affects the liver. - **Fructose intolerance:** This is the most common symptom. - **Hepatic accumulation of fructose-1-phosphate (F1P):** F1P cannot be further phosphorylated, which leads to its accumulation in the liver. - **Hypoglycemia:** Low blood sugar levels are observed. - **Symptoms:** Nausea, vomiting, and abdominal pain are common symptoms. - **Treatment:** Avoiding fructose and sucrose is crucial. ### Galactosemia - **Galactosemia:** This is a genetic disorder characterized by the deficiency in one of the enzymes needed to metabolize galactose. - **Symptoms:** Symptoms include failure to thrive, jaundice, vomiting, and liver damage. - **Treatment:** Restricting galactose and lactose intake, and in some cases, lactose-free formulas are recommended. ### Lactose - **Hydrolysis of lactose:** Lactose is broken down into glucose and galactose via the enzyme lactase. - **Origin of lactose:** Primarily from dairy products. - **Galactose enters glycolysis:** Galactose enters the glycolytic pathway through the same pathways as glucose. ### Galactose Toxicity - **Galactose toxicity:** Excessive galactose can lead to health problems. - **Galactose accumulation:** Can lead to the accumulation of galactitol in tissues like the lens of the eye or the brain. - **Treatment:** Limiting dairy products, fruits, and vegetables rich in galactose is recommended. - **Cataracts:** Accumulation of galactitol in the lens of the eye.