MCAT Biochemistry
The metabolism of fructose and sorbitol plays a crucial role in how our body processes sugars found in fruits, honey, and root vegetables. Fructose can enter glycolysis as glyceraldehyde-3-phosphate (G3P) or be metabolized into glycerol. The process begins with fructokinase converting fructose into fructose-1-phosphate (F1P), followed by aldolase B cleaving F1P into dihydroxyacetone phosphate (DHAP) and glyceraldehyde. These are then converted into G3P before entering glycolysis. Alternatively, glyceraldehyde can be converted to glycerol for triglyceride synthesis.
Two disorders related to fructose metabolism are essential fructosuria and hereditary fructose intolerance. Essential fructosuria is a benign condition caused by a deficiency in fructokinase, resulting in elevated blood and urine fructose levels. Hereditary fructose intolerance is more severe and is caused by a deficiency in aldolase B, leading to an excess of fructose-1-phosphate. This condition requires avoidance of dietary fructose. Sorbitol metabolism is connected to glucose processing; aldose reductase converts glucose into sorbitol, and sorbitol dehydrogenase then converts it to fructose. In uncontrolled type 2 diabetes, excess sorbitol may accumulate in organs without sorbitol dehydrogenase, causing tissue damage.
Lesson Outline
<ul> <li>Fructose Metabolism</li> <ul> <li>Fructose is a naturally occurring sugar in fruits, honey, and root vegetables</li> <li>Fructose enters glycolysis as G3P or metabolized into glycerol</li> <li>Fructokinase converting fructose to F1P using ATP</li> <li>Aldolase B cleaving F1P into DHAP and glyceraldehyde</li> <li>Triose phosphate isomerase converting DHAP to G3P</li> <li>Triose kinase converting glyceraldehyde to G3P using ATP</li> </ul> <li>Glyceraldehyde Conversion and Triglyceride Synthesis</li> <ul> <li>Glyceraldehyde directly converted to glycerol</li> <li>Glycerol combined with 3 fatty acids to form a triglyceride</li> </ul> <li>Inborn Errors of Fructose Metabolism</li> <ul> <li>Autosomal recessive conditions</li> <li>Essential fructosuria: benign condition with fructose present in blood and urine</li> <li>Hereditary fructose intolerance: life-threatening metabolic disease, causing vomiting, dehydration, hypoglycemia, hepatomegaly, and jaundice</li> </ul> <li>Sorbitol Metabolism</li> <ul> <li>Aldose reductase converting glucose to sorbitol using NADPH</li> <li>Sorbitol dehydrogenase converting sorbitol to fructose using NAD+</li> </ul> <li>Tissues and Organ Damage from Sorbitol Accumulation</li> <ul> <li>Type 2 diabetes and the increased risk of cataracts, retinopathy, nephropathy, and peripheral neuropathy</li> </ul> </ul>
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FAQs
Fructose metabolism mainly includes enzymes like fructokinase, aldolase B, and triose kinase. Fructokinase phosphorylates fructose to fructose-1-phosphate. Aldolase B splits fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate. Then, triose kinase phosphorylates glyceraldehyde to glyceraldehyde-3-phosphate, which can enter glycolysis and be further metabolized for energy production or other processes.
Sorbitol is metabolized by two enzymes: aldose reductase and sorbitol dehydrogenase. Aldose reductase converts glucose to sorbitol using NADPH, while sorbitol dehydrogenase converts sorbitol to fructose using NAD+. In type 2 diabetes, high glucose levels cause an increase in aldose reductase activity, leading to increased sorbitol production. Accumulation of sorbitol can result in osmotic stress, contributing to diabetic complications such as cataracts, retinopathy, neuropathy, and nephropathy.
Essential fructosuria is a benign, hereditary metabolic disorder caused by a deficiency of fructokinase, the enzyme responsible for converting fructose to fructose-1-phosphate in the liver. This deficiency results in incomplete metabolism of fructose, causing it to be excreted in urine rather than used for energy production. Essential fructosuria is generally asymptomatic and does not require treatment, as the body can still obtain energy from other dietary sources such as glucose or galactose.
Hereditary fructose intolerance is a potentially serious genetic disorder caused by a deficiency of the enzyme aldolase B. This enzyme is responsible for breaking down fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate. The deficiency leads to an accumulation of fructose-1-phosphate, causing toxic effects in various tissues, especially in the liver. Symptoms of hereditary fructose intolerance include vomiting, hypoglycemia, jaundice, liver enlargement, and kidney dysfunction. Treatment for hereditary fructose intolerance involves strict avoidance of fructose in the diet.
Sugar alcohols, also known as polyols, are a class of compounds derived from sugars and contain a hydroxyl group in place of the aldehyde or ketone group found in monosaccharides. Common sugar alcohols include sorbitol, xylitol, and erythritol. They are often used as sugar substitutes due to their lower caloric content and reduced effects on blood glucose levels compared to sugars like fructose. The metabolism of sugar alcohols, such as sorbitol, occurs through specific enzymatic pathways like sorbitol metabolism and can impact overall glucose metabolism, especially in individuals with type 2 diabetes.
