Monosaccharide and Disaccharide Metabolism: Biochemistry Study Notes

Monosaccharide and Disaccharide Metabolism

Overview of Monosaccharide and Disaccharide Metabolism

Monosaccharides and disaccharides are essential carbohydrates in human nutrition and metabolism. Glucose is the most common monosaccharide consumed, while fructose and galactose are also significant, contributing to energy metabolism and biosynthetic pathways. Mannose is important in glycoprotein synthesis.

• Monosaccharides: Simple sugars such as glucose, fructose, galactose, and mannose.

• Disaccharides: Composed of two monosaccharides (e.g., sucrose = glucose + fructose; lactose = glucose + galactose).

• Functions: Energy production, biosynthesis of macromolecules, and cellular signaling.

Fructose Metabolism

Sources and Transport of Fructose

Fructose is a major dietary sugar, primarily ingested as part of the disaccharide sucrose (table sugar), which yields equimolar amounts of glucose and fructose upon hydrolysis. Free fructose is also found in fruits, honey, and high-fructose corn syrup.

• Transport: Fructose enters cells via facilitated diffusion, independent of insulin.

• Insulin Response: Fructose does not stimulate insulin secretion.

Phosphorylation of Fructose

For metabolism, fructose must first be phosphorylated. This is primarily achieved by fructokinase in the liver, kidneys, and small intestine, producing fructose 1-phosphate.

• Fructokinase: High affinity (low $K_m$), high capacity (high $V_{max}$) for fructose.

• Hexokinase: Can phosphorylate fructose in most tissues, but with low affinity.

• Reaction: $\text{Fructose} + \text{ATP} \xrightarrow{\text{fructokinase}} \text{Fructose 1-phosphate} + \text{ADP}$

Cleavage of Fructose 1-Phosphate

Aldolase B cleaves fructose 1-phosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde, both of which can enter glycolysis or gluconeogenesis.

• Aldolase Isozymes:

  • Aldolase A: Muscle and most tissues

  • Aldolase B: Liver, kidney, small intestine (only isozyme that cleaves fructose 1-phosphate)

  • Aldolase C: Brain

• Reaction: $\text{Fructose 1-phosphate} \xrightarrow{\text{aldolase B}} \text{DHAP} + \text{Glyceraldehyde}$

Clinical Disorders of Fructose Metabolism

Deficiencies in enzymes of fructose metabolism can lead to metabolic diseases.

Disorder	Enzyme Deficiency	Symptoms	Inheritance
Essential Fructosuria	Fructokinase	Benign, fructose in urine	Autosomal recessive
Hereditary Fructose Intolerance (HFI)	Aldolase B	Severe hypoglycemia, vomiting, liver failure, jaundice, hyperuricemia, lactic acidemia	Autosomal recessive

• Diagnosis: Detection of fructose in urine, genetic testing.

• Treatment: Removal of fructose and sucrose from the diet.

Mannose Metabolism

Mannose Conversion to Fructose 6-Phosphate

Mannose is a C-2 epimer of glucose and a key component of glycoproteins. It is phosphorylated by hexokinase to mannose 6-phosphate, which is then isomerized to fructose 6-phosphate by phosphomannose isomerase.

• Reactions:

  • $\text{Mannose} + \text{ATP} \xrightarrow{\text{hexokinase}} \text{Mannose 6-phosphate} + \text{ADP}$

  • $\text{Mannose 6-phosphate} \xrightarrow{\text{phosphomannose isomerase}} \text{Fructose 6-phosphate}$

• Sources: Dietary intake is low; most mannose is derived from glycoprotein turnover.

Polyol Pathway: Glucose to Fructose via Sorbitol

Polyol (Sorbitol) Pathway

When glucose is abundant, it can be converted to sorbitol (a sugar alcohol) by aldose reductase. Sorbitol can then be oxidized to fructose by sorbitol dehydrogenase in certain tissues.

• Reactions:

  • $\text{Glucose} + \text{NADPH} + \text{H}^+ \xrightarrow{\text{aldose reductase}} \text{Sorbitol} + \text{NADP}^+$

  • $\text{Sorbitol} + \text{NAD}^+ \xrightarrow{\text{sorbitol dehydrogenase}} \text{Fructose} + \text{NADH} + \text{H}^+$

• Tissue Distribution: Sorbitol dehydrogenase is present in the liver, ovaries, and seminal vesicles.

• Clinical Relevance: In tissues lacking sorbitol dehydrogenase (e.g., lens, retina, Schwann cells), sorbitol accumulates during hyperglycemia, causing osmotic damage (cataracts, neuropathy, nephropathy, retinopathy in diabetes).

Galactose Metabolism

Sources and Transport of Galactose

Galactose is mainly obtained from the hydrolysis of lactose (milk sugar). It can also be released from glycoproteins and glycolipids. Transport into cells is not insulin-dependent.

Phosphorylation and Entry into Glycolysis

Galactose is phosphorylated by galactokinase to galactose 1-phosphate. This is then converted to UDP-galactose via the galactose 1-phosphate uridylyltransferase (GALT) reaction, using UDP-glucose as a nucleotide donor.

• Reactions:

  • $\text{Galactose} + \text{ATP} \xrightarrow{\text{galactokinase}} \text{Galactose 1-phosphate} + \text{ADP}$

  • $\text{Galactose 1-phosphate} + \text{UDP-glucose} \xrightarrow{\text{GALT}} \text{UDP-galactose} + \text{Glucose 1-phosphate}$

  • $\text{UDP-galactose} \xrightarrow{\text{UDP-hexose 4-epimerase}} \text{UDP-glucose}$

• Glucose 1-phosphate can enter glycolysis after conversion to glucose 6-phosphate.

Galactose in Biosynthesis

UDP-galactose serves as a donor of galactose units in the synthesis of lactose, glycoproteins, and glycolipids. If dietary galactose is insufficient, UDP-galactose can be synthesized from UDP-glucose by UDP-hexose 4-epimerase.

Clinical Disorders of Galactose Metabolism

Disorder	Enzyme Deficiency	Symptoms	Inheritance
Classic Galactosemia	GALT	Jaundice, hepatomegaly, cataracts, intellectual disability, risk of death if untreated	Autosomal recessive
Galactokinase Deficiency	Galactokinase	Cataracts, galactosemia, galactosuria	Autosomal recessive

• Pathophysiology: Accumulation of galactose 1-phosphate and galactitol (via aldose reductase) causes toxicity.

• Treatment: Removal of galactose and lactose from the diet.

Lactose Synthesis

Biosynthesis of Lactose

Lactose is a disaccharide of β-galactose and glucose, synthesized in the Golgi apparatus of lactating mammary glands by lactose synthase. This enzyme consists of two subunits: protein A (galactosyltransferase) and protein B (α-lactalbumin).

• Reaction: $\text{UDP-galactose} + \text{Glucose} \xrightarrow{\text{lactose synthase}} \text{Lactose} + \text{UDP}$

• Regulation: α-lactalbumin synthesis is stimulated by prolactin during lactation, altering the specificity of galactosyltransferase to favor lactose production.

• Other Functions: In non-lactating tissues, galactosyltransferase transfers galactose to N-acetylglucosamine, forming N-acetyllactosamine (important in glycoproteins).

Summary Table: Important Dietary Monosaccharides

Monosaccharide	Major Dietary Source	Main Metabolic Pathway
Glucose	Starch, sucrose, lactose	Glycolysis, glycogenesis, pentose phosphate pathway
Fructose	Fruits, honey, sucrose, high-fructose corn syrup	Fructolysis (fructokinase pathway), glycolysis
Galactose	Lactose (milk, dairy)	Galactose metabolism (Leloir pathway), glycolysis
Mannose	Glycoproteins (minor in diet)	Mannose metabolism, glycoprotein synthesis

Key Concepts and Clinical Correlations

• Fructose and galactose metabolism disorders can cause severe metabolic disturbances if not managed by dietary restriction.

• The polyol pathway is implicated in diabetic complications due to sorbitol accumulation.

• Lactose synthesis is hormonally regulated and essential for milk production.