Mono disaccharide metabolism

Monosaccharide and Disaccharide Metabolism

Plants Synthesize Glucose

Monosaccharide Diversity

Structure of Monosaccharides

  • Glucose:

    • Extensively studied.

  • Fructose:

    • Known as "fruit sugar".

    • Makes up sucrose when combined with glucose.

  • Galactose:

    • Known as "milk sugar".

    • Forms lactose in combination with glucose.

Fructose

  • Accounts for ~10% of daily caloric intake.

  • Found free in many fruits and honey.

  • Sucrose is cleaved in the intestine into fructose and glucose.

  • High-fructose corn syrup:

    • Comprises 55% fructose and 45% glucose.

    • Transport of fructose is insulin-independent, meaning it does not prompt insulin secretion.

Sugar Substitutes

  • Saccharin:

    • First produced in 1879.

    • 300 times sweeter than sucrose; no calories or nutritional benefit.

    • Linked to bladder cancer in lab animals; no potential risk for humans.

Aspartame (NutraSweet)

  • Composed of phenylalanine, aspartic acid, and methanol.

  • 180-200 times sweeter than sucrose.

  • Contains 4 kcal/gm, but only small amounts needed for sweetness.

  • Not heat-stable and can cause sensitivity reactions (e.g., headaches, nausea).

  • Not suitable for individuals with phenylketonuria (PKU).

Artificial Sweeteners

  • Classified as Generally Recognized as Safe (GRAS).

  • Subject to safety studies and some concerns over animal study irregularities and long-term high exposure effects.

Fructose Metabolism

  • First Step: Phosphorylation.

    • Hexokinase activated only when glucose is low.

    • Fructokinase: Primary mechanism for fructose metabolism.

      • Found in the liver, kidney, and small intestine; uses 1 ATP.

  • Pathway: Fructokinase pathway differs from glycolysis as it does not have a second phosphorylation step.

    • Production yields two 3-carbon molecules via Aldolase B.

    • These molecules are similar to glycolysis product but convert glyceraldehyde to G-3-P.

Disorders of Fructose Metabolism

  • Fructosuria:

    • Caused by fructokinase deficiency leading to fructose excretion.

    • Typically asymptomatic.

  • Fructosemia:

    • Due to Aldolase B deficiency; leads to elevated F-1-P and inhibits glycolysis.

    • Causes ATP deficiency resulting in potential growth and developmental issues; possible liver and kidney damage.

    • Recommendations to limit fructose (sucrose) intake.

The Polyol Pathway

  • Involves sorbitol, a sugar alcohol produced in the body.

  • Important in the retina, lens, kidneys, peripheral nerves, ovaries, and seminal vesicles.

Chronic Complications of Diabetes Mellitus

  • Hyperglycemia elevates intracellular osmotic pressure due to sorbitol and fructose.

    • Leads to cell injury, with notable effects in the eye, nerves, and kidneys.

  • Microvascular Diseases:

    • Retinopathy, diabetic nephropathy, and neuropathy.

Galactose Metabolism

  • Lactose: Cleaved into galactose and glucose in the intestine.

    • Lactase deficiency leads to lactose intolerance, which can be hereditary or acquired.

    • Symptoms include osmotic diarrhea, bloating, and gas.

Galactose Metabolism Steps

  • First Step: Phosphorylation by galactokinase, requiring 1 ATP.

  • Conversion into UDP-Galactose, with UDP-Glucose being a key step in glycogenesis.

  • Galactosemia:

    • High levels of galactose in the bloodstream due to deficiencies in galactokinase, GALT, or epimerase.

    • Symptoms include lethargy, jaundice, and potential brain, liver, and kidney damage.

Lactose Synthesis

  • Governed by endocrinological control involving:

    • Prolactin from the anterior pituitary for milk synthesis.

    • Oxytocin from the posterior pituitary for milk release.

  • Lactose Synthase: Enzyme critical for lactose synthesis in mammary glands.