Biochemistry Chapter: Disaccharides and Sugars

Disaccharides

• Composed of two monosaccharide units, disaccharides are glycosides prevalent in nature, providing significant dietary energy.
• Key examples include lactose, maltose, and sucrose, each playing distinct roles in nutrition.
• Sucrose: Common table sugar, made of α-glucose and β-fructose, produced in plant leaves, serves as energy transport for plants.
• Maltose: Known as malt sugar, formed by starch hydrolysis, contains α(1,4) glycosidic linkage between two glucose molecules.
• Lactose: Milk sugar, consists of galactose and glucose linked via a β-glycosidic bond, fundamental for energy in milk.

Digestion and Absorption

• Disaccharide digestion is facilitated by enzymes from the small intestine; enzyme deficiency can lead to digestive issues.
• Carbohydrate absorption mainly occurs in the large intestine, causing osmotic pressure that draws water, potentially leading to diarrhea.
• Gut bacteria digest disaccharides through fermentation, producing gas that may cause bloating and cramps.
• Lactose intolerance: Common deficiency in adults, inherited as autosomal recessive, varying dramatically in prevalence across populations (3% in Danes vs. 97% in Thais). Managed by dietary elimination or lactase enzyme treatment.

Polysaccharides

• Composed of numerous monosaccharide units linked by glycosidic bonds, resulting in compact structures important for living organisms.
• Cellulose: Structural component of plant cells, providing strength and rigidity, differing from starch in molecular orientation and bonding, leading to its fibrous quality.
• Distinct physical properties result from the orientation of glycosidic bonds; starch and glycogen form twisted chains while cellulose remains straight.

Homopolysaccharides vs. Heteropolysaccharides

• Homopolysaccharides: Enzymes can break down starch and glycogen, but cellulose is resistant, except in certain organisms (e.g., termites).
• Heteropolysaccharides: High molecular weight polymers composed of different monosaccharides, categorized into glycosaminoglycans, glycoproteins, and glycolipids.

Fatty Acids

• Long chain carboxylic acids, saturated fatty acids feature unbranched linear chains with only single carbon-carbon bonds, maximizing hydrogen atoms.
• Unsaturated fatty acids contain one or more carbon-carbon double bonds, classified as monounsaturated (one double bond) or polyunsaturated (multiple double bonds).
• Spatial configurations of fatty acids can be cis or trans; the cis configuration is predominant in biological systems, while trans unsaturated fats may have negative health effects.

Proteins

• Proteins are polymers of amino acids linked by peptide bonds, with 20 amino acids forming vast protein diversity.
• Amino acids are characterized by a central carbon with an amino group, carboxyl group, hydrogen atom, and variable R group, influencing protein behavior.
• Peptide bonds form through condensation reactions, leading to chains with distinct amino and carboxyl termini.
• Secondary structures: α-helix (spiral formation due to hydrogen bonding) and β-pleated sheet (folds formed by hydrogen bonding between adjacent chains).

Prions

• Prions are infectious, misfolded proteins causing transmissible spongiform encephalopathies in mammals, including BSE in cattle and CJD in humans.
• All prion diseases lead to brain structure abnormalities, are untreatable, and fatal, involving amyloid folds of beta sheets instead of typical alpha helices.