Bioquimica - Tema 2. Hidratos de Carbono

Hidratos de Carbono

  • Also known as carbohydrates, essential macronutrients.
  • Provide energy and have other crucial roles.

Funciones de los Hidratos de Carbono

  • Energy Storage: Act as short and long-term energy reserves (e.g., glycogen).
  • Structural Role: Essential components of cell membranes (e.g., glycolipids and glycoproteins).
  • Cell Signaling and Recognition: Act as signals on cell surfaces, facilitating cell communication.

COMPOSICIÓN QUIMICA DE LOS GLÚCIDOS

  • Composed of carbon (C), hydrogen (H), and oxygen (O).
  • Contain a carbonil group (aldehyde [-CHO] or ketone [-CO-]

Clasificación de los hidratos de Carbono

  • Monosaccharides: Basic building blocks with 3-7 carbon atoms.
  • Oligosaccharides: Formed by 1-10 monosaccharides.
  • Polysaccharides: Polymers with more than ten monomers, used for energy storage and structure.
  • Empirical formula: (CH<em>2O)</em>n(CH<em>2O)</em>n
  • Classified as aldoses (aldehyde group) or ketoses (ketone group).
  • Categorized by carbon atom number: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7C).

Isomería de los hidratos de Carbono

  • Isomers have the same chemical formula but different structures.
  • Carbohydrates exhibit stereoisomerism due to chiral carbons.
  • Number of stereoisomers: 2n2^n ('n' is the number of asymmetric carbons).

Tipos de esteroisómeros

  • Enantiomers: mirror images.
  • Diastereoisomers: Non-mirror image stereoisomers.
    • Epimers: Differ in OH orientation at one chiral carbon.
    • Other Diastereoisomers: Differ in OH orientation at two or more chiral carbons.

Esteroisómeros

  • Enantiomers: molecules related by mirror symmetry.
  • D and L designation: based on the OH orientation on the chiral carbon farthest from the functional group.
  • Enantiomers have similar chemical properties.
  • If the OH group on the furthest chiral carbon is on the left, it is an L enantiomer; if on the right, it is a D enantiomer.

Diasteroisomería y Epímeros

  • Diastereoisomers are not mirror images.
  • L or D designation based on OH orientation on the chiral carbon farthest from the functional group.
  • Epimers differ in the position of the OH group around one chiral center.
  • Epimers have different chemical properties and names.

Estructuras en Anillo – Ciclación de Aldosas

  • Aldotetroses and monosaccharides with 5+ carbons form cyclic structures in aqueous solutions.
  • Carbonyl group forms a hemiacetal with an OH group within the same molecule.
  • Generates a new chiral center, resulting in α (OH downwards) and β (OH upwards) anomers.
  • Groups on the left of the Fischer projection point upwards in the cyclic form.

Estructuras en Anillo – Ciclación de Cetosas

  • Cetohexoses (e.g., fructose) form cyclic structures via a hemicetal between the carbonyl group (C2) and a hydroxyl group (C5).
  • Anomers (α and β) interconvert via mutarotation, where the ring opens briefly and recloses with the C1 OH group in the opposite position.

Oxidación

  • R−CH2OM⟶R−C=0R-CH_2OM \longrightarrow R-C=0
  • R−CHOM⟶RR-CHOM \longrightarrow R
  • R−C−o⟶−01R-C-o \longrightarrow -01

Conformación en Silla de Monosacáridos

  • Cyclic monosaccharides adopt non-planar chair conformations.
  • Conformers are interconvertible without breaking covalent bonds.
  • 3D structure determines properties and biological functions of polysaccharides.

Modificaciones en la Estructura de Carbohidratos

  • Amino sugars: Hydroxyl groups replaced by amino groups (e.g., glucosamine).
  • Oxidation to Carboxyl Groups: Aldehydes oxidized to carboxylic acids (aldonic acids like gluconic acid); C-6 oxidation yields uronic acids.
  • Reduction of Sugars: Reduction forms sugar alcohols (e.g., glucose to sorbitol).
  • Esterification: Hydroxyl groups bind to phosphoric acids, forming high-energy carbohydrate phosphates (e.g., glucose-6-phosphate).

Disacáridos

  • Monosaccharides join via dehydration, forming O-glycosidic bonds.
  • Glycosidic bonds are sensitive to acidic conditions and high temperatures.
  • Oligosaccharides are short sugar chains, with disaccharides being the most common (two sugar monomers).
  • Examples: sucrose, maltose, and lactose.

Polisacáridos

  • Most carbohydrates in nature exist as polysaccharides.
  • Homopolysaccharides: energy storage or structural elements.
  • Heteropolysaccharides: structural elements for cells
  • Polysaccharides can be linear or branched.

Polisacáridos de almacenamiento

  • Storage polysaccharides include amylose and amylopectin (plants), and glycogen (animals and microorganisms).
  • All are polymers of α-D-glucopyranose (glucose).
  • Amylose: linear polymer with α(1->4) linkages.
  • Amylopectin and glycogen: branched polymers with α(1->4) and α(1->6) linkages.

Polisacáridos estructurales

  • Cellulose is the main polysaccharide in plants.
  • Linear polymer of D-glucose linked by β(1->4) bonds.
  • Forms extended chains stabilized by hydrogen bonds.
  • Human enzymes cannot hydrolyze β(1->4) glycosidic bonds.
  • Cellulase enzyme is required for cellulose digestion.

Glucosaminoglucanos

  • Glucosaminoglycans (mucopolysaccharides) are important structural polysaccharides in vertebrates.
  • Composed of repeating disaccharide units containing N-acetylglucosamine or N-galactosamine derivatives.
  • Acidic due to sulfate or carboxylate groups.
  • Hyaluronan: glucuronic acid and N-acetylglucosamine, acts as a lubricant in joints.

GLUCOCONJUGADOS

  • Polysaccharides and oligosaccharides carry information.
  • Involved in cell communication and protein targeting.
  • Oligosaccharide chains form a glycocalyx on the cell surface.
  • Carbohydrates bind to proteins or lipids, forming glycoconjugates.

PROTEOGLUCANOS

  • Proteoglycans are proteins with one or more covalently linked glycosaminoglycan chains.
  • Involved in tissue organization and cell adhesion.
  • Serine residue is the common attachment point, linked by a tetrasaccharide.
  • Some proteoglycans form large aggregates linked to hyaluronan.

MATRIZ EXTRACELULAR

  • Proteoglycan aggregates intertwine with proteins (collagen) to form mechanically resilient networks.
  • Fibronectin interacts with heparin sulfates and collagen.
  • Integrins link the cytoskeleton to the extracellular matrix.

GLUCOPROTEINAS

  • Glycoproteins are proteins with covalently linked oligosaccharides.
  • Oligosaccharides are linked to serine, threonine (O-linked), or asparagine (N-linked).
  • Many proteins are glycosylated.
  • Glycosylation with N-acetylglucosamine regulates protein activity.

GLUCOLÍPIDOS

  • Glycolipids are membrane sphingolipids with oligosaccharide head groups.
  • Gangliosides contain sialic acid and determine blood group.
  • Located on the external face of the plasma membrane.
  • Lipopolysaccharides form the outer membrane of Gram-negative bacteria.

Polisacáridos de almacenamiento

  • CHO - > Monosaccharides -> Absorpcíon -> Intestino delgado
  • CHO NO digeribles -> CO2, H2, CH4 -> Fermentación - Excreción

Digestión y absorción de carbohidratos

  • Gradual degradation → monosaccharides
  • Polysaccharides:
    • Boca: Masticación/hidratación -> a-amilasa salivar (Ptialina)
    • Estómago: Polisacáridos -> Oligosacáridos
  • Disacáridos:
    • Sacarosa
    • Lactosa
  • Monosacáridos
    • Lactosa

Digestión y absorción de carbohidratos

  • Intestino delgado:
    • Almidón -> Dextrinas -> Maltotriosa
    • Sacarosa -> Glucosa
    • Lactosa -> Galactosa
    • Fructosa
    • a-amilasa pancreática