carbohidratos-pdf

Química de carbohidratos

lunes 1 de marzo de 2010

2.5 Química de carbohidratos a. Monosacáridos, disacáridos b. Polisacáridos de reserva c. Polisacáridos estructurales d. Mucopolisacáridos

Carbohidratos

•Pertenecen al grupo de biomoléculas mas abundantes: Carbohidratos, Proteínas, Lípidos y Acidos Nucléicos

•Químicamente se definen como aldehidos o cetonas polihidroxiladas

•La materia orgánica mas abundante sobre la tierra

Photosynthesis converts morethan 109 metric tons of CO2 and H2O into celluloseand other plant products.

the word “saccharide” is derived from the Greek sakcharon, meaning “sugar”, a sweet crystaline substance derived from plants

•Función :

Almacén de energía, combustibles e intemediarios metabólicos

Ribosa y desoxiribosa, componentes esenciales de RNA y DNA

Almidón y celulosa

Elementos estructurales en paredes celulares de bacterias y plantas.

Reconocimiento o anclaje molecular

Diversidad estructural en carbohidratos :

Propiedad clave que determina su papel como mediadores de interacciones celulares

Construidos a partir de unidades de azúcares (monosacáridos) C3-C9

Tamaño, secuencia y estereoquímica definida

Unión (N-gly, α, β, ....) y composición molecular específica

Monosacáridos: los azúcares mas simples

Aldehidos o cetonas con uno o mas grupos hidroxilo

Formula general Cn(H2O)m donde n, m pueden ser iguales o diferentes y ≥ 3

If the carbonyl group is at an end of the carbon chain (aldehyde group) the monosaccharide is an aldose

Glyceraldehyde has a single asymmetric carbon and, thus, there are two stereoisomers of this sugar generating two series of aldoses D-aldosas y L-aldosas.

D-glyceraldehyde and L-glyceraldehyde are enantiomers, or mirror images of each other

If the carbonyl group is at any other position (ketone group) the monosaccharide is a ketose.

Glyceraldehyde and dihydroxyacetone are referred to as trioses

Simple monosaccharides with four, five, six, and seven carbon atoms are called tetroses, pentoses, hexoses, and heptoses, respectively.

Because these molecules have multiple asymmetric carbons, they exist as diastereoisomers, isomers that are not mirror images of each other, as well as enantiomers.

Sedoheptulosa

Neuraminic acid

Sialic acidlunes 1 de marzo de 2010

The symbols D and L designate the absolute configuration of the asymmetric carbon farthest from the aldehyde or keto group.

Enantiomers : non-superimposable COMPLETE mirror images

Epimers: sugars that differ in configuration at ONLY 1 POSITION

Two sugars that differ only in the configuration around one carbon atom are called epimers.

Monosaccharides with four, five, six, and seven carbon atoms in their backbones are called, respectively, tetroses, pentoses, hexoses, and heptoses.

a) There are aldoses and ketoses of each of these chain lengths: aldotetroses and ketotetroses, aldopentoses and ketopentoses, and so on.

b) The hexoses, which include the aldohexose D-glucose and the ketohexose D-fructose, are the most common monosaccharides in nature.

c) The aldopentoses D-ribose and 2-deoxy-D-ribose are components of nucleotides and nucleic acids.

Three ways to represent the two stereoisomers of glyceraldehyde.

The stereoisomers are mirror images of each other. Ball and-stick models show the actual configuration of molecules.

By convention, in Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader; vertical bonds project behind the plane of the paper, away from the reader.

Recall that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away.

Formation of hemiacetals and hemiketals.An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal. A new chiral center at the carbonyl carbon is created. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is aglycosidic bond

In Solution, The Common Monosaccharides Have Cyclic Structures

The keto group in the open chain form of a ketohexose such as fructose, can form an intramolecular hemiketal by reacting with either the C-6hydroxyl group to form a six-membered cyclic hemiketal (pyranose) or the C-5 hydroxyl group of to form a five-membered cyclyc hemiketal(furanose)

Two cyclic forms of D-glucose. Reactionbetween the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the and anomers, which differ only in the stereochemistry around the hemiacetal carbon.

The interconversion of α and β anomers is called mutarotation.

Son aquellos azúcares que poseen al menos un carbonilo (grupo funcional) intacto, es decir no formando ni cetal o acetal.

Azúcares que en solución alcalina forman un aldehido o cetona

Reductores: Glucosa, fructosa, gliceraldehido, lactosa, arabinosa, maltosa…..

No reductores: Sacarosa, trealosa

Las cetosas no se oxidan directamente, mas bien al transformarse en aldehidos por tautomerismo ceto-enólico

Conformational formulas of pyranoses. (a) Two chair forms of the pyranose ring. Substituents on the ring carbons may be either axial (ax), projecting parallel to the vertical axis through the ring, or equatorial (eq), projecting roughly perpendicular to this axis. Two conformers such are these are not readily interconvertible without breaking the ring. However, when the molecule is “stretched” (by atomic force microscopy), an input of about 46 kJ of energy per mole of sugar can force the interconversion of chair forms. Generally, substituents in the equatorial positions are less sterically hindered by neighboring substituents, and conformers with bulky substituents in equatorial positions are favored.

Tautomerismo ceto-enólico

Tautomerismo ceto-enólico en fructosa bajo condiciones alcalinas

Disacáridos

•Carbono anómericode un azucar se une a –OH de otro azúcar formando enlace O-glicosídico

•Enlace N-glicosídico C-anomérico => N de proteína, nucleótido o azúcar

•Sensibles a hidrólisis ácida y resistentes a hidrólisis básica

•Extremo reductor, presencia de carbonilo intacto, no formando ni cetal o acetal.

Maltosaα-D-Glucopyranosyl-

(1→4)-D-glucose

Sacarosaβ-D-fructofuranosyl-(2→1)-α-D-glucopyranoside

Lactosaβ-D-galactopyranosyl-(1→4)-D-glucose

Maltosa

Configuración α de carbono anomérico, forma enlace α

The glucose residue with the free anomeric carbon is capable of existing in α- and β-pyranose forms

Kojibiosa α(1→2)

Isómeros α de maltosa (Glc-Glc)

Maltosa α(1→4)

Nigerosa α(1→3)

Isomaltosa α(1→6)

Trealosa α(1→1)

Isómeros β de maltosa

celobiosa β(1→4)

Soforosa β(1→2)Laminaribiosa β(1→3)

Gentobiosa β(1→6)

β,β-Trealosa β(1→1)

Polisacáridos

lineal homogéneo

ramificado homogéneo

alternado

bloques

lineal heterogéneo

ramificado heterogéneo

interumpido ramificado

Polisacáridos

Glicógeno glucosa α-(1-4) ramificado en α-(1-6) cada 8-12 résidus

Almidón, formado por dos homopolímeros, amilosa (no ramificada α-(1-4)) y amilo pectina (ramificado en α-(1-6) cada 24-30 résiduos)

amilosa no ramificada α-(1-4)

amilo pectina ramificada en α-(1-6) cada 24-30 résiduos

Almidón : motivo de repetición la maltosa

Celulosa : motivo de repetición la celobiosa

Hélices abiertas mejor almacén de energía

Cadenas lineales mejor soporte estructural

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