L17/C7: Carbohydrates

Biochemistry I

Carbohydrates or saccharides

• most abundant biological molecules

• essential components of living organisms

Monosaccharide

• a single saccharide

• can associate to form very diverse polymers, which can either be linear or branched, and include different types of monosaccharides (different chemical properties)

• aldehyde or ketone derivatives of straight chain polyhydroxyl alcohols containing at leas three carbon atoms

Oligosaccharide

• few linked monosaccharides; they can be associated with proteins (glycoproteins) or lipids (glycolipids)

Polysaccharides

• consist of many monosaccharides connected via glycosidic bonds

• ex: cellulose and glycogen

• can be Homopolysaccharides or Heteropolysaccharides

• structure and placement controlled enzymatically

• are also known as glycans

• can form branched and well as linear polymers

• exo- and endo- glycosidases are enzymes that will hydrolyze monosaccharide units in much the same way that exo and endo- peptidases cleave peptide bonds in proteins

Aldoses

The carbonyl group is an aldehyde (CHO)

Ketoses

The carbonyl group is a ketone (CO)

D-sugars

• those that have the same stereochemical configuration at the asymmetric center farthest from their carbonyl group as does D-glyceraldehyde

• much more prevalent than L-sugars, so D-prefix might be omitted from common sugars (ex: Glucose and Fructose)

Epimers

Sugar molecules differing in stereochemical configuration at only one carbon atom

• particular case of diastereomers

Pyranose

The cyclic six-membered ring form of sugars

Furanose

The cyclic five-membered ring form of sugars

a Anomer

OH down, OH opposite side of ring from CH2OH (hydroxymethyl)

B-anomer

OH up, OH same side of ring from CH2OH (hydroxymethyl)

Glycosidic Bond

The bond linking the anomeric carbon to the oxygen of the OH substituent

• can either be in the a or B anomeric configuration and can be linked through the 1-2, 1-4, 1-6 linkage

Reducing Sugar

Those bearing an unlinked anomeric alcohol group

• can reduce an oxidizing agent

Non-reducing sugar

The anomeric alcohol group is linked with another sugar or molecule

• cannot reduce an oxidizing agent

Cellulose structure

• polymer of glucose units linked by B-D-glucose (1,4)

Starch

A mixture of a-amylose and amylopectin that plants synthesize for energy reserve

a-amylose

linear polymer of several thousand glucose residues connected via a(1-4) bonds

• polymer forms a left-handed helix

Amylopectin

Mainly a(1-4) glucose with occasional a(1-6) branches every 24-30 glucoses

Glycogen

Storage polysaccharide of animals

• structurally similar to amylopectin, but more highly branched

Glycosaminoglycans

• unbranched polysaccharides that compose a gel-like matrix used to fill extracellular spaces (particularly connective tissue)

• can be associated with proteins in the extracellular matrix to form proteoglycans

• proteoglycans form highly hydrated gels and serving as shock absorber and lubricant

Peptidoglycan (Bacterial Cell Wall)

• mostly impermeable to drugs and stabilize bacteria against hypotonic (salt gradients) and osmotic shock

• composed of disaccharide with amino acid chain and pentaglycine bridge

• D-amino acids render them resistant to proteases, which are specific for L-amino acids

Glycoproteins

• oligosaccharide modifications for proteins can have critically important biological implications

  • protein structure and biological stability

  • recognition events

    • virus infection

    • immune recognition

    • antigenic determinants (blood type)

• modifications of proteins by polysaccharide units have specific or less specific requirements

  • Sequence: Asn-X-Ser/Thr for N-linked oligosaccharides

  • Secondary or Tertiary structure for O-linked oligosaccharides, accessibility dictated