Carbohydrates

Structure of Carbohydrates

Properties of Carbohydrates Most abundant class of organic molecules Source: Photosynthesis Classification –Monosaccharides Stereoisomers Aldehydes (Aldose) or Ketones (Ketose) Number of Carbons (ie 3=triose; 6=hexose) Combined: Aldotriose/Ketotetrose –Polymers Oligosaccharides (2- ~20 sugars) Polysaccharides (> ~20 sugars)

Biological Roles of Carbohydrates Energy source Energy storage Cell walls Recognition events –Between proteins (targeting) –Between cells Signalling Components of other biological molecules –Antibiotics –Enzyme cofactors –Nucleic Acids

Monosaccharides (Sugars)

Classes of Monosaccharides

Chirality D- versus L- determined by chirality of highest number carbon (from aldehyde or ketone)

Figure 8-1 Aldoses

Figure 8-1 Aldoses

Figure 8-2 Ketoses

Figure 8-2 Ketoses

Epimers (stereoisomers differing by configuration of only one of several chiral centers)

Enantiomers (mirror images)

Mutarotation Creation of new chiral center

Formation of Hemiacetal

Formation of Hemketal

Anomeric Carbon Atom Mutarotation Reversible Creation of new asymmetric center

Cyclization of D-Glucose

Anomers Anomeric carbon atom –Most oxidized carbon atom –Shares electrons with 2 oxygen atoms  -configuration has -OH on opposite side of ring from CH 2 OH group at chiral center that designates D - or L -

Cyclization of D -Fructose (biologically relevant forms)

Nomenclature

Examples of Nomenclature  - D -glucopyranose  - D -fructofuranose Configuration of anomeric carbon Configuration of sugar Sugar prefix Ring Type *not required Anomeric carbon modification: ose: reducing oside: non-reducing

Cyclization of D -Fructose (biologically relevant forms)

Figure 8-5 Chair Conformations of  - D -glucopyranose Chair and Boat Forms Equitorial and Axial Substituents Steric Crowding: equitorial more stable EquatorialAxial

Derivatives of Monosaccharides

Phosphate Esters

Deoxy Sugars Note: 5-membered ring form is used in biological systems

Amino Sugars (e.g. GlcNAc-6-P)

Sugar Alcohols

Glycosides

Structure of Glycosides

Glycosidic Linkages (glycoside) Acetal Stable: no mutarotation Non-reducing sugar (no free anomeric C atom)

Nomenclature

Reducing test Free Aldehydes are reductants If free to mutarotate sugar is a reductant –Must have only –OH at anomeric carbon Cupric oxide brick-red precipitate Cu 2 O

Disaccharides

Sucrose (non-reducing) OR: Glc(α1 β2)Fru

Sucrose OR: Glc(α1 β2)Fru

 -Maltose Glc(α1  4)Glc

 -Lactose Gal(β1  4)Glc

Nomenclature 1.Recognize individual monosaccharides 2.Drop the –se and add root for rings –6 member: pyran –5 member: furan 3.Attach : –ose: can mutarotate –oside: canNOT mutarotate –osyl: not terminal residu e 4.Indicate carbon to carbon number linkage (#  #) 5.Label each residue with D or L and α or β

Oligosaccharides Generally complex –Heteropolymers –Branched Various Cellular Functions –Receptors –Antigens –Signal transduction –Trafficking

O-linked Oligosaccharides (serine/threonine)

N-linked Oligosaccharides (asparagine)

Sugar groups on glycoproteins frequently function in recognition

Polysaccharides Simpler structures –Homopolymers –Less branching Limited Cellular Functions –Structural/Protective –Energy Storage

Linear Polysaccharides

Branched Polysaccharides

Functions of Polysaccharides Structural - e.g. plant cell walls, cement between cells (animals):  -linkages stable to enzymatic cleavage Storage - e.g. glycogen as energy reserves:  -linkages are readily cleaved Potential osmotic problem Accessibility for energy production  -linkages Branching

Cellulose (plant cell walls)

Chitin  (1—>4)-linked homopolymer of N-acetylglucosamine Exoskeletin of invertebrates (e.g. crustacians, insects, and spiders) Cell wall (most fungi and some algae)

Glycogen (storage) Linear: α1  4 Branches: α1  6

Starch (plants) linear branched (similar to glycogen, but fewer branches)