Carbohydrates and Monosaccharides

Carbohydrates

• Carbohydrates are based on the formula (CH2O)n, where n is greater than or equal to 3
• Simple sugars are called monosaccharides
• Polymers of simple sugars are called oligo- and polysaccharides

Monosaccharide Classification

• Monosaccharides are classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group)
• They are also classified based on the number of carbon atoms they contain: triose (3 carbons), tetrose (4 carbons), pentose (5 carbons), hexose (6 carbons), etc.

Stereochemistry

• Monosaccharides with 3 or more carbons (aldoses) and 4 or more carbons (ketoses) have asymmetric carbons (chiral centers)
• D- and L- configurations refer to the configuration of the highest numbered asymmetric carbon
• D- and L- forms are mirror images and are called enantiomers
• Molecules with more than one asymmetric carbon can be diastereomers: they differ in configuration at one or more chiral carbons but are not mirror images
• Epimers are two molecules that differ in configuration about a single asymmetric carbon

Ring Structures

• Pyranoses are six-membered, oxygen-containing rings
• Furanoses are five-membered, oxygen-containing rings
• The ketone or aldehyde carbon that becomes chiral upon ring formation is called the anomeric carbon
• Anomers (α and β) differ in configuration about the anomeric carbon
  • In the α-configuration, the OH of the anomeric carbon is on the same side as the OH of the highest numbered asymmetric carbon in a Fischer projection
  • In the β-configuration, the OH of the anomeric carbon is on the opposite side as the OH of the highest numbered asymmetric carbon in a Fischer projection
• Haworth projections provide a three-dimensional representation of ring structures: groups to the right in a Fischer projection are drawn down in the Haworth projection
• Chair and boat conformations are possible due to ring pucker, with axial and equatorial orientation of groups attached to the ring

Sugar Derivatives

• Oxidation of the free anomeric carbon to carboxylate results in acids
• Other derivatives include alcohols, deoxysugars, esters (like ATP and GTP), amino sugars (like glucosamine), muramic acid, and glycosides (anomeric carbon reacted with an alcoholic function)
• Sugars with a free anomeric carbon are reducing sugars
• The end of a sugar polymer with a free anomeric carbon is called the reducing end; the opposite end is the nonreducing end

Oligosaccharides

• Disaccharides include:
  • Maltose (diglucose)
  • Lactose (galactose and glucose)
  • Sucrose (fructose and glucose) - a nonreducing sugar

Storage Polysaccharides

• Starch
  • Composed of α-amylose and amylopectin
  • α-amylose is a linear chain of α(1→4)D-glucose
  • Amylopectin is also a linear chain of α(1→4)D-glucose but with branches every 12 to 30 residues, created by α(1→6)D-glucose linkages
  • In animals, α-amylase hydrolyzes internal α(1→4) glycosidic linkages in starch
  • Microorganisms use α-amylase as an exoamylase, releasing the disaccharide maltose
• Glycogen, similar to amylopectin, is composed of α(1→4) D-glucose chains with α(1→6) D-glucose branches every 8 to 12 residues
• Dextrans, produced by bacteria, are α(1→6) D-glucose polymers

Structural Polymers

• Cellulose: a linear polymer of β(1→4) D-glucose, forming the plant cell wall
• Chitin: a β(1→4) N-acetyl-D-glucosamine polymer, found in exoskeletons and fungal cell walls

Glycosaminoglycans

• These are polymers with repeating disaccharide units and are negatively charged

Peptidoglycan of Bacterial Cell Walls

• Gram-negative bacterial cell walls
  • Contain NAM-NAG repeats crosslinked with a tetrapeptide
  • The tetrapeptide is attached to an outer cell wall via a 57-amino acid protein
  • This protein is linked to aminopimelic acid (which replaces 10% of D-alanines)
  • The outer cell wall contains lipopolysaccharides, which are antigenic
• Gram-positive bacterial cell walls
  • Contain NAM-NAG repeats crosslinked with tetrapeptides and pentaglycines
  • These bacteria lack an outer cell wall
  • They contain teichoic acids: polymers of ribitol phosphate or glycerol phosphate

Glycoproteins

• Functions include structural support, enzyme activity, receptors, transport, and immunoglobulins
• Linked by:
  • O-linked: the amino acids serine, threonine, or hydroxylysine are linked to sugars, with N-acetylgalactosamine being the most common
  • N-linked: asparagine is linked to sugars, playing roles in protein folding and turnover

Proteoglycans

• These are glycoproteins
• Contain glycosaminoglycans as their sugar component
• Interact with a variety of molecules