Skip to content

Carbohydrates

Definition

  • General formula: \((CH_2O)_n\), with \(N \ge 3\)

Roles

  • energy
  • signaling
  • adhesion
  • lubricants
  • protein trafficking
  • structural

Carbonyl chemistry in sugars

  • carbonyl group: \(C=O\)

Ketone vs aldehyde

  • ketone: carbonyl with two R groups

  • keto- / ketose (when the \(C=O\) in a saccharide is a ketone)

  • aldehyde: carbonyl with one R group and one H

  • aldo- / aldose

Monosaccharides

  • simplest carbohydrates

Examples:

  • ketone with two \(CH_2OH\) groups: dihydroxyacetone
  • glucose: aldose
  • fructose: ketose

By carbon number:

  • triose (3C)
  • tetrose (erythrose, erythrulose)
  • pentose (ribose)
  • hexose (glucose, galactose, fructose)

Key rule:

  • count carbon from the carbonyl end

Stereoisomers

Enantiomers

  • all chiral centers are opposite (all groups on chiral carbons must be opposite of each other)

Fischer projection

  • up/down bonds point back
  • left/right bonds point front

D and L naming

Rule :

  • if the hydroxyl group on the chiral carbon is on the right in the Fischer projection → D
  • if on the left → L
  • if there are multiple chiral carbons, use the one furthest from the ketone/aldehyde

Diastereoisomers

  • 2 or more chiral centers are opposite, but not all are opposite

Epimers

  • only one chiral center is opposite

Penicillin :

  • stereoisomer shift

Cyclization of monosaccharides

Hemiacetal vs hemiketal

  • aldehyde + alcohol = hemiacetal
  • ketone + alcohol = hemiketal

Glucose ring formation

  • \(C_1\) reacts with \(C_5\) for glucose

  • forms either:

  • \(\alpha\)-D-glucopyranose (OH below the ring)

  • \(\beta\)-D-glucopyranose (OH above the ring, stable)

Key terms:

  • the new chiral carbon is the anomeric carbon
  • conformations: chair (stable), boat

Fructose rings

  • 6-member ring: pyranose
  • 5-member ring: furanose

Glycosidic bonds

  • “glycosidic” = sugar bonded to something else
  • includes sugar–sugar linkages

Example: \(\alpha(1\to4)\) glycosidic bond

  • anomeric \(C_1\) (alpha OH) to \(C_4\) OH

Other examples

  • cytidine: sugar to cytidine, \(\beta\)-N-1,1
  • isomaltose: \(\alpha\)-O-1,6

Reducing vs non-reducing sugars

  • reducing sugar: has a free anomeric OH
  • non-reducing sugar: does not have a free anomeric OH

Disaccharides

Lactose

  • galactose \(\beta(1\to5)\) glucose

Sucrose

  • glucose \(\alpha(1\to2)\) \(\alpha\) fructose

Maltose

  • glucose \(\alpha(1\to4)\) glucose

Isomaltose

  • glucose \(\alpha(1\to6)\) glucose

Lactase and lactose intolerance

  • lactase digests lactose in the small intestine → monosaccharides
  • lactose may not be digested if lactose intolerant
  • fermentation occurs in the large intestine by microbiota
  • diarrhea and may cause inability to absorb nutrients

Polysaccharides

Starch

  • composed of \(\alpha\)-D-glucose polymers:

  • amylose (~20%): linear, \(\alpha\)-O-1,4

  • amylopectin (~80%): branched, \(\alpha\)-O-1,4 and \(\alpha\)-O-1,6

Digestion

Saliva

  • contains \(\alpha\)-amylase
  • endoglycosidase (can attach to branching point of amylopectin)
  • hydrolyzes \(\alpha\)-O-1,4

Stomach

  • forms alpha-dextrins
  • inactivates \(\alpha\)-amylase

Small intestine

  • \(\alpha\)-amylase with \(HCO_3^-\)
  • forms tri- and oligosaccharides, maltose, isomaltose
  • maltase, isomaltase, lactase, sucrase break sugars into monosaccharides for absorption

Glycemic index

  • sugars go to liver after intestine

Glycogen

  • branching frequency: about 8–12 glucoses per branch point
  • stored in glycogen granules
  • has a center part of two enzymes and all attached to it