Glycosides - Medical Botany - Lecture Slides, Slides of Botany and Agronomy

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Glycosides

Glycosides

• Secondary products formed from one or more

sugar molecules added to a non-sugar

biologically active molecule

• Sugar most commonly found is glucose
• Non-sugar molecule known as the aglycone and may be - amino acid derivatives - steroids - triterpenes

Saponins

• Terpene glycosides
  • Steroid glycosides
  • Steroid-alkaloid glycosides
  • Triterpene glycosides
• May also occur as the aglycone and are called

sapogenins

Properties of Saponins

• Combination of hydrophobic terpene with a

hydrophillic sugar produces cmpds that act as

surfactants (detergents)

• When agitated in water, saponins form a

soapy lather

Useful saponins

• Saponins from yam - Disocorea spp. are the source of steroids used for human hormones - Birth control pills - most synthetic but some still use natural hormones - Anti-inflammatory steroids
• Licorice - Glycyrrhiza glabra produces saponins that possibly help gastric ulcers because they suppress prostoglandins

Cardiac glycosides

• Structurally similar to saponins with similar

detergent properties

• Distinguished by presence of a lactone ring

and “rare” sugars

• Wide distribution - found in over 200 species

(55 genera in 12 families)

• Best known are digitoxin and digoxin from

Digitalis (foxglove) – 30 glycosides

Digitoxin

Sugars in digitoxin

• 2 molecules of digitose
• 1 molecule of 1-acetyl digitose
• 1 molecule of glucose

Digitose

Poisonous Plants with Cyanogenic Glycosides

• Foxglove - Digitalis purpurea
• Oleander - Nerium oleander
• Lily of the Valley - Convallaria sp.
• Milkweed - Asclepias spp

Oleander

Milkweeds

• Another common source of glycosides are milkweeds
  • produce latex rich in cardiac glycosides
• Milkweed leaves host for monarch butterfly larvae
• Glycosides sequestered in body of larvae and retained through metamorphosis and present in butterfly
• Butterflies toxic to birds that feed on them causing vomiting - birds avoid butterflies

Cyanogenic glycosides

• About 150 spp of plants produce glycosides that release hydrogen cyanide - HCN
• Most cyanogenic glycosides are derived from 4 amino acids (phenylalanine, tyrosine, valine, or isoleucine) or from nicotinic acid
• Cyanogenic glycosides are not toxic by themselves but when a plant is damaged by an herbivore, glycoside is broken down by enzymes and HCN is released

Effects of HCN

• HCN inhibits ETS
• HCN completely inhibits the transfer of

electron to oxygen by cytochrome oxidase –

final step in the ETS

• This can lead to cell death and possibly even

the death of the organism if the concentration

is high enough

Electron Transport System