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Flashcards in Carbohydrates Deck (19)
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____ form of sugars is the predominant form in humans

D-form of sugars


contrast aldoses and ketoses

  • aldoses have an aldehyde group on C1 whereas ketoses have a keto group on C2


name 3 conditions where sugars are present in urine

  1. diabetes mellitus
  2. fructosuria and hereditary fructose intolerance
  3. galactosemia

Lactose intolerance patients do not have sugars in urine


name the C-4 epimers

glucose and galactose


name the C-2 epimers

glucose and mannose


describe sorbitol

  • sugar alcohol formed from glucose
  • formed in the nerve tissues, retina and the lens of the eye when the blood glucose is elevated (prolonged hyperglycemia in diabetics)


describe galactitol

galactitol is formed from galactose in the lens in children with untreated galactosemia


describe glucuronic acid

  • glucuronic acid is derived by oxidation of C-6 of glucose 
  • UDP-glucuronic acid is used in conjugation rxns (bilirubin, steroids)
  • glucuronic acid is a component of GAGs (hyaluronic acid, heparin) and contributes to the negative charge 


name the aldoses and ketoses


describe lactose

  • disaccharide made up of 2 monosaccharides linked by a glycosidic linkage
    • galactose + glucose linked by B1->4 glycosidic linkage
  • lactose is a reducing sugar (positive Benedict's test)


describe sucrose

  • glucose + fructose
  • non-reducing sugar since the C1 of glucose and C2 of fructose are involved in the glycosidic linkage (aldehyde and keto groups are not free)


describe fructose

  • fructose is a ketohexose
  • HFCS contains 55% fructose and 42% glucose (used as sucrose substitute in soft drinks)


describe maltose

  • maltose = 2 glucose units
    • linked by an a1->4 glycosidic linkage
  • maltose is a reducing sugar


contrast homopolysaccharides and heteropolysaccharides

  • homopolysaccharides: similar monosaccharide units
    • starch, glycogen, cellulose (made up of glucose units)
  • heteropolysaccharides: different monosaccharide units
    • glycosaminoglycans/mucopolysaccharides


describe glycogen

storage polysaccharide

  • found in liver and muscles as cytosolic glycogen granules
  • the glucose units in the linear chain are linked by a1->4 glycosidic linkages and glucose units at the branch points are linked by a1->6 glycosidic linkages
  • glycogenin is the core protein surrounded by branches of glycogen with about 30,000 glucose units


describe starch

dietary polysaccharide

  • starch is made up of 2 components:
    • amylose (linear unbranched polymer of glucose)
    • amylopectin (branched polymer of glucose)
  • starch is made up of glucose units in the linear chain linked by a1->4 linkages. Glucose units at the branch point in amylopectin are linked by a1->6 linkages
  • starch is the major dietary polysaccharide digested by salivary and pancreatic amylase


contrast amylopectin and glycogen

  • amylopectin has fewer number of branches compared to glycogen
  • enzymes of glycogen metabolism act on the ends of the branches of glycogen and sequentially remove or add glucose from the ends of the glucose molecule


describe cellulose

  • majority of the dietary fiber is undigestible carbohydrate
    • examples are cellulose and pectin
  • cellulose is an unbranched homopolysaccharide containing glucose units linked by B1->4 linkages 
    • B1->4 linkages cannot be cleaved by digestive enzymes
  • lactase can hydrolyze the B1->4 linkages only in the dissacharide lactose--not in cellulose


describe glycoproteins

  • glycosylation may occur as either one type of linkage or with both on the same protein
  • O-linked
    • glycosylation on the OH group of Ser/Thr
    • often found as extracellular proteins/membrane bound
    • glycan groups always face extracellular side
  • N-linked
    • glycosylation on the Asn residue