Carbohydrates

Question 1

____ form of sugars is the predominant form in humans

Answer 1

D-form of sugars

Question 2

contrast aldoses and ketoses

Answer 2

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

Question 3

name 3 conditions where sugars are present in urine

Answer 3

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

Lactose intolerance patients do not have sugars in urine

Question 4

name the C-4 epimers

Answer 4

glucose and galactose

Question 5

name the C-2 epimers

Answer 5

glucose and mannose

Question 6

describe sorbitol

Answer 6

• 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)

Question 7

describe galactitol

Answer 7

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

Question 8

describe glucuronic acid

Answer 8

• 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

Question 9

name the aldoses and ketoses

Answer 9

Question 10

describe lactose

Answer 10

• 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)

Question 11

describe sucrose

Answer 11

• 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)

Question 12

describe fructose

Answer 12

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

Question 13

describe maltose

Answer 13

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

Question 14

contrast homopolysaccharides and heteropolysaccharides

Answer 14

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

Question 15

describe glycogen

Answer 15

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

Question 16

describe starch

Answer 16

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

Question 17

contrast amylopectin and glycogen

Answer 17

• 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

Question 18

describe cellulose

Answer 18

• 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

Question 19

describe glycoproteins

Answer 19

• 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