Monosaccharides and Glycosides

Question 1

Sugars and sugar derivatives play numerous additional roles besides simply providing chemical energy for the synthesis of ATP. This includes functional and structural roles in

Answer 1

Glycosaminoglycans, glycolipids, and proteoglycans

Question 2

The simple sugars galactose, glucose, mannose, and fructose have the same chemical formula (C6H12O6). Hence, these four sugars are

Answer 2

Isomers

Question 3

Among isomers, a geometric difference at any single carbon other than the carbonyl carbon results in

Answer 3

Epimers

Question 4

By virtue of intramolecular reactivities, sugars can exist in either straight chain or ring conformations. The predominant structure of aldoses such as glucose and galactose is a

Answer 4

Six membered hemi-acetal ring

Question 5

This six-membered hemi-acetal ring is also referred to as a

Answer 5

Pyranose ring

Question 6

Adopt either a five membered hemiketal ring structure (4 carbons, 1 oxygen), also called a furanose ring or a six-­membered ring

-ex: Fructose

Answer 6

Ketoses

Question 7

Form a six membered (5 carbons, 1 oxygen) hemiacetal ring, also referred to as a pyranose ring

-ex: Glucose and Galactose

Answer 7

Aldoses

Question 8

In all these cases, ring formation will result in two alternative configurations at the

Answer 8

Anomeric carbon (Carbon #1 of aldoses and #2 of Ketoses)

Question 9

Structural variants about the anomeric carbon are called

Answer 9

Anomers

Question 10

Anomers, (expressed as a and B) equilibrate with each other via the

Answer 10

Open chain intermediate

Question 11

Mirror image structural variants are called

-ex: D- and L-Glucose

Answer 11

Enantiomers

Question 12

A trisaccharide of galactose, glucose, and fructose

Answer 12

Raffinose

Question 13

A Disaccharide of glucose

Answer 13

Trehalose

Question 14

Extensively polymerized sugars are also prevalent in our diets, including

Answer 14

Amylose, amylopectin, and glycogen

Question 15

While simple sugars are absorbed directly, disaccharides and polysaccharides must first be

Answer 15

Hydrolyzed to simple sugar constituents

Question 16

Digestion of start begins with

Answer 16

Salivary a-amylase

Question 17

Continues this process once the acidic contents of the stomach have been neutralized

Answer 17

Pancreatic a-amylase

Question 18

The bulk of CHO digestion (hydrolysis) occurs at the mucosal lining of the jejunum, with the secretion of additional digestive enzymes, such as

Answer 18

Glucoamylase/maltase (a-1,4), Sucrase/isomaltase (a-1,6), and lactase

Question 19

The products of these normally quite efficient hydrolyzing enzymes are the three simple sugars

Answer 19

Glucose, fructose, and galactose

Question 20

ABSORPTION of these three sugars, in the intestinal epithelium, is the responsibility of which three transporters?

Answer 20

SGLT1, GLUT5, and GLUT2

Question 21

The principal lumenal transporters

Answer 21

SGLT1 and GLUT5

Question 22

The principle transporter into the capillaries

Answer 22

GLUT2

Question 23

They arise by a number of means, with their root cause being the incomplete digestion

Answer 23

Di- and oligosaccharides

Question 24

The passage of significant quantities of osmotically active complex sugars into the large intestine results in

Answer 24

Bloating, dehydration, and gas (i.e. CO2 and H2 production)

Question 25

Failure to fully digest CHO can stem from genetic deficiency for one of the above hydrolases, temporary loss of hydrolase activity through other intestinal pathology, and

Answer 25

"Adult" onset of enzyme deficiency

Question 26

Perhaps the most common of these age-­dependent onset phenomena

Answer 26

Lactase deficiency

Question 27

Up to half of all adults, and up to 90% of blacks and Asians are deficient for

Answer 27

Lactase

Question 28

Diagnosis of these enzyme deficiencies is done by

Answer 28

Oral tolerance tests

Question 29

How do cells essentially "capture" incoming sugars?

Answer 29

Phosphorylation

Question 30

Isn’t transported by SGLT1, GLUT2, or GLUT5, and the presence of the phosphate makes passive transit of the membrane an extremely low probability event

Answer 30

Glucose-6-phosphate

Question 31

Glucose and galactose have the same chemical formula and are both aldoses. How does the cell convert one into the other?

Answer 31

Epimerization

Question 32

Because glucose and fructose differ only in the location of the carbonyl carbon, how can the cell convert one to the other?

Answer 32

Isomerization

Question 33

An additional transformation that sugars undergo is their conversion to

Answer 33

Nucleotide linked forms (ex: UDP glucose)

Question 34

The integration of fructose into mainstream CHO metabolism requires only two steps, occurring principally in the

Answer 34

Liver, intestines, and kidneys

Question 35

Whereas intestinal fructose absorption is relatively slow, the first step in fructose metabolism is a very

Answer 35

Fast step

Question 36

Both recruit dietary fructose as a substrate, to produce fructose 6-­phosphate and fructose 1-­phosphate (F-1-­P), respectively

Answer 36

Hexokinase and Fructokinase

Question 37

Hexokinase is comparatively quite inefficient at this, hence the significant majority of fructose is converted to fructose 1-­phosphate by

Answer 37

Fructokinase

Question 38

Only makes substantial contribution to fructose metabolism at very high substrate concentrations

Answer 38

Hexokinase

Question 39

Cleaves Fructose1-­P to D-glyceraldehyde and dihydroxyacetonephosphate (DHAP)

Answer 39

Aldolase B

Question 40

Can enter pathways for glycolysis or gluconeogenesis

Answer 40

DHAP

Question 41

Can enter glycolysis or be transformed into glycerol, the backbone for lipids

Answer 41

D-glyceraldehyde

Question 42

Pathologies associated with dietary fructose take which two basic forms?

Answer 42

1. ) Excessive intake | 2. ) Hereditary deficiency for specific enzymes

Question 43

In both cases, fructose toxicity is related to the formation of large amounts of

Answer 43

F-1-P

Question 44

In both cases, fructose toxicity is related to the formation of large amounts of fructose 1-­phosphate, with very slow turnover into

Answer 44

D-glyceraldehyde and DHAP

Question 45

Because of the comparatively slow rate of catalysis by aldolase B (compared to that for fructokinase), under conditions of high dietary fructose intake there can be a significant build up of

Answer 45

F-1-P

Question 46

A buildup of F-1-P causes a sequestration of the cell’s supply of

Answer 46

Pi

Question 47

This, in turn, causes a decrease in available ATP levels, with concomitant increases in

Answer 47

ADP and AMP

Question 48

Deamination of ADP and AMP results in

Answer 48

Hyperuricemia

Question 49

Eventual action of aldolase B on this built up Fructose 1-­P supply results in elevated levels of

Answer 49

Pyruvate and lactate

Question 50

Also builds up by virtue of its competition with uric acid for filtration by the kidney

Answer 50

Lactate

Question 51

Also increases under these high fructose conditions

Answer 51

Fatty acid and TAG synthesis

Question 52

Because of these several adverse effects of high fructose in the diet, early theories that fructose is a ‘safe’ sugar for diabetics to use in place of glucose have largely been

Answer 52

Abandoned

Question 53

Apart from the adverse effects of elevated uric and lactic acids, and elevated levels of fatty acids and triacylglycerols, the liver also metabolizes fructose-­derived trioses via gluconeogenesis, thereby

Answer 53

Raising blood glucose anyway

Question 54

Inborn errors in fructose metabolism fall into which two groups?

Answer 54

Essential fructosuria and hereditary fructose intolerance

Question 55

Results from the absence of fructokinase. -This is a rare and asymptomatic deficiency

Answer 55

Essential Fructosuria

Question 56

Can be quite serious and has biochemical consequences very similar to that described above for excessive fructose intake in a normal individual

Answer 56

Hereditary Fructose Intolerance (HFI)

Question 57

High levels of Fructose1-­P build up (up to 10 mM) even in the absence of high fructose (or sucrose) intake with

Answer 57

HFI

Question 58

Clinical manifestations of HFI include

Answer 58

Hypoglycemia, nausea, and vomiting after mild fructose exposure

Question 59

Infants with HFI struggle with a failure to thrive and develop, including longer term concerns for

Answer 59

Liver damage and dysfunction

Question 60

What is the aim of treatment for HFI?

Answer 60

Elimination of fructose from the diet

Question 61

Prevents inhibition of glucokinase by fructose 6-­phosphate

Answer 61

F-1-P

Question 62

The principal monosaccharide in seminal fluid

Answer 62

Fructose

Question 63

Fructose is synthesized in the seminal vesicles (along with other tissues, in both sexes) by the so-­called

Answer 63

Polyol pathway

Question 64

This two step conversion of glucose to fructose (polyol pathway) is driven forward by high concentrations of

Answer 64

NADPH and NAD

Question 65

The intermediate compound of the polyol pathway

Answer 65

Sorbitol

Question 66

Has a high Km for glucose -the first enzyme in the polyol pathway

Answer 66

Aldose reductase

Question 67

Sorbitol accumulates in the lens and in other tissues in which aldose reductase is found in

Answer 67

Diabetics

Question 68

Increased intracellular sorbitol concentrations cause

Answer 68

Tissue damage

Question 69

The integration of galactose into mainstream liver CHO metabolism offers a few branch points, but also includes only a few

Answer 69

Enzymatic Steps

Question 70

Catalyzes the formation of galactose 1-­phosphate (gal1- | P)

Answer 70

Galactokinase

Question 71

Gal1-­P is then converted into

Answer 71

UDP Galactose

Question 72

Gal1-­P is then converted into UDP-­galactose by an exchange reaction with UDP-­glucose, catalyzed by

Answer 72

Galactose 1-­phosphate uridyl transferase

Question 73

The final step is the epimerization of UDP- | galactose to

Answer 73

UDP-glucose

Question 74

The final step is the epimerization of UDP-galactose to UDP-­glucose, carried out by

Answer 74

UDP-galactose 4-epimerase

Question 75

There are alternative fates for this product UDP-­glucose. It may be converted into

Answer 75

Glycogen or G-1-P

Question 76

UDP-­glucose has yet other fates in the synthesis of

Answer 76

Lactose, glycolipids, glycoproteins, and proteoglycans

Question 77

Formation of essential components of cell walls and membranes are the more critical fates for

Answer 77

Galactose

Question 78

It is possible to completely eliminate dietary galactose and yet still supply cells with all that they require through the reversible action of

Answer 78

UDP-­galactose 4-­epimerase

Question 79

Caused by a deficiency in Gal1-­P uridyl transferase, leading to a build up of both galactose and gal1-­P

Answer 79

Galactosemia

Question 80

Classical Galactosemia is caused by a deficiency in

Answer 80

Gal-1-P Uridyl transferase

Question 81

Partly because of the evident lack of toxicity in high galactose levels, it is believed most negative effects seen in galactosemia are attributable to

Answer 81

Gal-1-P

Question 82

Another problem with galactosemia is buildup in the lens of

Answer 82

Galactitol

Question 83

Can lead to cataracts by virtue of the production of galactitol in the lens, via aldose reductase

Answer 83

Galactokinase deficiency

Question 84

Galactokinase deficiency can lead to cataracts by virtue of the production of galactitol in the lens, via

Answer 84

Aldose reductase