sakai-Carbohydrate Structure and Function Flashcards
Outline the classification of monosaccharides based on number of carbon atoms, name four hexoses. Which pentoses found in RNA and DNA and which linkage is used in nucleosides?
Trioses (3C), Tetroses (4C), Pentoses (5C), Hexoses (6C), Heptoses (7C).
Hexoses are glucose, fructose, galactose, and mannose. RNA contains ribose and DNA contains deoxyribose, these pentose sugars are linked to the purine or pyrimidine base by a -N-glycosidic linkage.
Which functional group is characteristic for aldoses in comparison to ketoses? Name three sugars that are substrate for aldose reductase and explain why this enzyme does not use fructose.
Aldoses contain an aldehyde group on carbon #1, ketoses contain a keto group on carbon #2.
Aldose reductase forms sugar alcohols and it can use glucose, galactose or mannose as substrates. It cannot use fructose as substrate as fructose is a ketose and not an aldose.
What is mutarotation? Which form of glucose is predominant in solution? What are enatiomers? Is glucose in the L- form or is it the D-form that is predominant in humans? What is a pyranose and what is a furanose?
Mutarotation is the process of the inter-conversion of the two cyclic forms ( and ) when the ring is opened at the anomeric carbon.
The predominant form of glucose in solution is the -form.
Enantiomers are mirror images of each other (L-form and D-form).
The D-form of sugars is the predominant form in humans. (note and compare: it is the L-form for amino acids)
The cyclic form of monosaccharides allows the distinction into pyranose: which refers to a six-membered ring (glucopyranose) and also into furanose: which refers to a five-membered ring.
What is a reducing sugar? Is the disaccharide sucrose a reducing sugar? Is the disaccharide lactose a reducing sugar? Explain!
Monosaccharides are reducing sugars as they can open their ring structure and react with their functional group (aldehyde or keto group) and reduce a specific chromogenic agent.
The disaccharide sucrose is not a reducing sugar, as both anomeric carbons of glucose (C#1) and fructose (C#2) are linked to each other via an 1,2 glycosidic bond and the opening of the ring structure is not possible.
The disaccharide lactose is a reducing sugar, as although the anomeric carbon of galactose (C#1) is in the -1-4 linkage, the anomeric carbon (C#1) of glucose is not locked in the glycosidic bond and can open and react with its aldehyde group (Benedicts test).
When you find a reducing sugar in urine, which abnormality and clinical condition can be the reason? Name three.
Sugars are not normally present in urine but can be detected in clinical conditions.
- Glucose is found in urine in diabetic patients.
- Fructose is found in urine in fructosuria or in patients with hereditary fructose
intolerance ( fructokinase deficiency or aldolase B deficiency) - Galactose is found in urine in patients with galactosemia or with classical
galactosemia (galactokinase deficiency or uridyl transferase deficiency).
⦁ In a diabetic patient, what is the respective sugar alcohol of glucose that can accumulate in lens and nerve endings and kidney? Which enzyme is involved?
High concentrations of serum glucose as found in diabetic patients, leads in the lens, nerve endings and kidney to accumulation of the sugar alcohol sorbitol, formed by aldose reductase from glucose. This reaction is minute at normal glucose concentrations in these cells but is increased at elevated blood glucose levels. Aldose reductase has a large Km for glucose and acts at high intracellular glucose levels.
[note: Sorbitol formation is wanted in hepatocytes after a carbohydrate-rich meal . Aldose reductase in the liver forms sorbitol at high glucose levels. The liver contains as follow-up the enzyme sorbitol dehydrogenase which is not found in the lens or nerve endings or kidney.
Sorbitol dehydrogenase forms fructose from sorbitol. This fructose is trapped in the liver by fructokinase and fructose 1-P is formed.
Aldose reductase and glucokinase trap glucose in the liver cell (sorbitol and glucose 6P) at high blood glucose concentration.]
[Note: sorbitol dehydrogenase is also found in seminal vesicle cells and allows there the eventual formation of fructose from glucose via sorbitol. GLUT-5 in these cells facilitate the release of fructose into the seminal fluid. Sperm cells use fructose for their energy metabolism.]
⦁ In a patient with classical galactosemia, which enzyme is deficient, what is the respective sugar alcohol of galactose and where does it accumulate?
Classical galactosemia results from the deficiency of galactose 1-P uridyl transferase.
High blood levels of galactose are an abnormality and in these patients, galactose is substrate for aldose reductase. Galactitol accumulates in the lens, nerve ending and kidney and also in the liver.
[In contrary to sorbitol, which does not accumulate in the liver, galactitol accumulates also in the liver, there is no liver enzyme to get rid of galactitol. This sugar alcohol was not meant to be formed by aldose reductase and damages the liver cells.]
⦁ Does sucrase generate equal amounts of glucose and fructose? What is HFCS 55?
Sucrose contains equal amounts of glucose and fructose and is cleaved by sucrase in the digestive tract.
HFCS 55 is High Fructose Corn Syrup with 55% fructose and 42% glucose. It is special that HFCS is a mixture of the monosaccharides fructose and glucose.
[note: The monosaccharide fructose is perceived as tasting sweeter than glucose or sucrose. Fructose is used as sweetener in soft drinks or it is found naturally in higher concentration in sweet fruits, fruit sugar]
⦁ Which sugar is the C-4 epimer to glucose? Which sugar is the C-2 epimer to glucose? Which sugar is an isomer form of glucose?
Glucose is C-4 epimer to galactose and glucose is C-2 epimer to mannose. Glucose is an isomer form of fructose.
[note: UDP-glucose can be changed in humans to UDP-galactose in a reversible reaction by an UDP-hexose-4-epimerase.
Glucose 6-P can be changed in a reversible reaction to fructose 6-P by phosphoglucose isomerase]
⦁ Which digestive enzyme generates maltose and which monosaccharides are found in maltose? What is isomaltose?
Maltose is formed during the digestion of starch and glycogen by salivary and pancreatic -amylase. Maltose contains two glucose units linked by an (1-4) glycosidic bond.
Isomaltose contains glucose units linked by an (1-6) glycosidic bond which formed the branch point of starch or dietary glycogen.
⦁ Which food is rich in lactose and which monosaccharides are found in lactose? Which enzyme cleaves lactose? When is this enzyme most active?
Lactose is found in higher quantities in milk (milk sugar) with about 11g per cup.
Lactose contains galactose and glucose in a 1-4) glycosidic bond.
Lactase cleaves lactose at the brush border of intestinal mucosal cells.
Lactase is most active after birth and important for milk digestion.
[Natural decline of lactase activity can lead to primary lactose intolerance in children and adults. Loss of lactase due to severe diarrhea can lead to secondary lactose intolerance.]
⦁ Can humans form a sugar alcohol from fructose? Explain!
No, fructose is a ketose and it is not a substrate for aldose reductase.
Aldose reductase forms sugar alcohols from aldoses [glucose (sorbitol), galactose (galactitol) and mannose (mannitol)]
⦁ Describe the composition of starch
Starch is a plant polymer of glucose composed of amylose [linear, unbranched, 1-4) linkages] and amylopectin [branched with 1-6) linkages at branch points].
⦁ Describe the composition of glycogen
Glycogen is an animal polymer of glucose with linear (1-4) glycosidic linkages and with (1-6) linkages at branch points. It is used for storage of glucose in animals and it is found in cytosolic glycogen granules that are highly branched.
⦁ Which type of linkages do you find at branch points of starch and glycogen?
Branch points contain (1-6) glycosidic linkages.
[note: In starch or dietary glycogen, these linkages are not cleaved by -amylase during digestion but later on by isomaltase at the mucosal brush border]
[note: In the cellular metabolism of cytosolic glycogen, these linkages are not cleaved by glycogen phosphorylase during glycogen degradation and need the glycogen debranching enzyme for cleavage]
