TOPIC 8 MONO- AND DISACCHARIDES METABOLISM OBJs

Question 1

1. Explain the location and role of disaccharidases in digestion

Answer 1

• Disaccharidases are glycoside hydrolase enzymes found in the intestinal brush border that are responsible for the breakdown of disaccharides into monosaccharides.

Question 2

2. Identify the causes of lactose and sucrose intolerance

Answer 2

• Due to not having the enzymes lactase and sucrase

Question 3

Other main mono sugars here are

Answer 3

Fructose and galactose, different due to no insulin dependence to enter the cell

Question 4

3. Describe the two routes of fructose metabolism

Answer 4

1) phosphorylated by hexokinase -

Fructose + ATP — Hexokinase—> Fructose-6-P + ADP

• F-6-P can feed straight into glycolysis

2) Fructokinase in the liver - phosphate added

Fructose + ATP –Fructokinase–> Fructose-1-P + ADP

• F-1-P goes to glyceraldehyde or DHAP and then to G-3-P which feeds directly into glycolytic pathway

Question 5

4. Explain the biochemical basis of fructosuria

Answer 5

• Lack fructokinase

Question 6

5. Explain the biochemical basis of hereditary fructose intolerance and why it can lead to hypoglycemia

Answer 6

• Defective phosphofructoaldolase
• Consuming fructose-containing sugars leads to an increase in Fructose-1-P
• Leads to Hypoglycemia (low blood glucose)
• Problem is due to Fructose-1-P being an inhibitor of two enzymes
• Effects glycolysis - result: reduction of glucose supplied from gluconeogenesis and glycogen breakdown in the liver, causing the drop in blood glucose levels.

Question 7

6. Describe the route of galactose metabolism

Answer 7

• Major route is by a specific kinase = galactokinase

Question 8

7. Explain the biochemical basis of galactosemia and why if a child with galactosemia survives into adolescence, how can the metabolic block in galactose metabolism be overcome

Answer 8

• In galactosemia, you lack uridyl transferase
• Can’t go to UDP-galactose
• If infant survives, can overcome galactose intolerance
• Increase in activity of UDP galactose pyrophosphorylase, which is virtually absent from foetal and infant liver.

Question 9

8. Describe the route of mannose metabolism

Answer 9

• Mannose is phosphorylated by hexokinase to mannose-6-P
• Mannose-6-P is then isomerised to fructose-6-P, to feed into glycolysis, by phosphomannose isomerase

Question 10

Majority of fructose metabolised by

Answer 10

fuctokinase

Question 11

fructokinase

Answer 11

• phosphorylates
• low Km or high affinity
• liver, kidney and small intestine

Question 12

major carbohdrate for sperm cell

Answer 12

fructose

Question 13

Galactose initially broken down by which enzyme

Answer 13

galactokinase

Question 14

For galactose to be useful in the cell it needs to be converted to UDP-galactose

Answer 14

true

Question 15

disaccharides

Answer 15

maltose
isomaltose
sucrose
lactose

Question 16

lactose is _____ active

Answer 16

osmotically -> brings water to intesting, bloating and diarrhoea

Question 17

monosaccharides

Answer 17

glucose
fructose
galactose
mannose

Question 18

Fructosuria

Answer 18

lack of fructokinase

cant convert fructose to fructose-1-phosphate

Question 19

hereditary fructose intoleterance (HFI)

Answer 19

lack phosphofructoaldolase
- deficient aldolase
- so fructose is still converted to Fructose-1-P, but can’t go any further
- F-1-P ^ leads to hypoglacema (low blood sugar)

Question 20

Galactosemia

Answer 20

lack of uridyl transferase