WEEK 5 (Monosaccharide Metabolism)

Question 1

Which Monosaccharides are isomers of Glucose?

Answer 1

Fructose & Galactose

Question 2

Which monosaccharides make up Sucrose and Lactose?

Answer 2

SUCROSE = GLUCOSE + FRUCTOSE
LACTOSE = GLUCOSE + GALACTOSE

Question 3

Galactose and Glucose are taken up by which transporter?

Answer 3

SGLT1 which is a Na+ dependent transporter

Question 4

Fructose is taken up by which transporter?

Answer 4

GLUT 5 by Facilitated diffusion

Question 5

How do Galactose, Glucose & Fructose leave enterocytes?

Answer 5

By GLUT 2

Question 6

What is Fructose?

Answer 6

A Ketohexose found in honey and a wide variety of fruits and vegetables

Question 7

Describe Fructose Metabolism

Answer 7

1) FRUCTOSE -> FRUCTOSE-1-PHOSPHATE via FRUCTOKINASE (use of an ATP)
2) FRUCTOSE-1-PHOSPHATE -> DIHYDROXYACETONE PHOSPHATE + GLYCERALDEHYDE via ALDOLASE B
3) DIHYDROXYACETONE PHOSPHATE -> GLYCERALDEHYDE-3-PHOSPHATE via TRIOSE PHOSPHATE ISOMERASE or GLYCERALDEHYDE -> GLYCEROL (use of NADH) or GLYCERALDEHYDE -> GLYCERALDEHYDE-3-PHOSPHATE via TRIOSE KINASE (use of an ATP)
4) GLYCERALDEHYDE-3-PHOSPHATE -> GLYCOLYSIS

Question 8

Why is the rate of fructose metabolism more rapid than that of glucose?

Answer 8

Triose phosphate production from FRUCTOSE-1-PHOSPHATE bypasses PHOSPHOFRUCTOKINASE-1 which is the major rate-limiting step in glycolysis

Question 9

Describe the disease ‘Essential Fructosuria’

Answer 9

• Involves a defect in FRUCTOKINASE
• Autosomal recessive
• A BENIGN, ASYMPTOMATIC CONDITION since fructose is not trapped in cells

SYMPTOMS:
- Fructose appears in blood and urine

Question 10

Describe the disease ‘Hereditary Fructose Intolerance’ aka Aldolase B Deficiency

Answer 10

An autosomal recessive disorder where patients lack the enzyme to break down FRUCTOSE-1-PHOSPHATE (ALDOLASE B)

SYMPTOMS:
- Hypoglycaemia, Seizures & vomiting after fructose ingestion
- Failure to thrive
- Liver and renal failure
- Hepatomegaly

TREATMENT:
Avoid fructose, sucrose & sorbitol

Question 11

Fructose is the primary source of energy for ________________

Answer 11

Spermatozoa

Question 12

Fructose can be produced from glucose via ____________ without using ATP

Answer 12

Sorbitol

Question 13

What happens during hyperglycaemic states?

Answer 13

Excess glucose is shunted from the hexokinase/glycolysis pathway into another metabolic pathway -> Cells containing ALDOSE REDUCTASE convert glucose to SORBITOL which is converted to Fructose via SORBITOL DEHYDROGENASE

Question 14

What happens during hyperglycaemic states?

Answer 14

Excess glucose is shunted from the hexokinase/glycolysis pathway into another metabolic pathway -> Cells containing ALDOSE REDUCTASE convert glucose to SORBITOL which is converted to Fructose via SORBITOL DEHYDROGENASE

Question 15

What happens when organs lack Sorbitol dehydrogenase?

Answer 15

Sorbitol accumulates -> Excess sorbitol causes osmotic damage -> Changes seen in hyperglycaemic diabetic patients e.g diabetic cataracts, diabetic retinopathy, diabetic neuropathy & diabetic nephropathy

Question 16

Most galactose ingested by humans is in the form of ______________

Answer 16

Lactose

Question 17

How is lactose converted to glucose?

Answer 17

Lactose is hydrolysed to galactose and glucose by LACTASE located on the microvillar membrane of the small intestine -> Following absorption, galactose is transported to the LIVER where it is converted to glucose

Question 18

What are the symptoms of lactase deficiency after ingestion of lactose?

Answer 18

• Diarrhoea
• Bloating
• Cramps

Question 19

What happens if galactose accumulates in the blood?

Answer 19

Galactose is transferred to the lens where Galactose is converted to GALACTITOL via ALDOSE REDUCTASE -> Galactitol trapped in the lens causes swelling and cataracts

Question 20

What can happen to galactose in the well-fed state?

Answer 20

Galactose can enter glycolysis or contribute to glycogen storage

Question 21

Describe Galactose-1-Phosphate Uridyltransferase deficiency

Answer 21

An autosomal recessive disorder where there is an absence of galactose-1-phosphate uridyltransferase. Damage is caused by accumulation of toxic substances (e.g galactitol) & symptoms develop when infant begins feeding.

SYMPTOMS:
- Cataracts early in life
- Vomiting, diarrhoea following lactose ingestion
- Lethargy
- Liver damage
- Hyperbilirubinemia (a condition in which there is a build up of bilirubin in the blood, causing yellow discoloration of the eyes and skin, called jaundice)
- Mental retardation

COMPLICATIONS:
Can lead to E Coli sepsis in neonates

Question 22

Describe Galactokinase deficiency

Answer 22

An autosomal recessive disorder where there is a deficiency in GALACTOKINASE which causes Galactose to be converted to GALACTITOL in the eye lens.

SYMPTOMS:
- Galactose appears in blood (GALACTOSEMIA) & urine (GALACTOSURIA)
- Cataracts in early life

Question 23

What are cataracts?

Answer 23

Clouding of the eye lens that affects vision

Question 24

Which enzymes are correlated to mild galactosemia and classic galactosemia?

Answer 24

MILD GALACTOSEMIA = GALACTOKINASE

CLASSIC GALACTOSEMIA = GALACTOSE-1-PHOSPHATE URIDYLTRANSFERASE

Question 25

Where does Lactase function?

Answer 25

On the intestinal brush border to digest lactose

Question 26

What does an insufficient lactase enzyme lead to?

Answer 26

Lactase enzyme deficiency -> No monosaccharides (Glucose + Galactose) -> Dietary lactose intolerance

Question 27

What are the different Lactase deficiency types?

Answer 27

• PRIMARY
• SECONDARY
• CONGENITAL

Question 28

What are the properties of Primary Lactase Deficiency

Answer 28

• Age-dependent decline after childhood
• Common in people of Asian, African or Native American descent

Question 29

What are the properties of Secondary Lactase Deficiency?

Answer 29

• Autoimmune disease
• Loss of intestinal brush border due to gastroenteritis

Question 30

What are the properties of Congenital Lactase Deficiency?

Answer 30

• Rare
• Due to defective gene

Question 31

Describe Lactase deficiency

Answer 31

TESTING:
- Stool demonstrates decreased pH and breath shows increased hydrogen content with LACTOSE HYDROGEN BREATH TEST
- Intestinal biopsy reveals normal mucosa in patient with hereditary lactose intolerance

SYMPTOMS:
- Bloating
- Cramps
- Flatulence
- Osmotic diarrhoea

TREATMENT:
- Avoid dairy products/add lactase pills to diet
- Lactose-free milk