Hexose Metabolism

Question 1

Fructose is a….., the tissues that can use it are……

Answer 1

Ketohexose
Liver, kidney, intestinal mucosa, seminal vesicles, adipose tissue but not brain

Question 2

Describe the fructokinase reaction

Answer 2

It produces fructose 1-phosphate from fructose using ATP as phosphate donor, insulin-independent, its actovity depends on fructose conc.

Question 3

Describe the role of fructose in seminal vesicles

Answer 3

The energy for sperm mobility is derived from fructose.

Question 4

Explain atherogenic effect of fructose

Answer 4

Glucokinase and PFK1 are imp rate limiting steps in glucose metabolism which are absent in fructose. Thus fructose rapidly enters the tissues and enhances FA synthesis by giving DHAP converted to glycerol 6-phosphate used for TAG synthesis thus in TAG and LDL in blood so cause harmful atherogenic effect.

Question 5

Describe role of fructose in diabetic diet

Answer 5

Fructose is recommended instead of sucrose in diabetes as its metabolism is inuslin independent, but it should be taken in small amounts as large amouts cause liver damage by ATP depletion.

Question 6

The enzyme deficient in heredotary fructose intolerance is…..
Describe its pathogenesis

Answer 6

Aldolase B causing:
1. Fructose 1-phosphate accumulated leading to depletion of inorganic phosphate and ATP
2. It also inhibits glycogen phosphorylase leading to hypoglycemia and accumulation of liver glycogen.
3. AMP is degraded leading to hyperuricemia and lactic acidosis
4. Dec availability of hepatic ATP leads to dec gluconeogenesis thus hypoglycemia and vomiting and dec protein synthesis thus dec of clotting factors and essential proteins
5. Kidney function is also affected.

Question 7

…..is deficient in fructosuria

Answer 7

Fructokinase

Question 8

Define the polyol pathway

Answer 8

Pathway in which glucose is converted to sorbit9l by aldose reductase present in lens, retina, schwann cells, kidney, placenta, liver, RBCs, ovaries and seminal vesicles. A second enzyme, sorbitol dehydrogenase converts sorbitol to fructose present only in liver, seminal vesicles and ovaries.

Question 9

The major dietary source of galatose is……, the first enzyme in its metabolism is…… converts it to….., while the rate-limiting enzyme is…..

Answer 9

Lactose in milk
Galactokinase
Galactose 1-phosphate
Galactose 1-phosphate uridyltranserase

Question 10

Galactose enter into structure of……….

Answer 10

Glycoproteins, glucolipids, lactose during lactation

Question 11

Describe the biochemical basis of:
1. Mild galactosemia
2. Severe galactosemia

Answer 11

1. Deficiency of galactokinase leading to galatctose conversion to galactitol which accumulates in lens of eye causong cataract
2. Galatose 1-P uridyltransferase deficiency leads to accumulation of galactoae 1-P in liver and inhibition of glycogen phosphorylase this leads to hypoglycemia, liver enlargement, jaundice, mental retardation.

Question 12

Describe the synthetic processes involving galactose

Answer 12

1. Lactose is synthesized by UDP-galactose: glucose galactosyltransferase or lactose synthase for short. It is formed of protein A (b-D-galactosyltransferase) which transfers galactose form its UDP for to N-acetylglucosamine to form N-acetyllactosamine which is a component of glucoproteins present in tissues other than mammary gland.
2. During lactation protein B is formed (a-lactalbumin) and its synthesis is stimulated by prolactin, it changes the enzyme specificity to form lactose instead of N-acetyllactosamine