Metabolism Of Other Mono And Disaccharides

Question 1

An important component in structural carbohydrates

Answer 1

Galactose

Question 2

Why is fructose metabolized more rapidly than glucose?

Answer 2

It bypasses the PFK1 step

Question 3

Fructose metabolism steps

Answer 3

1. Phosphorylation of fructose

2. Cleavage of fructose-1-phosphate

Question 4

Enzymes of Phosphorylation of Fructose

Answer 4

Step 1 of Fructose Metabolism

Fructokinase:

• in liver, kidney, and small intestinal mucosal cells
• uses ATP (IRREVERSIBLE)

Hexokinase

• has a high Km/low affinity for fructose, therefore is does not contribute significantly
• acts in all other tissues

Question 5

Aldolase B is the only aldolase enzyme that can..

Answer 5

Cleave fructose 1-P

Question 6

All aldolase enzyme isoenzymes can cleave…

Answer 6

Fructose 1,6-BP

Question 7

Products from Cleavage of Fructose 1-P

Answer 7

Glyceraldehyde (or glyceraldehyde 3-p) and DHAP (dihydroxyacetone phosphate)

Question 8

After cleaved, glyceraldehyde can go onto what what process?

Answer 8

Glycolysis or gluconeogenesis

After it is phosphorylated by triose kinase to glyceraldehyde 3-P

Question 9

Glyceraldehyde can also be converted to…. To enter other pathways?

Answer 9

Glycerol

Question 10

What is sorbitol?

Answer 10

An intermediate between the conversion of glucose to fructose

Question 11

Conversion of glucose to fructose via sorbitol

Answer 11

Aldose reductase

• converts glucose to sorbitol
• found in lens, retina, liver, kidney, cells of ovaries and seminal vesicles

Sorbitol dehydrogenase

• convert sorbitol to fructose
• only found in liver, ovaries, and seminal vesicles

Question 12

Why do the seminal vesicles have sorbitol dehydrogenase?

Answer 12

The sperm uses fructose

Question 13

How can tissues that express aldose reductase end up damaging the tissue itself?

Answer 13

By producing sorbitol from glucose, but not able to make fructose from sorbitol.

Will be damaged by long term elevation of blood glucose

Question 14

Excess sorbitol can result in..

Answer 14

Osmotic uptake of water, which can cause cataracts, retinopathy, nephropathy, and peripheral neuropathy

Question 15

Conversion of mannose to fructose 6-P

Answer 15

Important component of glycoproteins

Substrate for Hexokinase
- mannose -> mannose 6-P

Phosphomannoseisomerase
- mannose 6-P -> fructose 6-P

Question 16

Essential Fructosuria

Answer 16

Fructokinase deficiency

Autosomal recessive

Fructose accumulates in the urine

Question 17

Hereditary Fructose Intolerance

Answer 17

Aka Fructose Poisoning

Fructose is phosphorylated and trapped in the cell and builds up

The Fructose ties up most of the available phosphate, which means liver and kidney cells are compromised

Leads to hepatomegaly, jaundice, hypoglycemia, and renal dysfunction

(Hyperuricemia can occur too due to AMP buildup due to lack of ATP production)

Question 18

What is the source of galactose?

Answer 18

Lactose, from milk and milk products

Some from lysosomal degradation of complex carbs

Question 19

Galactose Metabolism steps

Answer 19

1. Phosphorylation of galactose
   Enzyme: galactokinase
   Galactose -> galactose 1-P

2.Formation of UDP-galactose
Enzyme: GALT (galactose 1-P uridyltransferase)
Galactose 1-P can be converted into UDP-galactose or UDP-glucose

Question 20

What are the possible products of Galactose Metabolism?

Answer 20

UDP-Galactose and UDP-Glucose

Question 21

Elevated glucose can cause cataracts due to what disease?

Answer 21

Galactokinase Deficiency (galactose is not getting phosphorylated)

Question 22

Accumulation of galactose 1-P can cause liver damage, retardation, and cataracts due to GALT deficiency is what disorder?

Answer 22

Classic Galactosemia

Question 23

Elevated levels of galactitol in lens causing cataracts is cause by what enzyme disorder?

Answer 23

Aldose Reductase, converts galactitol to galactose

Question 24

Where is Lactose synthesized?

Answer 24

Golgi apparatus

Question 25

What is Lactose?

Answer 25

Milk sugar produced by lactating mammary glands

Question 26

Enzyme for lactose synthesis

Answer 26

Lactose synthase (DUH)

Complex of 2 enzymes:
Beta-D-galactosyltransferase
alpha-lactalbumin

Question 27

What is the synthesis for alpha-lactalbumin stimulated by? Where is it only found?

Answer 27

Prolactin, peptide hormone

Mammary glands

Question 28

Some important epimers of glucose

Answer 28

C-2 epimer - mannose

C-4 epimer - galactose