Interconversion of Sugars

Question 1

PPP has 2 roles

Answer 1

1. Producing NADPH (used for fatty acid synthesis. glutathione reduction)
2. Producing ribulose phosphate, converted to ribose, needed to make nucleotides, cofactors, ATP etc

Question 2

PPP has 2 phases

Answer 2

Oxidative

Non-oxidative - way of interconverting 5-C sugars eg ribose into intermediates of glycolysis

Question 3

What is UDP-Glucose

Answer 3

UDP-glucose is a glucose donor

UDP-glucose can be epimerized to UDP-galactose

UDP-glucose can be oxidized to a sugar acid, glucuronic acid

Question 4

Galactose and fructose metabolism

Answer 4

Galactose -> UDP –> UDP-glucose –> glucose 6-P (then ultimately pyruvate)

or Galactose –> UDP-Glucose –> Glycogen

Fructose –> DHAP + GLY 3-P –> Pyruvate

Question 5

What a the key points of the PPP?

Answer 5

• an alternative pathway for glucose oxidation, occurs in the cytosol
• two phases: oxidative and non-oxidative
• entry into the PPP is determined by the ratio of NADP+/NADPH
• active in tissues that are rapidly growing (bone marrow, skin, intestinal mucosa), actively synthesizing fatty acids, cholesterol, steroids (liver, lactating mammary gland, adrenal gland, gonads), exposed to O2 (RBC, cells of the lens and cornea)

Question 6

Is the oxidative state reversible or irreversible

Answer 6

irreversible

Question 7

what are the products and enzymes of the oxidative phase

Answer 7

glucose is oxidized (not by removing a C, no ATP generated)

NADPH, ribulose-5-phosphate and CO2 are generated

enzymes: glucose-6-phosphate dehydrogenase (rate limiting, first step)

Question 8

Is the non oxidative state reversible or irreversible

Answer 8

reversible

Question 9

what are the products and enzymes in the non-oxidative state

Answer 9

interconverts 5-C sugars such as ribose-6-phosphate with 6-C and 3-Csugars such as glucose-6-phosphate

enzymes: transketolase (requires thiamine) and transaldolase –> unique to the PPP)

Question 10

Importance of NADP

Answer 10

oxidized NADP+ accepts a hydride ion (2e-, 1H+) to form reduced NADPH

NADPH is a strong reductant

The NADPH/NADP+ ratio in the cytosol is around 200 to 1, Therefore it will be involved in reduction biosynthesis

Question 11

Functions of NADPH

Answer 11

1. Reductive biosynthesis
   - fatty acid synthesis
   - cholesterol synthesis
   - steroid synthesis
   - deoxyribonucleotide synthesis
2. Detoxification
   - reduction of oxidized gluthatione
   - cytochrome P450 monooxygenase
3. Phagocytosis
   - NADPH Oxidase

Question 12

Non-oxidative pathway from the oxidative pathway products

Answer 12

The oxidative phase produces a keto sugar (ribulose-5-phosphate) which is converted to an aldo- sugar (ribose 5-phosphate) by an isomerase.
This can be used to nucleotides and cofactors

However, to enter the non-oxidative phase, ribulose-5-phosphate needs to be converted to xylulose 5-phosphate, by an epimerase

Question 13

How does transketolase work?

Answer 13

It catalyzes the transfer of a 2-C unit from a ketose donor to an aldose acceptor

It requires the cofactors TPP and MG

Erythrocyte transketolase activity may be used as a measure of thiamine status

Question 14

How does transaldolase work?

Answer 14

Catalyzes the transfer of a 3-C unit from a ketose donor

Question 15

What is the general scheme of the PPP

Answer 15

• NADPH formed in the ox-phase is used to reduce oxidized glutathione and support biosynthesis
• The other product of the oxidative phase, ribose-5-phosphate, serves as a precursor for nucleotides coenzymes, DNA and RNA’

In cells not using the ribose-5-phosphate for synthesis, the non-ox phase recycles 6 pentose molecules into 5 hexose molecules

Question 16

Why is the is the PPP important in RBCs?

Answer 16

• the most imporant pathways of carbohydrate metabolism in erythrocytes are glycolysis, PPP and generation of 2,3-BPG
• Glycolysis produces ATP by substrate level phosphorylation for the maintenance of ion pumps
• ROS cause lipid peroxidation and “leaky” membranes which leads to hemolysis. PPP can help produce reduced glutathione which can degrade ROS

Question 17

What is glutathionine

Answer 17

• A tripeptide containing cysteine
• It is reduced when its cysteine is reduced (tripeptide is alone and its a SH group)
• It is important in cells to maintain a reducing environment and degrade ROS

Question 18

How is the cell protected against ROS

Answer 18

ROS - results when O2 interacts with substances that give up e-

Reduced glutathione (GSH) destroys ROS like H2O3 reducing it to water

NADPH maintains GSH

The PPP is the major source of NADPH for reduction of glutathione (GSSG –> 2 GSH)

Question 19

What are some antioxidant scavenging enzymes?

Answer 19

• CAT (catalase) - reduces -H2O2 to water
• SOD (superoxide dismutase) reduces superoxide to hydrogen peroxide
• GPX (glutathione peroxidase) reduces hydrogen peroxide to water

Question 20

What happens during a G6PD deficiency?

Answer 20

G6PD deficiency is one of the most common inherited enzyme disorders occurring in 100 million people

symptoms are related to hemolysis of RBC bc of the inability to produce adequate amounts of cytosolic NADPH

Symptoms are revealed only under oxidative stress:

• treatment with antimalarial gents
• infections
• fava beans (generates ROS)

Question 21

How does hemolysis by ROS occur in RBC

Answer 21

• ROS is generated from non-enzymatic oxidation of Hb to Met Hb
• The PPP generates NADPH which is used to regenerate GSH
• When G6PDH is defective, NADPH is limited the ROS increase
• ROS lead to hemolysis

Question 22

Phagocytosis and PPP

Answer 22

• In response to an infectious agent, phagocytic cells rapidly consume O2 (respiratory burst) to produce ROS
• The ROS aids in killing invading pathogens

The respiratory burst results from NADPH oxidase

W/i a minute following the phagocytic response, there is an 20-200 fold increase in O2 consumption and 10-20 fold increase in PPP

The PPP provides the NADPH for the respiratory burst

-In phagocytosis, flux through the PPP increases to provide NADP to NADP oxidase which generates ROS in phagocytosis

Question 23

Why is UDP-glucose a good glucose donor?

Answer 23

The high energy phosphoester provides energy when hydrolyzed and favors the donation of glucose to other molecules

Question 24

UDP-glucose metabolism: making lactose

Answer 24

-The galactose in UDP-galactose can be donated to glucose to form lactose during lactation

Question 25

Lactose Synthesis

Answer 25

- synthesized only in the mammary gland of adults - lactose synthase catalyzes the last step of lactose synthesis and has two subunits: 1. Galactosyltransferase (GT): normally transfers galactose to glycoproteins 2. alpha-Lactalbumin: a modifier protein that allows GT to transfer galactose from UDP-galactose to glucose to form lactose

Question 26

When is alpha-lactalbumin made

Answer 26

after parturition in response to prolactin

Question 27

What are glycosyltransferases

Answer 27

They catalyze the transfer of sugars from nucleotide sugars like UDP-glucose to AA residues on proteins to form glycoproteins

Question 28

UDP-glucose metabolism: making glucuronic acid

Answer 28

UDP-glucose can be oxidized to a sugar acid, glucuronic acid

Question 29

UDP-Glucoronate Pathway

Answer 29

an important to drug detoxification and excretion of bilirubin - UDP-glucuronate can donate glucuronic acid to other -OH containing compounds to form glucuronide - The negatively charged carboxyl group in glucuronide increases water solubility of non-polar compounds which can be excreted in the urine or bile

Question 30

How are glucuronides formed

Answer 30

Glucuronic acid can be conjugated with endogenous or exogenous substances to form a group of molecules known as glucuronides glucuronides are more water soluble at physiological pH

Question 31

Importance of glucuronides

Answer 31

important in the metabolism of bilirubin (breakdown product of heme and is very nonpolar/water insoluble, When glucuronic acid gets incorporated in bilirubin, it is now water soluble and can be excreted), drug detoxification and steroid excretion

Question 32

Metabolism of galactose

Answer 32

- Found in dairy - When it gets into the cell, it is phosphorylated by a galactokinase. It forms forms galactose 1-phosphate and cant leave the cell - Galactose 1-P reacts with UDP glucose to form glucose 1-P and UDP-galactose (uridylyltransferase is involved) - UDP-galactose is epimerized to UDP-glucose - Glucose-1-P can enter glycolysis (as G6P) or other metabolic routes for glucose

Question 33

What are galactosemias

Answer 33

Enzyme deficiency - results from the inability to metabolize galactose - these defects may involve galactokinase, uridylyltransferase or 4-epimerase - most serious one, classical galactosemia is a deficiency in uridylyltransferase is life-threatening to neonate, leads to irreversible severe cognitive disability - high galactose in the blood leads to cataracts

Question 34

What is classical galactosemia?

Answer 34

Deficiency in Galactose 1-phosphate uridylyltransferase transfer of UDP to galactose is inhibited and galactose 1-P accumulates Treatment - galactose free diet

Question 35

Pathway for fructose metabolism

Answer 35

- Gets into the liver cell by GLUT2 - bypasses hexokinase and PFK-1 regulatory step in glycolysis - high levels of fructose coming into the cell, can lead to more lactic acidosis and fatty acid synthesis in the liver

Question 36

Defects in fructose metabolism

Answer 36

essential Fructosuria - deficiency in fructokinase -asymptomatic because it doesn't get trapped in the cell, not getting phosphorylated so can just leave the liver cell ``` Hereditary Fructose Intolerance - Aldolase B deficiency - symptoms when fructose is ingested -severe hypoglycemia - F1P builds up and ATP - Elevated F!P leads to hypohlycemia F1P also inhibits the pathway that make glucose ```

Question 37

The Polyol Pathway for fructose synthesis

Answer 37

- Glucose/sugars are reduced to alcohol sugars by aldose reductase. ​NADPH is consumed - The alcohol sugar of glucose is sorbitol - sorbitol doesn't readily cross the cell membrane bc it is not transported by the transporters well. Now it accumulates in the cell. - build of of sugar alcohol, and consumption of NADPH can lead to tissue damage that diabetics experience especially in the lens, nerve and glomerulus

Question 38

What happens when sorbitol accumulates?

Answer 38

- Reduction of glucose to sorbitol consumes NADPH - Depletion of NADPH results in accumulation of ROS - Increased sorbitol leads to osmotic pressure - Glycation of lens proteins leads to opaque cloudiness of the lens cataracts

Question 39

Mechanism of Tissue injury by hyperglycemia

Answer 39

``` Watch slide 43 Sorbitol pathway will dominate during Hyperglycemia - sorbitol and fructose -osmotic effects - oxidative effects ``` Glycation pathway - glycated proteins - altered function or turnover

Question 40

The non-oxidative portion of pathway

Answer 40

A keto-sugar (ribulose-5-phosphate) is converted to an aldo sugar (ribose 5-phosphate) by an isomerase A keto sugar (ribulose 5-phosphate) is epimerized to another keto-sugar (xyulose-5-phosphate) by an epimerase

Question 41

What is the difference between nonclassical galactosemia and classical galactosemia?

Answer 41

Non-classical - metabolism of galactose is blocked during the conversion of galactose to galactose 1-phosphate Classical galactosemia - galactose 1-phosphate can't be converted further (to glucose 1 phosphate and ultimately glucose)