Interconversion of Sugars Flashcards
PPP has 2 roles
- Producing NADPH (used for fatty acid synthesis. glutathione reduction)
- Producing ribulose phosphate, converted to ribose, needed to make nucleotides, cofactors, ATP etc
PPP has 2 phases
Oxidative
Non-oxidative - way of interconverting 5-C sugars eg ribose into intermediates of glycolysis
What is UDP-Glucose
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
Galactose and fructose metabolism
Galactose -> UDP –> UDP-glucose –> glucose 6-P (then ultimately pyruvate)
or Galactose –> UDP-Glucose –> Glycogen
Fructose –> DHAP + GLY 3-P –> Pyruvate
What a the key points of the PPP?
- 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)
Is the oxidative state reversible or irreversible
irreversible
what are the products and enzymes of the oxidative phase
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)
Is the non oxidative state reversible or irreversible
reversible
what are the products and enzymes in the non-oxidative state
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)
Importance of NADP
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
Functions of NADPH
- Reductive biosynthesis
- fatty acid synthesis
- cholesterol synthesis
- steroid synthesis
- deoxyribonucleotide synthesis - Detoxification
- reduction of oxidized gluthatione
- cytochrome P450 monooxygenase - Phagocytosis
- NADPH Oxidase
Non-oxidative pathway from the oxidative pathway products
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
How does transketolase work?
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
How does transaldolase work?
Catalyzes the transfer of a 3-C unit from a ketose donor
What is the general scheme of the PPP
- 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
Why is the is the PPP important in RBCs?
- 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
What is glutathionine
- 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
How is the cell protected against ROS
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)
What are some antioxidant scavenging enzymes?
- CAT (catalase) - reduces -H2O2 to water
- SOD (superoxide dismutase) reduces superoxide to hydrogen peroxide
- GPX (glutathione peroxidase) reduces hydrogen peroxide to water
What happens during a G6PD deficiency?
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)
How does hemolysis by ROS occur in RBC
- 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
Phagocytosis and PPP
- 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
Why is UDP-glucose a good glucose donor?
The high energy phosphoester provides energy when hydrolyzed and favors the donation of glucose to other molecules
UDP-glucose metabolism: making lactose
-The galactose in UDP-galactose can be donated to glucose to form lactose during lactation
Lactose Synthesis
- 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
When is alpha-lactalbumin made
after parturition in response to prolactin
What are glycosyltransferases
They catalyze the transfer of sugars from nucleotide sugars like UDP-glucose to AA residues on proteins to form glycoproteins
UDP-glucose metabolism: making glucuronic acid
UDP-glucose can be oxidized to a sugar acid, glucuronic acid
UDP-Glucoronate Pathway
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
How are glucuronides formed
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
Importance of glucuronides
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
Metabolism of galactose
- 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
What are galactosemias
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
What is classical galactosemia?
Deficiency in Galactose 1-phosphate uridylyltransferase
transfer of UDP to galactose is inhibited and galactose 1-P accumulates
Treatment - galactose free diet
Pathway for fructose metabolism
- 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
Defects in fructose metabolism
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
The Polyol Pathway for fructose synthesis
- 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
What happens when sorbitol accumulates?
- 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
Mechanism of Tissue injury by hyperglycemia
Watch slide 43 Sorbitol pathway will dominate during Hyperglycemia - sorbitol and fructose -osmotic effects - oxidative effects
Glycation pathway
- glycated proteins
- altered function or turnover
The non-oxidative portion of pathway
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
What is the difference between nonclassical galactosemia and classical galactosemia?
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)
