HMP Shunt Flashcards
What is the HMP shunt? What are other names for it? Where does it occur? What is the subcellular site?
An alternate pathway for glucose oxidation
Also known as
- pentose phosphate pathway
- phosphogluconate pathway
Site:
All cells, especially important in
- liver, adipose tissue, lactating mammary glands
- testes, ovaries, placenta, adrenal cortex
- RBCs
- WBCs
Subcellular site
Cytosol
Why is the HMP shunt important? (7) What are the major coenzymes required for HMP shunt?
•Provides NADPH for
üFatty acid synthesis in liver, adipose tissue & lactating mammary glands & cholesterol synthesis in liver
üHydroxylation reactions in liver
üSteroid synthesis in testes, ovaries, placenta & adrenal cortex
üDetoxification of reactive oxygen species in RBCs
üPhagocytosis in WBCs
üNitric oxide synthesis
•Source of ribose 5-P for nucleotide synthesis in dividing cells
Major coenzymes required for HMP shunt
ØNADP (from niacin)
TPP (from thiamine
What are the two phases of HMP SHUNT?
- *•Oxidative phase**
- irreversible reactions, convert glucose 6-P to a pentose phosphate
- Generate NADPH
- Important in tissues that need significant amounts of NADPH
- *•Non-oxidative phase**
- reversible interconversion of sugar phosphates ultimately yielding glycolytic intermediates
- Can produce ribose phosphate without the need for oxidative phase
- Active in all cells synthesizing nucleotides & nucleic acids
Draw the oxidative phase of HMP shunt
•Draw the non-oxidative reaction of HMP shunt! And name all the enzymes used in each step
Draw out the HMP shunt in a summary. Which ways can Glucose 6 phosphate enter?
What is the significance of HMP shunt? Does the HMP Shunt require any energy or generate ATP? What is the HMP Shunt regulated by? What inhibits the regulator of HMP shunting ? What upregulates the expression of the regulator?
- Source of NADPH
- Source of ribose 5-phosphate required for biosynthesis of nucleotides
- Utilization of 5-carbon sugars obtained from the diet or degradation of structural carbohydrates in the body
HMP shunt
- Can use or reproduce intermediates of glycolysis; ‘shunting’ glycolytic intermediates through an alternative path to yield NADPH or ribose-P
- Does not directly consume or generate ATP
Regulation of HMP shunt
- Is regulated at the G6PD reaction
- NADPH inhibits G6PD activity (high NADPH/NADP ratio slows down G6PD reaction)
- Insulin upregulates expression of G6PD gene (induction; enzyme synthesis)
What are the functions of NADPH( 2)? What does NADPH do in the second function? What uses HMP shunt mainly as a source of NADPH?
1. Reductive biosynthesis
Biosynthesis of fatty acids, cholesterol
2. Reduction of hydrogen peroxide
- Reactive oxygen species (ROS) produced as byproduct of aerobic metabolism or through reactions with drugs/toxins
- ROS can react with membrane lipids, proteins, DNA – cause harmful changes, implicated in pathologic processes
- Antioxidants neutralize ROS; cellular defense mechanism
- H2O2 Reduced to water by glutathione (antioxidant) which itself gets oxidized
- NADPH required to keep glutathione in reduced state (important in RBCs since HMP shunt only source of NADPH)
- NADPH important in RBCs ability to detoxify ROS
Where does the free radicals comes from?
How is Hydrogen peroxide neutralized? Explain how NADPH works
Smoking
Macrophages
UV light
Air pollution
Oxidative stress
In the body oxygen becomes water in the ETC. If there is incomplete reduction of oxygen by an addition of one H as a time you will get an intermediate such as hydrogen peroxide, superoxide , hydroxyl radical . To neutralize then you have enzymes such as superoxide dismutase, which converts superoxide to oxygen and hydrogen peroxide . Hydrogen peroxide can be neutralized by catalases and glutathione peroxidases
Glutathione ( is a tripeptide). The thiol group is very important in the antioxidant property . Hydrogen peroxide and glutathione in the reduced state become water. The two glutathione join by the disulfide linkage which is called the oxidized glutathione which is reduced again by glutathione reducase where NADPH donates a H+ ( coupled reaction ). NADPH is from the HMP shunt
Oxidative stress can be seen if there is less detoxification of the free radical. When is this seen? When there is less production of NADPH
What is the cytochrome P450 monooxygenase system ? How does NADPH help the system? Why is Mitochondrial cytochrome P450 system important? (3) Why is Microsomal cytochrome P450 system (in liver) important?
3. Cytochrome P450 monooxygenase system
- Create a hydroxyl group in the substrate (hydroxylation reactions)
- NADPH provides reducing equivalents
Mitochondrial cytochrome P450 system
- Hydroxylation of steroids – adrenal cortex, gonads, placenta
- Bile acid synthesis – liver
- Activation of vitamin D – kidney
Microsomal cytochrome P450 system (in liver)
- Detoxification of xenobiotics (foreign compounds) including drugs, pollutants & pesticides
- Increase the solubility of these compounds, facilitate their excretion
Explain the phagocytosis by WBCS and How NAHPH comes into play
- Occurs in neutrophils & macrophages (monocytes)
- Microorganisms, foreign particles & cell debris ingested by receptor-mediated endocytosis are destroyed by oxygen-dependent & independent mechanisms
- Oxygen-dependent system generates oxygen-derived free radicals using NADPH
- Bacteria are destroyed by the free radicals
Draw out the Phagocytosis by WMCs ( role of NADPH)
- NADPH oxidase generates superoxide using O2 & NADPH (respiratory burst)
- Superoxide generates H2O2 either spontaneously or via SOD
- Myeloperoxidase generates HOCl from H2O2
- H2O2 can generate hydroxyl radical through Fenton reaction
- Nitric oxide synthase (NOS) produces NO using NADPH
- NO combines with superoxide to form peroxynitrite (ONOO-)
Bacteria destroyed by the free radicals
SOD: superoxide dismutase
During phacytosis, the membrane invaginates & superoxide is released into the space within the phagosome
iNOS: Inducible nitric oxide synthase
HOCl (hypochlorous acid) – a component of household bleach
What does the deficiency of NADPH oxidase causes? What is this disease characterized by?
- Genetic deficiency of NADPH oxidase causes chronic granulomatous disease
- Characterized by severe, persistent infections
- Formation of granulomas (nodular areas of inflammation) that sequester the bacteria which were not destroyed
How is Nitric Oxide synthesized? What are the functions are nitric oxide?(4)
Synthesized by nitric oxide synthases (NOS)
L-arginine + NADPH + H+ + O2 -> L-citrulline + NO + NADP+ NO
-Relaxes vascular smooth muscle
-prevents platelet aggregation
-Functions as a neurotransmitter in brain
Mediates bactericidal & tumoricidal actions of macrophages
- HMP shunt-clinical significance:
- What is G6PD deficiency ? Where is It most common? What type of inheritance is it? What type of anemia does it cause? What does this deficiency impair? What is most affected by the deficiency and why?
•Most common disease-causing enzyme abnormality in humans
•High prevalence in middle-east, tropical Africa & Asia, Mediterranean
•Many different mutations in G6PD gene known; X-linked recessive inheritance
•Mutant enzymes may show decreased activity or stability, decreased binding affinity for substrates
•Major cause of hemolytic anemia (usually precipitated by drugs)
•Deficiency impairs ability of cells to form NADPH; inability to keep glutathione in reduced state
•Impaired detoxification of reactive oxygen species
•RBCs most affected by the deficiency, HMP only source of NADPH
What is required to keep the integrity of the RBC? Explain with a diagram the effects of G6PD deficiency on erythrocytes
What is required to keep the integrity of the RBC? Energy ( ATP via Na/K pump) and NADPH to combat the ROS’s . Both of these come from glucose . The picture show that via glycolyisis ATP and NADPH is producedd and via HMP shunt, NADPH is produced -> needed to reduce the glutathione into reduced form which inturn will reduce the hydrogen peroxide to water. This is a coupled reaction. These are all linked to Glucose-6- phosphate dehydrogenase.
So if there is a absence of G6PD what is likely to happen? Dec in the NADPH-> dec in reduced from of glutathione -> hydrogen peroxide will accumulate -> form other radical-> hemolysis
Glutathione is important for hemoglobin ( has many thiol groups that are kept in the reduced form by glutathione ) . If it is not possible to keep them in reduced state they oxidize and cross-link to form Heinz-bodies ( clumps of oxidized protein)
What triggers these features? Infections, certain drugs, fava beans
