Pentose Phosphate pathway and NADPH Flashcards
What’s another name for the pentose phosphate pathway?
Hexose MonoPhosphate Shunt
HMP shunt
What is the HMP shunt?
a branch off from the glycolytic pathway
what purposes does HMP shunt serve?
- generation of NADPH
- generation of the 5-carbon sugar ribose, to be used in the synthesis of nucleotides
Does HMP shunt produce NADPH and ribose or one or the other?
can produce both or one or the other depending on the need of the cell
HMP shunt and ATP
NO ATP used or produced during this process
Step 1 of HMP shunt
- dehydrogenation of glucose 6-P
- enzyme Glucose 6-phosphate dehydrogenase (G6PD)
- oxidative irreversible reaction
- RATE LIMITING STEP
- major point of regulation
- the flux through the pathway increases in an absorptive state
glucose 6-phosphate dehydrogenase
- enzyme for the dehydrogenation of glucose 6-P
- NADP+ is required enzyme
glucose 6-phosphate dehydrogenase inhibitor
NADPH is potent competitive inhibitor
expression of glucose 6-phosphate dehydrogenase
up-regulated by insulin
Step 2 of HMP shunt
- hydrolysis to 6-phosphogluconate
- enzyme-6-phosphogluconolactone hydrolase
- IRREVERSIBLE
- not rate limiting
- produces 1 NADPH
- oxidative
step 3 of HMP shunt
oxidative decarboxylation of 6-phosphogluconate
- enzyme-6-phosphogluconate heydrogenase
- IRREVERSIBLE
- produces 1 NADPH
- oxidative
steps 4-8 of HMP shunt
- nonoxidative reversible reactions
- interconversions of sugar molecules
- interconvert sugars with 3-7C-atoms
- permit synthesis of ribose 5-P used for nucleotide production
- enzyme-transketolase
transketolase
- requires TPP (from thiamine)
- important in diagnosing thiamine deficiency
- done by measurment of its activity in RBCs
structures of NADH and NADPH
very similar, just a small difference
-both e carriers, although not interchangeable
NADPH functions
- electron donor for the “reductive” biosynthesis of FA, cholesterol, and steroids
- E donor for the neutralization of reactive oxygen species
- provides reducing equivalents for cytochrome P450 monooxygenase system (for a biosynthesis of steroids and detox of xenobiotics)
- Play role in phagocytosis
- substrate for the synthesis of NO
NADPH role in neutralization of ROS
- tripeptide: gamma-glutamylcysteinglycine (G-SH)
- Glutathion (GSH/GSSG) is the major antioxidant system
ROS produced
- during aerobic metabolism
- through reactions with drugs and toxins
- when levels of antioxidants are diminished
Monooxygenase
enzyme that will incorporate 1 O-atom from molecular oxygen (creating OH group) with the other being reduced to H2O
Mitochondrial Ct P450
- synthesis of steroids
- inner mitochondrial membrane
- steroidogenic tissue (placenta, ovaries, testes, adrenal cortex) uses NADPH for synthesis of steroid hormones
- liver uses the same system to synthesize bile acids and vit D3
- in kidney converts vit D3 to its active form
Microsomal systme Cyt P450
- membranes of smooth ER in liver cells
- detox of drugs, toxins, chemicals
- by adding O, the compound may be inactivated, made more soluble, or provide a reactive group (hydroxyl) for attachment of other compounds
phagocytosis in WBC
neutrophils and mactophages (monocytes)
-generation of O free radicals aid in killing the microorganisms that the have engulfed
The MPO system
combo of NADPH oxidase and myeloperoxidase are used to generate the O free radicals to aid in the destruction of microorganisms
Chronic Granulomatous disease (CGS) caused by what?
rare genetic NADPH oxidase deficiency
Chronic Granulomatous disease (CGS)
- persistent serve infections due to the inability to kill bacteria forming granulomas
- the granuloma is formed as a defense where the body attempts to “wall off” the collection of cells from the rest of the body
Nitric oxide
- NO
- synthesized from argenine, oxygen, and NADPH
- very short half-life
- very reactive
- diffuses into cells easily since it is a gas
NO-synthase (NOS)
3 enzymes product of different genes
- nNOS (neural tissues)
- eNOS (endothelial cells)
- iNOS (inducible)
What NOS is constitutively expressed at constant levels?
nNOS
eNOS
biological activity of NO
- smooth muscle relaxant
- used by macrophages to generate free radicals to kill microorganisms
- inhibits platelet aggregation
- functions as a neurotransmitter in brain
nitroglycerin action
converted into NO to relax vascular smooth muscles
Glucose 6-Phosphate dehydrogenase (G6PD) deficiency
- inability to detoxify oxidizing agents
- some protection against malaria
Where is the gene for G6PD?
on X chromosome (defects seen mostly in males)
-in some areas 25% of males are affected
what populations have the highest incidence of G6PD deficiency?
african, mediterranean, asian
What is the incidence of G6PD among american blacks?
11%
G6PD deficiency precipitating factors
usually only symptomatic when experiencing an oxidative stress
- infection (O radicals generated by macrophages)
- drugs that produce an oxidative stress
- fava beans (only some deficiencies) also called favism
clinical manifestations of G6PD deficiency
- almost exclusively in RBCs as an episodic hemolytic anemia in adults
- neonatoal jaundice in newborns
- effects on other organs/tissues are due to hemolytic anemia
- shortened life span in individuals with the severe form
Why G6PD deficiency in RBCs?
- BC in RBCs, the only pathway to make NADPH is via the HMP pathway
- in other cell types, other pathways contribute to NADPH formation
- RBCs cannot synthesize more G6PD because they have no nucleus.
Most of the mutations that result in G6PD deficiency …
affect the stability of the enzyme, which means over the life cycle of a RBC the enzyme is lost and not replaced
characteristic of G6PD deficiency in RBCs
presence of Heinz bodies in RBCs
Heinz bodies
precipitates of oxidized hemoglobin in RBCs
recovery from G6PD deficiency
if oxidative stress is removed, usually the patient will recover as new RBCs are made
