HMP - Abali 3/8/16 Flashcards
HMP basics
hexose monophosphate pathway
aka…
- pentose monophosphate pathway
- phosphogluconate pathway
- hexose monophosphate shunt
- alt route for metabolism of glucose
- produces NADPH and ribose (key for nt synthesis)
- no ATP directly consumed or produced
involves mitochondria and pyruvate/malate shuttle
functions of HMP
1. generation of NADPH
- req for reductive biochem rxns (FA synth, chol synth)
2. generation of ribose
- req for nt and nucleic acid synth
3. in RBCs, serves a regenerative fx for glutathione
- NADPH regenerates reduced form of glutathione (antioxidant, protects cells from ROS)
NADPH fx
diff b/w NADPH and NADH
reductive agent in many anabolic pathways
- NADPH has a P group that NADH does not have
what processes/where is HMP involved?
HMP makes NADPH, which is key for many anabolic rxns
steroid synthesis
- adrenal gland, testes, ovaries
FA synthesis
- aidpose tissue, mammary gland, liver
cholesterol synthesis
- liver
maintenance of reduced glutathione stock (antiox)
- RBCs
stages of HMP
oxidative phase
- produce NADPH
nonoxidative phase (reversible sugar interconversions)
- ribulose-5-phosphate converted into…
- ribose 5 P [nt synth]
- glyceraldehyde 3 P [glycolysis int]
- fructose 6 P [glycolysis int]
- linked with needs of glycolysis, gluconeogenesis
how is the HMP linked with glycolysis
transketolase [needs thiamine pyrophosphate (TPP) prosthetic group] : catalyze transfers of 2C
transaldolase : catalyze transfers of 3C
*thiamine/B1 also critical for PDH complex, alpha ketoglutarate DH, branched chain alpha ketoacid DH)
flux/regulation of HMP
short term
-
glucose 6 phosphate dehydrogenase (G6PDH)
- feedback inhibition via NADPH (competitive inhibitor)
long term
- insulin → upreg transcription of G6PDH
TPP and Wernicke Korsakoff syndrome
transketolase requires TPP
if TPP binding is defective, can cause Wernicke Korsakoff syndrome and assoc neuropsych symptoms
- paralysis of eye movements
- abnormal gait
- decreased mental fx
- severely impaired memory
fates of HMP, determined by cellular need for:
NADPH only
FA synthesis, detox
- ox rxns produce NADPH
- non ox rxns convert ribulose-5-P → G6P to produce more NADPH
fates of HMP, determined by cellular need for:
NADPH
ribose 5 P
rapidly dividing cells
- ox rxns produce NADPH + ribulose 5 P
- isomerase converts ribulose-5-P → ribose 5 P
fates of HMP, determined by cellular need for:
NADPH
pyruvate
RBCs
- ox rxns produce NADPH + ribulose 5 P
- nonox rxns convert ribulose-5-P → fructose 6 P, glyceraldehyde 3 P
- these ints are shuttled into glycolysis → pyruvate
pathways requiring NADPH
- reductive biosynthesis
- cytochrome P450 monooxygenase
3. phagocytosis by WBCs
- NO synthesis
- reduction of hydrogen peroxide (ROS)
NAPDH use in:
reductive biosynth
NAPDH is required in anabolic pathways that synthesize
- FAs
- cholesterol
- steroids
- [regenerate] reduced form of glutathione antioxidant
NAPDH use in:
cyt P450s
cyt P450s typically add an -OH to compounds to make them more soluble
mitochondrial system
- steroid biosynth (adrenals, testes/ovaries, placenta)
- bile acid, active vit D3 biosynth (liver)
microsomal system
- detox (liver)
NAPDH use in:
phagocytosis in WBCs
pathogens are phagocytosed → brought into phagolysosome
respiratory burst: O2 → superoxide radical O2- [NAPDH oxidase]
- O2- → H2O2 → HOCl (hypochlorous acid aka Clorox) [myeloperoxidase]
- toxic to bacteria!
importance of H2O2 degradation
- ROS can damage proteins, DNA, unsaturated lipids → lead to cell death
- implicated in cancer, aging, inflammatory injury
glutathione peroxidase and catalase convert H2O2 → H2O
NAPDH use in:
NO synthesis
L-Arg + O2 → L-citrulline + NO [NO synthase]
- NADPH oxidized
***breakdown pathway (PDE5) is inhibited by Viagra!
NO fx
- sm muscle relaxant
- prevents platelet aggregation
- neurotransmitter
- mediates macrophage bactericidal/tumericidal activity
glutathione peroxidase
tripeptide: Gly, Cys, Glu
* when oxidized via glutathione peroxidase, it dimerizes : no protective properties
in order to regain anti-ROS properties, it needs to be reduced
- reduction occurs via glutathione reductase + NADPH
importance of reduced glutathione (GSH) in RBCs
RBC is a highly oxidizing environment
every day, spontaneous conversion of Hb-Fe+2 → Hb-Fe+3 + superoxide (rate: 1%/hour)
- only Fe+2 can bind oxygen! need to get the Fe+3 reduced → Fe+2
- also, superoxide → H2O2 → membrane damage!
need GSH handy to break down the ROS before membrane integrity is compromised
- NADPH and glutathione reductase are constantly required for this process
- cell burns up to 10% of its ATP on the HMP pathway to make sure NADPH stocks are sufficient
how/why does G6PDH deficiency affect RBCs?
G6P + NADP → 6 P gluconate + NADPH
RBCs with G6PDH deficiency can’t make the NADPH they need to keep GSH (reduced glutathione) levels up high enough to prevent membrane damage
→ hemolytic anemia (caused by intensification of oxidative environment in RBC: drugs, chemical in fava beans)
- sign: Heinz bodies (GS-SG + MetHb)
Heinz bodies
seens with G6PDH deficiency
ox damage to Hb → cross-linking and precipitation of Hb seen as small inclusions : Heinz bodies
G6PDH deficiency
aka favism
- most common genetic enzymopathy (400 variants)
- X-linked recessive
homozygous recessive
- high levels of hemolysis, anemia
heterozygous
- typically asymptomatic unless exposed to compounds that increase oxidative stress/ROS
- drugs (anti-malarial primaquine)
- fava beans
**condition selected for by conferred relative resistance to infection by Plasmodium falciparum
G6PDH deficiency
G6PDH A- vs. G6PDH Mediterranean
type I (most severe) → type IV (least severe)
type III G6PDH A- (African) : young cells can provide some level of enzyme activity
type II G6PDH Mediterranean : most cells provide negligible enzyme activity
A- starts at a higher enzyme activity, lasts longer than Mediterranean
G6PDH A- vs. G6PDH Mediterranean
comparison of…
- chronic hemolysis
- degree of hemolysis
- G6PDH defect
- enzyme half life
- hemolysis with drugs
- hemolysis with infection
G6PDH A- /// G6PDH Mediterranean
chronic hemolysis : none /// none
degree of hemolysis : moderate /// severe
G6PDH defect : old RBC /// all RBC
enzyme half life : 14 days /// hrs [normal approx 60 days]
hemolysis with drugs : unusual /// common
hemolysis with infection : common /// common
other sources of NADPH for tissues
(besides HMP)…
- malic enzyme
- nicotinamide nucleotide transhydrogenase
need mitochondria for both of these means of producing NADPH
