L04 - RBC metabolism Flashcards
2 sources of oxidative damage to RBCs?
- Neutrophils, macrophages, endothelial cells release reactive oxygen species (probably H2O2): diffuse into red blood cell
- In red blood cell: deoxyhemoglobin (Hb)(Fe2+)
Fe2+»_space; Fe3+ generates H202 from O2
Explain how superxoides are formed inside the RBC?
Increase O2 inside RBC
e- from Fe2+ is transferred to O2 to form superoxide as Fe2+ is oxidised to Fe3+
Superoxide > H2O2 > Oxidative damage
2 major approaches to suppress formation of Hb (Fe3+)/ prevent oxidative damage ?
- Chelation of Fe2+ to His. residue in Hb»_space; Steric hinderance blocks access of H+ to initiate Fe2+ oxidation
- RBC protect itself via glycolysis and pentose phosphate pathway
Pentose phosphate pathway sequence?
Glucose >> Glucose 6-phosphate >> [G6PD + make NADP] >> 6 - phosphogluconate + NADPH >> [NADP] >> Ribulose 5- phosphate + NADPH >> Ribose 5-phosphate
What is the fate of R5P in RBC?
R 5-P not used for nucleotide synthesis in mature RN which is anuclear
Recycled for glycolysis
Explain how the pentose phosphate pathway is used to overcome oxidative stress in RBC?
Oxidative phase of pentose phosphate pathway (PPP) produces 2 NADPH:
> > serves as a donor of electrons to make GSSG into Glutathione(GSH) via Glutathione reductase enzyme
1) GSH turns H2O2 into water via GSH peroxidase
2) GSH reduces hydroxyl free radicals
Why is GSH such a good anti-oxidant?
GSH = glutamate + cysteine + glycine
Cysteine sulphur has many oxidation states available for oxidation»_space; used in reduction of ROS
GSH is dimerized to GSSH after reducing ROS
RBC has higher chance of oxidation to Hb (Fe3+) in the lungs than in peripheral tissue. T or F? Why?
True
PO2 in lungs is high and amount of Hb loaded with O2 is also high
What protein is used to protect against increased risk of oxidative damage in RBC in the lungs?
Band 3 protein cluster
i) anchors spectrin skeleton to the plasma membrane
ii) C- terminus is a competitive binding site for DeoxyHb and several glycolytic enzymes
Explain the Band 3 mechanism in RBC in increased or decreased O2 levels?
C- terminus is a competitive binding site for DeoxyHb and several glycolytic enzymes
- Increasing O2 level»_space; more oxyHb formed»_space; incresae glycolytic enzyme binding to Band 3»_space; reduce glycolysis and more pentose phosphate pathway»_space; make more NADPH, GSH
- Decreasing O2 level»_space; more deoxyHb formed»_space; detach glycolytic enzymes from band 3»_space; more glycolysis and less PPP»_space; less NADPH, GSH, more ATP
Why does the level of NADPH and ATP made at various O2 levels important for oxidative stress?
High O2»_space; More NADPH made»_space; adapt to high oxidative stress
Low O2»_space; Less NADPH made, More ATP made for responding to O2 demand of nearby cells/ Unload O2 ATP- Purigenic receptor pathway at endothelium
Production of G 6-P is dependent on the O2 level to cope with various oxidative stress. T or F?
False
Glucose»_space; G 6-P is independent of O2 level, conversion is constant
Depends on the [glycolytic enzymes] released from Band 3 protein to determine what to make from the G6P
Pyruvate kinase deficiency can cause a shift in the O2 dissociation curve in which direction?
To the right = Decrease affinity of Hb to O2 in addition to co-operative binding
due to Accumulation of glycolytic intermediates
How does glycolysis relate to O2 transport by erythrocytes?
Via A special glycolytic metabolite in erythrocyte
2,3- Bisphosphoglycerate (BPG)
Pathway of forming 2,3 BPG?
1,3- Bisphosphoglycerate --[bisphosphoglycerate mutase]--> 2,3 Bisphosphoglycerate --[2,3- bisphosphoglycerate phosphatase]--> 3- Phosphoglycerate
What determines the concentration of 2,3 - BPG in RBC?
Balance between bisphosphoglycerate mutase enzyme activity vs 2,3- bisphosphoglycerate phosphatase enzyme activity
Mechanism of 2,3 - BPG in shifting the O2 dissociation curve? *
Act as an allosteric regulator for Hb, changing the affinity of Hb for oxygen
2,3-BPG preferentially binds to stabilize T-state of haemoglobin (Hb) in erythrocytes
(R-state Hb central cavity is smaller and cannot bind 2,3 BPG)
> > reduces the affinity of Hb for Oxygen and encourage unloading
The extent of O2 dissociation curve shift cause by 2,3 BPG depends on what?
Extent of shifting depends on [BPG]
Which shifts the O2 dissociation curve more to the right: Hb + CO2, or Hb + BPG, or Hb + CO2 + BPG?
Increasing shift:
Hb + CO2
> Hb + BPG
> Hb + CO2 + BPG
How does high altitude shift the O2 dissociation curve to the right via glycolytic intermediate? (Note: Not asking about EPO and more erythropoiesis) exam
High altitude/ hypoxic
1) Low O2 = More deoxyHb displace ALDOLASE enzyme from Band 3
ALDOLASE = F-1,6-BP»_space;> Glyceraldehyde 3-Phosphate (precursor for 2,3 BPG)
2) AMP-activated protein kinase (AMPK) increase
» phosphorylates and activates BPG Mutase
> > > Increase 2,3 - BPG level = T-state stabilized, decrease affinity of Hb to O2
List the sequence of reactions involved with 2,3 BPG, starting with Glyceraldehyde 3-phosphate?
Glyceraldehyde 3- Phosphate --[GAPDH]--> 1,3- Bisphosphoglycerate --[BPG mutase]--> 2,3 BPG --[2,3-BPG phosphatase]--> 3- Phosphoglycerate --[PGM]--> 2 - Phosphoglycerate
*1,3 - BPG can bypass synthesis of 2,3- BPG by directly going to 3-phosphoglycerate via PGK enzyme
Mechanism of increasing supply of precursors to 2,3- BPG in hypoxia?
Hypoxia
> > More deoxyHb formed
More glycolytic enzyme liberated from Band 3 i.e. Aldolase
Increase catalysis of F 1,6-BP
Increase supply of precursor Gylceraldehyde 3- phosphate
GAPDH + BPG mutase = 2,3- BPG
Mechanism of lipid involvement in RBC under hypoxic conditions?
Hypoxia
> more deoxyHb made
Increase production of sphinogosin 1- phopshate (S1P)
S1P binds to deoxyHb and recruits it to plasma membrane to facilitate interaction with Band 3
more deoxyHb bind to Band 3, displace more glycolytic enzymes
More glycolysis = more precursor for 2,3 BPG
Mechanism of vasodilation of arterioles to local hypoxic tissue?
Local hypoxia
> Local demand of O2 exceed total O2 carrying capacity of local RBC
Secretion of ATP by RBC **
Stimulation of PURINERGIC RECEPTOR **on endothelium
Signals generated, dilatation of arterioles upstream to increase blood flow
Increase O2 delivery
