P: Haemoglobin & myoglobin Flashcards
What are globular haemeproteins?
group of specialised proteins that contain haeme as a tightly bound prosthetic group.
Haeme group functions:
Haemoglobin + myoglobin
Soluble guanylyl cyclase
Catalase
Cytochrome
Haemoglobin + myoglobin: oxygen binding
Soluble guanylyl cyclase: binding of vasodilator - nitric oxide
Catalase: binding + decomposition of hydrogen peroxide
Cytochrome: electron binding in electron transport chain.
Haeme structure, location and binding:
- Complex of ___ and ___
- In myoglobin & haemoglobin, ___ in ___ ring forms two additional bonds with ___ in ___ protein + ___
- protoporphyrin IX, ferrous iron (Fe2+)
- Fe2+, porphyrin, histidine, globin, oxygen
Myoglobin
- ___ haemeprotein located within ___ cells.
- ___ of O2 within muscle cells to drive muscle contraction during ___
- ___ can also scavenge excess reactive ___ species that can damage cells
- monomeric, skeletal, cardiac + smooth muscle
- reservoir, arterial hypoxaemia
- Haeme group, oxygen
Structure of myoglobin
- 80% arranged in 8 ___ A-H
- ___ + ___ residues on exterior
- ___, ___ residues in interior
- Tethered into hydrophobic cleft in protein formed by ___ helices which each contain histidine residue:
1. ____ binds ___ in haeme
2. ____ helps stabilize ___, allowing ___ binding of ___ to ___.
- alpha-helices
- polar, charged
- nonpolar, hydrophobic
- E+F
- Proximal histidine (F8), Fe2+
- Distal histidine (E7) ferrous form or iron, reversible, O2, ferrous iron
Haemoglobin location, function and structure
Only found in RBCs, transports oxygen and CO2 in circulatory system.
Haemoglobin structure:
- Four protein subunits with associated haeme group - each structurally similar to myoglobin
- Two alpha and two beta subunits
- One molecule of haemoglobin can transport 4 molecules of oxygen.
Deoxyhaemoglobin structure
- Haeme group is nonplanar
- Fe2+ is pulled out of the plane of porphyrin towards histidine residue
- Fe2+ lies approximately 0.4Å outside porphyrin plane
Oxyhaemoglobin structure
- Fe2+ is pulled into plane of porphyrin ring.
- Haeme group is planar.
T form deoxyhaemoglobin
- Hydrophobic + ionic bonds contstrain movement of four subunits
- Low affinity form
R form oxyhaemoglobin
- ___ bound to ___ on protein subunit
- What happens to Fe2+ and structure of haemoglobin?
- Breaking of ____ bonds
- Subunits now have more ____
- ___ oxygen affinity
- Fe2+, F8 histidine
- Movement of Fe2+ into plane of haeme group upon O2 binding causes changes in quaternary structure of haemoglobin
- hydrophobic/ionic bonds
- movement
- High
What does oxygen binding to one haemoglobin subunit induce?
T-R conformational changed caused by oxygen binding to one subunit is transmitted to other 3 monomers in tetramer.
Binding of oxygen to one subunit induces increased binding to the other three subunits.
Pulmonary capillaries:
- pO2 = ___ mmHg
- ___ facilitates rapid binding to HgB
- ___ transitions increase affinity of other HgB subunits for ___
- Saturation = ___
- 100
- cooperativity
- T —> R, O2
- 100%
Systemic capillaries:
- pO2 = ___ mmHg
- How many O2 release from each HgB molecule?
- Saturation %?
- Built-in ___ capacity?
- pO2 = 40mmHg
- On average, one O2 is released from each HgB molecule
- Saturation = 75%
- Sufficient for oxygenation of tissues under resting conditions
- Built-in reserve capacity.
Tissue hypoxia:
- Tissue hypoxia pO2 < ___ mmHg
- What happens with O2 release?
- How/why?
- Tissue hypoxia pO2 < 40mmHg
- More O2 is rapidly released from HgB delivering more O2 to tissues
- R-T transitions reduce affinity of other Hb subunits for O2 —> more O2 unloaded from subunits —> O2 unloading from Hb makes it “easier” for O2 to be released from other subunits (reverse cooperativity)
Sigmoidal nature of oxyhaemoglobin dissociation curve:
- Haemoglobin is ___: exhibits ___ of O2 binding and release
- Allows HgB to act as a ___ of oxygen
- High affinity for O2 in ___ and reduced affinity in ___
- tetrameric, cooperativity
- transporter
- lungs, tissues
Hyperbolic nature of myoglobin dissociation curve:
- Myoglobin is ___: no ___–> curve is hyperbolic
- Allows myoglobin to act as an O2 ___ within muscle cells.
- High affinity for O2 at ____ levels
- Rapid release of O2 only at ____ levels.
- monomeric, cooperativity
- reservoir
- normal muscle PO2
- very low muscle PO2
Factors that modulate affinity of haemoglobin for oxygen:
- pO2
- pH of blood
- Carbon dioxide (PCO2)
- 2,3 diphosphoglycerate (2,3-DPG): when bound to haemoglobin, it stabilizes T form and reduces affinity of O2 for haemoglobin —> shift of HgB curve to the right (a metabolite of glycolysis pathway)
Increased metabolic activity results in an increase in these factors:
- Directly interacts + causes structural change in haemoglobin
- Reduces affinity of haemoglobin for oxygen
- Increased unloading of oxygen into tissues
- Alleviates hypoxia caused by increased metabolic activity
p50:
- Definition
- What happens when p50 increases?
- What happens when p50 decreases?
- pO2 at which Hb is 50% saturated.
- P50 increases –> higher pO2 required for 50% saturation –> more O2 unloaded from Hb (right shift)
- P50 decreases –> lower pO2 required for 50% saturation –> less O2 unloaded from Hb (left shift)
Bohr effect: effect of ___ + ___ on ___ dissociation curve.
Increased metabolic activity:
- Increased ___ production due to anaerobic respiration
- Increased ___ production –> increases acidity
- Catalyzed by ___ in RBCs
- acidity, CO2, oxyhaemoglobin
- lactic acid
- CO2
- carbonic anhydrase
Bohr effect: reduction in pH causes ____ of histidine residues on Hb
- Additional ___ bonds to form between ___ subunits: stabilizes ___ form of HB —> lower ___ and increased ___ of O2
- ___ can also bind directly to free amino groups on Hb forms negatively charged ___ groups + more ionic bonds between ___ subunits stabilizing ___ form
protonation
- ionic, Hb, T form of Hb, affinity, release
- CO2, carbamate, Hb, T form
Effect of 2,3 diphosphoglycerate:
- ___ pathway is the only source of ATP in hypoxic tissues
- 2,3 DPG binds to pocket of positively charged ____ formed by ___ of Hb: this stabilizes ___ form of Hgb, lowering ___ and increased release of ___
- High [O2] in lungs expels ___ –> ___ transition.
- glycolytic
- amino acids, beta-subunits, T, affinity, O2
- 2,3-DPG, T –> R
Carbon monoxide + haemoglobin:
- Hb has much ___ affinity for CO than O2 (binds to the ___ on the haeme group)
- CO ___ oxygen, forming ____
Toxic effects:
- Reduces overall ___ in blood (increase in [CO] reduces ___ of Hb —> can be lethal at ___ ppm)
- At lower concentrations, binding of CO shifts conformation of other subunits to ___ form: subunits have higher affinity for ___ —> ___ oxygen released to tissues
- Results in ____
- higher, Fe2+
- displaces, monoxyhaemoglobin (CO-Hb)
- [O2], O2 saturation, 800
- R, O2, less
- tissue hypoxia
Forms of haemoglobin
- Haemoglobin A (HbA): major form in adults (90%)
- Haemoglobin A2 (HbA2): minor adult form (2-5%)
- Haemoglobin Gower 1: produced during early embryonic development
- Haemoglobin F (HbF): synthesised during foetal development.
Foetal haemoglobin:
- HbF has ___ affinity for O2 compared to HbA
- This is due to weak binding of ___ to ___ as subunits lack some positively charged amino acids required to bind ___, which promotes ___ transition
- ___ O2 affinity of HbF facilitates ___ of O2 across placenta from maternal red blood cells.
- higher
- 2,3-DPG, HbF, 2,3-DPG, T –> R
- Higher, transport
Haemoglobinopathies
Family of genetic diseases that result in the production of:
1. Insufficient quantities of haemoglobin molecules
2. Structurally abnormal/defective haemoglobin molecules.
Results in anaemia
- Not enough functional Hgb to transport O2 from lungs to tissues –> tissue hypoxia
Causes of sickle cell anaemia
- Autosomal recessive disorder
- Point mutation in B-globin gene: GAG–> GTG, so VAL is encoded instead of GLU
- In deoxy-HgS: Val6 interacts with Phe85 and Val88 of B-chain, causing aggregation/clumping of HbS molecules.
- In oxy-HgS: In R conformation, Phe85 and Val 88 are buried inside molecule, so no interaction or clumping.
Sickle cell anaemia consequences
- Lower PO2 levels in systemic capillaries: aggregation of HgS –> red blood cells deform to sickle-shape –> obstruction of capillaries + restriction of organ blood flow –> haemolysis
- Anaemia
- Pain + fever
- Organ damage
- Severe infections
Thalassaemia
No production/reduced production of globin subunits. Caused by gene deletions or mutations that block/reduce transcription or translation.
B- thalassaemia
- One copy on each chromosome 11
- Heterozygotes: B thalassaemia trait
- Homozygous: unpaired alpha subunits cause death of progenitor RBC –> erythropoiesis + severe anaemia
A-thalassaemia
- Two copies of alpha-genes on each chromosome 16
- Severity depends on number of defective genes: asymptomatic - severe haemolytic anaemia - foetal death
