P: Haemoglobin & myoglobin Flashcards
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
Complex of protoporphyrin IX and ferrous iron (Fe2+)
In myoglobin & haemoglobin, Fe2+ in porphyrin ring forms two additional bonds with histidine in globin protein + oxygen.
Myoglobin
monomeric haemeprotein located within skeletal, cardiac + smooth muscle cells.
Function:
- Reservoir of O2 within muscle cells to drive muscle contraction during arterial hypoxaemia
- Haeme group an also scavenge excess reactive oxygen species that can damage cells.
Structure of myoglobin
- 80% arranged in 8 alpha-helices A-H
- Polar + charged residues on exterior
- Nonpolar, hydrophobic residues in interior
- Tethered into hydrophobic cleft in protein formed by E+F helices which each contain histidine residue:
1. Proximal histidine (F8) binds Fe2+ in haeme
2. Distal histidine E7 helps stabilize ferrous form of iron, allowing reversible binding of oxygen to ferrous ion.
Haemoglobin
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
- Fe2+ found to F8 histidine on protein subunit
- Movement of Fe2+ into plane of haeme group upon O2 binding causes changes in quaternary structure of haemoglobin
- Breaking of hydrophobic/ ionic bonds
- Subunits now have more movement
- High oxygen affinity
Oxygen binding to haemoglobin
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 = 100mmHg
- Cooperativity facilitates rapid binding to HgB
- T-R transitions increase affinity of other HgB subunits for O2
- Saturation = 100%
Systemic capillaries
- 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 < 40mmHg
- More O2 is rapidly released from HgB delivering more O2 to tissues
- R-T transitions reduce affinity of other Hb subunits for O2
- Reduced affinity, more O2 unloaded from subunits.
- O2 unloading from Hb makes it “easier” for O2 to be released from other subunits
Sigmoidal nature of oxyhaemoglobin dissociation curve:
- Allows HgB to act as transporter of oxygen
- High affinity for O2 in lungs and reduced affinity in tissues
Hyperbolic nature of myoglobin dissociation curve:
- Allows myoglobin to act as an O2 reservoir within muscle cells.
- High affinity for O2 at normal muscle pO2 levels
- Rapid release of O2 only at very low muscle pO2 levels.
Factors that modulate affinity of haemoglobin for oxygen:
- pH of blood
- Carbon dioxide
- 2,3 diphosphoglycerate: a metabolite of glycolysis pathway.
Increased metabolic activity increases:
- 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
pO2 at which Hb is 50% saturated.
P50 increases –> higher pO2 required for 50% saturation –> more O2 unloaded for Hb
P50 decreases –> lower pO2 required for 50% saturation –> less O2 unloaded from Hb.
Bohr effect
effect of acidity + carbon dioxide on oxyhaemoglobin dissociation curve.
Increased metabolic activity: increased lactic acid production & increased carbon dioxide production
Bohr effect: reduction in pH
Causes protonation of histidine residues on Hb.
This allows additional ionic bonds to form between Hb subunits
- Stabilizes T form of Hb
- Lower affinity and increased released of O2
Co2 can also bind directly to free amino groups on Hb
- Forms negatively charged carbamate groups
- More ionic bonds between Hb subunits stabilizing T form.
Effect of 2,3 diphosphoglycerate:
- Glycolytic pathway is the only source of ATP in hypoxic tissues
- 2,3 DPG binds to pocket of positively charged amino acids formed by beta-subunits of Hb: this stabilizes T form of Hgb, lowering affinity and increased release of O2
- High [O2] in lungs expels 2,3-DPG –> t-R transition.
Carbon monoxide + haemoglobin
Carbon monoxide displaces oxygen, forming carbon monoxyhaemoglobin.
At lower concentrations, binding of CO shifts conformation of other subunits to R form: subunits have higher affinity for O2, so less oxygen released to tissues.
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 - affinity to oxygen and why?
- HbF has higher affinity for O2 compared to HbA
- This is due to weak binding of 2,3-DPG to HbF as subunits lack some positively charged amino acids required to bind 2,3-DPG, which promotes T-R transition
- Higher O2 affinity of HbF facilitates transfer of O2 across placenta from maternal red blood cells.
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.
Causes of sickle cell anaemia
- 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 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 chromosome 11
- Heterozygotes: B thalassaemia trait
- Homozygous: unpaired alpha subunits cause death of progenitor RBC –> ineffective erythropoiesis + severe anaemia
A-thalassaemia
- Two copies of a-genes on each chromosome 16
- Severity depends on number of defective genes: asymptomatic - severe haemolytic anaemic -foetal death).
