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
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: 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
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
