Haemoglobin and Myoglobin

• Haemoglobin and myoglobin both bind and transport molecular oxygen

Oxygen Carriers

• Oxygen carriers are essential because oxygen is non-polar and poorly soluble in water
• Oxygen is poorly diffusible, meaning that in large multicellular organisms it will not reach target tissues by diffusion alone

Myoglobin

• Myoglobin is found in muscle tissue
• Myoglobin stores and facilitates the diffusion of oxygen in the muscle
• It has a high affinity for oxygen
• Myoglobin responds to the muscle's oxygen needs
• Myoglobin consists of 153 amino acids, is compact, and tightly folded
• 75% of myoglobin is -helical, consisting of 8 helices
• Histidine 93 in the 8th -helix is covalently linked to Fe
• Haem is linked into the Fe helix by the proximal and distal histidine
• Oxygen binding to myoglobin shows a hyperbolic dependence on oxygen concentration

Haemoglobin

• Haemoglobin is found in the blood, specifically in red blood cells
• Haemoglobin is responsible for the transport of oxygen throughout the body, responding to oxygen availability
• It binds oxygen in the lungs
• Haemoglobin permits the transfer of oxygen from the blood to cells and tissues
• It allows the transfer of oxygen from blood haemoglobin to muscle myoglobin
• Adult haemoglobin is an alpha 2 beta 2 tetramer
• Haemoglobin has four polypeptide chains: two alpha chains (141 aa) and two beta chains (146 aa)
• Each chain contains a haem prosthetic group that binds one O2
• The conformation of each polypeptide chain is very similar to that of myoglobin, sharing similar amino acids
• X-ray crystallography reveals that deoxyhaemoglobin can exist in a low-affinity T state or a high-affinity R state
• Oxygen binding promotes the stabilization of the R state and a 15% rotation in the molecule that enhances further oxygen binding
• The oxygen binding curve for haemoglobin is sigmoidal because of cooperative binding of oxygen

Oxygen Binding

• Binding affinity for oxygen increases as more oxygen molecules bind to Hb subunits
• The binding of the first O2 molecule to one subunit is difficult, showing low affinity
• The binding of the last O2 molecule to the fourth subunit is very easy, thus showing high affinity
• Deoxyhaemoglobin has a low affinity for O2
• Successive O2 binding incrementally increases the affinity for the other sites
• The steepest part of the oxygen binding curve of haemoglobin coincides with a pO2 common to tissues
• The sigmoidal binding curve of haemoglobin means that O2 can be efficiently carried from the lungs to the tissues
• Haemoglobin is a mix of low and high affinity states, resulting in a physiologically functional intermediate state
• Haemoglobin is more sensitive to small differences in O2 concentrations
• The cooperative nature of O2 binding to Hb means single unoccupied heme groups have a high affinity for O2
• This allows for the capture of maximal O2 as it diffuses across the blood-gas interface
• Oxygen concentrations in tissues allow oxygen delivery to respond to small changes in pO2
• The oxygen dissociation curve explains Hb’s ability to bind oxygen in the lung and release it to the tissues
• Transfer of oxygen is facilitated by the steepness of the pressure gradient between blood and mitochondria, where it is used for oxidative phosphorylation

Normal Values

• PO2 in mixed alveolar gas is 100mmHg (13KPa)
• PCO2 in mixed alveolar air is 40 mmHg (5.3KPa)
• PO2 in venous blood is 40 mmHg (5.3KPa)
• PCO2 in venous blood is 45 mmHg (6kPa)
• PO2 in arterial blood is 75-100mmHg (11-13 KPa)
• PCO2 in Arterial blood is 35-45 mmHg (4-6KPa)
• PO2 in peripheral tissues is 40 mmHg (5.3 KPa)
• PCO2 in peripheral tissues is 45-50mmHg (6-6.5KPa)
• In the lungs, mixed venous blood in the distributaries of the pulmonary arteries has a pO2 at 40 mmHg (5.3 KPa) and saturation of approximately 75%
• The pO2 of the air in the alveoli is 100mmHg (13KPa). There is a gradient.
• The oxygen dissociation curve for haemoglobin plateaus around 60 mm Hg (8KPa)

Haem

• Haem consists of a porphyrin ring and an Fe atom bound to 4 N atoms of the ring
• Fe2+ can make bonds to oxygen on either side of the plane, though only one at a time
• One molecule of O2 binds to the haem group in myoglobin and haemoglobin
• The Fe atom is bound to the protein via a histidine residue (proximal histidine) on the other side of the ring

Haldane Effect

• Haemoglobin gives up CO2 when pO2 rises (lungs) and binds CO2 when pO2 falls (tissues)
• Red cell carbonic anhydrase also facilitates haemoglobin and carbon dioxide transport

Bohr Effect

• H+ and CO2 both bind to haemoglobin molecules
• The binding of H+ and CO2 lowers the affinity of haemoglobin for O2
• Metabolically active tissues produce large amounts of H+ and CO2 (HCO3-)
• H+ facilitates O2 unloading by shifting the dissociation curve to the right, increasing CO2 carrying capacity
• The Bohr effect ensures the delivery of O2 is coupled to demand

2,3-Bisphosphoglycerate (BPG)

• 2-3 diphosphoglycerate decreases haemoglobin’s oxygen affinity
• Chronic hypoxemia caused by pathological lung conditions or high altitude stimulates 2-3 DPG (2 3BPG), an intermediate from glycolysis
• Stimulating 2-3 diphosphoglycerate shifts the haemoglobin oxygen curve to the right, increasing tissue accessibility to oxygen
• It is present in red blood cells at approximately 5 mM
• One BPG binds per haemoglobin tetramer and decreases the affinity for O2
• BPG concentration increases at high altitudes, promoting O2 release at the tissues, also with Sickle Cell Anaemia

Carbon Monoxide (CO)

• Carbon monoxide is a poison because it combines with ferromyoglobin and ferrohaemoglobin and blocks oxygen transport
• CO binds to haemoglobin 250x more readily than O2
• Carbon monoxide poisoning is fatal when COHb is >50%
• CO binding also acts to increase the affinity for oxygen for unaffected subunits, preventing oxygen delivery to the tissues
• Faulty boilers can lead to high levels of carbon monoxide in homes
• Carbon monoxide is Colourless, Odourless, and Tasteless
• The two types of CO poisoning are acute poisoning and chronic poisoning
• Common symptoms of carbon monoxide poisoning include headache, dizziness and fatigue, blurry or double vision, shortness of breath, confusion, chest pain, nausea, and vomiting
• CO poisoning can turn skin a bright cherry color which persists after death

Abnormal Red Blood Cell Morphologies

• Abnormal red blood cell morphologies are associated with various types of anaemia
• These include Sickle cell disease, Thalassemia, Megaloblastic anaemia, Iron deficiency, Hereditary spherocytosis, Hereditary elliptocytosis, Haemoglobin C disease, Acanthocytosis, DIC, TTP, and Heart valve prosthesis

Conversions

• 1 torr = 1 mmHg = 0.133 KPa