Haemoglobin structure and gas transport – Biochemistry/Physiology

What is haemoglobin?

A hemeprotein found exclusively in red blood cells (RBC); main function is transport of O₂ from the lungs to the capillaries of the tissues.

What are the components of haemoglobin?

Globin (protein) plus heme.

What are hemeproteins?

Specialised proteins that contain heme as a tightly bound prosthetic group.

Name examples of hemeproteins.

Haemoglobin, myoglobin, cytochromes, catalase.

Describe the structure of haemoglobin.

A globular protein consisting of four polypeptide chains forming a tetramer.

What is the major haemoglobin in adults?

Haemoglobin A (HbA).

What is the globin chain composition of HbA?

α₂β₂ (2 alpha chains and 2 beta chains).

What level of protein structure does α₂β₂ represent?

Quaternary structure.

How are the globin chains in haemoglobin held together?

By non-covalent interactions.

What is a haemoglobin subunit?

Each individual polypeptide chain.

How is the haemoglobin tetramer organized?

Into two identical dimers: (αβ)₁ and (αβ)₂.

What forms the strong association between α and β subunits in a dimer?

Interchain hydrophobic interactions.

What additional interactions occur within a haemoglobin dimer?

Ionic and hydrogen bonds.

How are the two haemoglobin dimers held together?

Primarily by polar bonds.

Why can the two haemoglobin dimers move relative to one another?

The interactions between dimers are weaker than those within dimers.

What causes deoxyhaemoglobin and oxyhaemoglobin to occupy different relative positions?

Weaker interactions between the mobile dimers.

What is the T (taut) form of haemoglobin?

The deoxy form of haemoglobin.

What is the oxygen affinity of the T form?

Low oxygen affinity.

How do αβ dimers interact in the T form?

Through a network of ionic and hydrogen bonds that constrain movement of the polypeptide chains.

What is the R (relaxed) form of haemoglobin?

The high oxygen-affinity form of haemoglobin.

What causes conversion of T form to R form?

Binding of O₂ to haemoglobin.

How does O₂ binding promote the R form?

It ruptures some ionic and hydrogen bonds between αβ dimers.

What happens to polypeptide chain movement in the R form?

Chains have greater freedom of movement.

What is heme?

A complex of protoporphyrin IX and ferrous iron (Fe²⁺).

Where is iron located within the heme molecule?

At the centre of the porphyrin ring.

How is Fe²⁺ held within the heme molecule?

By bonds to the four nitrogens of the porphyrin ring.

How many additional bonds can Fe²⁺ form?

Two additional bonds, one on each side of the planar porphyrin ring.

What occupies one additional Fe²⁺ coordination position?

A histidine residue of the globin molecule.

What occupies the other additional Fe²⁺ coordination position?

Oxygen.

What is methaemoglobin?

Haemoglobin in which iron has been oxidized from Fe²⁺ to Fe³⁺.

Can methaemoglobin bind oxygen?

No.

What occupies the sixth coordinate position of Fe³⁺ in methaemoglobin?

Water.

What can cause oxidation of haemoglobin to methaemoglobin?

Certain drugs or H₂O₂.

Which enzyme corrects occasional oxidation of haemoglobin iron?

NADH-cytochrome b₅ reductase.

What are the three functions of haemoglobin?

Transport O₂ from lungs to tissues, transport CO₂ from tissues to lungs, and buffer blood pH.

How is molecular oxygen carried in blood?

Bound to haemoglobin and dissolved in plasma.

What percentage of oxygen is transported dissolved in plasma?

About 1.5%.

Why is only a small amount of oxygen dissolved in plasma?

Oxygen is poorly soluble in water.

How many O₂ molecules can one haemoglobin molecule bind?

Four.

Describe oxygen binding to haemoglobin.

Rapid and reversible.

What is oxyhaemoglobin?

Haemoglobin combined with oxygen (HbO₂).

What is deoxyhaemoglobin?

Reduced haemoglobin that has released oxygen (HHb).

What reaction occurs in the lungs?

HHb + O₂ → HbO₂ + H⁺.

What is heme-heme interaction?

Cooperative binding in which binding of one O₂ increases affinity of remaining heme groups for oxygen.

What type of binding does haemoglobin exhibit?

Cooperative binding.

What structural change occurs after the first oxygen binds?

T form changes to R form.

How much greater is the affinity for the last O₂ molecule than for the first?

Approximately 300 times greater.

What effect does unloading one oxygen molecule have?

Enhances unloading of subsequent oxygen molecules.

What is fully saturated haemoglobin?

All four heme groups are bound to oxygen.

What is partially saturated haemoglobin?

One, two, or three oxygen molecules are bound.

What largely determines oxygen binding to haemoglobin?

Partial pressure of oxygen (PO₂).

What does PO₂ indicate?

Amount of oxygen dissolved in plasma, not oxygen bound to haemoglobin.

What is the oxygen-haemoglobin dissociation curve?

A plot of haemoglobin saturation against PO₂.

What is the shape of the oxygen-haemoglobin dissociation curve?

Sigmoid (S-shaped).

Why is the oxygen-haemoglobin dissociation curve sigmoid?

Because of cooperative oxygen binding (heme-heme interaction).

What is haemoglobin's oxygen affinity at PO₂ < 10 mmHg?

Very low.

What happens between PO₂ 10-50 mmHg on the oxygen dissociation curve?

Oxygen affinity increases substantially and the curve is steep.

Where is PO₂ 10-50 mmHg generally found?

In the tissues.

What happens between PO₂ 70-100 mmHg on the oxygen dissociation curve?

The curve flattens rapidly toward 100% saturation.

Where is PO₂ 70-100 mmHg generally found?

In the alveoli of the lungs.

What does a left shift of the oxygen-haemoglobin dissociation curve indicate?

Increased haemoglobin affinity for oxygen.

What is the consequence of a left shift of the oxygen dissociation curve?

Haemoglobin does not readily release oxygen to tissues.

What does a right shift of the oxygen-haemoglobin dissociation curve indicate?

Decreased haemoglobin affinity for oxygen.

What is the consequence of a right shift of the oxygen dissociation curve?

Increased delivery of oxygen to tissues.

What four factors affect haemoglobin affinity for oxygen?

Temperature, blood pH, carbon dioxide, and 2,3-bisphosphoglycerate (2,3-BPG).

How does increased temperature affect the oxygen-haemoglobin dissociation curve?

Shifts it to the right.

How does increased temperature affect haemoglobin affinity for oxygen?

Decreases affinity.

What is the physiological advantage of increased temperature decreasing oxygen affinity?

More oxygen is released to active, warmer tissues.

How does decreased blood pH affect the Hb-O₂ bond?

Weakens the bond.

How does decreased pH affect haemoglobin affinity for oxygen?

Decreases affinity.

How does decreased pH affect oxygen unloading?

Increases oxygen unloading to tissues.

What is the Bohr effect?

Reduced haemoglobin affinity for oxygen caused by decreased pH or increased H⁺ concentration.

How does increased PCO₂ affect haemoglobin affinity for oxygen?

Decreases affinity.

How does increased PCO₂ affect oxygen unloading?

Increases oxygen unloading.

Why is the effect of increased PCO₂ beneficial?

Active tissues release more CO₂ and H⁺, promoting oxygen release from oxyhaemoglobin.

Why is glucose metabolism entirely anaerobic in RBCs?

RBCs lack mitochondria.

How is 2,3-BPG produced?

It is synthesized from an intermediate of glycolysis.

How does increased 2,3-BPG affect the oxygen dissociation curve?

Shifts it to the right.

How does increased 2,3-BPG affect haemoglobin oxygen affinity?

Decreases affinity.

How does increased 2,3-BPG affect oxygen unloading?

Increases oxygen unloading.

When does 2,3-BPG concentration increase?

During chronic hypoxia, obstructive pulmonary emphysema, high altitudes, and chronic anaemia.

What is the chain composition of fetal haemoglobin (HbF)?

α₂γ₂.

How does the oxygen affinity of HbF compare with HbA?

HbF has a higher affinity for oxygen.

Why does HbF have a higher oxygen affinity than HbA?

HbF binds only weakly to 2,3-BPG.

What is the shape of the oxygen dissociation curve for myoglobin?

Hyperbolic.

Why is the myoglobin oxygen dissociation curve hyperbolic?

Myoglobin binds a single oxygen molecule.

What is the oxygen binding reaction of myoglobin?

Mb + O₂ ⇌ MbO₂.

What is the role of myoglobin in muscle?

Binds oxygen released by haemoglobin at low PO₂.

When does myoglobin release oxygen?

When PO₂ becomes very low and oxygen demand increases.

In what three forms is carbon dioxide transported in blood?

Dissolved in plasma, chemically bound to haemoglobin, and as bicarbonate ion.

What percentage of carbon dioxide is transported dissolved in plasma?

7-10%.

What percentage of carbon dioxide is transported bound to haemoglobin?

20-30%.

What is carbaminohaemoglobin?

Haemoglobin carrying carbon dioxide (HbCO₂).

Where is carbaminohaemoglobin carried?

Within RBCs.

What reaction forms carbaminohaemoglobin?

CO₂ + Hb ⇌ HbCO₂.

Where does CO₂ bind in carbaminohaemoglobin?

Directly to amino acids of the globin.

What is the major form of carbon dioxide transport?

Bicarbonate ion (HCO₃⁻).

What percentage of carbon dioxide is transported as bicarbonate?

60-70%.

Where is bicarbonate formed?

Inside red blood cells.

What happens when CO₂ enters RBCs?

It combines with water to form carbonic acid (H₂CO₃).