Describe the structure and types of haemoglobin. Add a note on its abnormal derivatives.

Definition and Structure

• Molecular weight 64,500 Da; a tetramer of four polypeptide chains, each with one haem
• Normal concentration 13–17 g/dL (male), 12–15 g/dL (female)
• 1 g of haemoglobin carries 1.34 mL of oxygen

Haem

• Protoporphyrin IX + ferrous iron (Fe2+)
• Iron is held by four bonds to pyrrole nitrogens, a fifth to the proximal histidine (F8) of globin, and the sixth is free to bind oxygen
• Iron must remain in the Fe2+ state — oxidation to Fe3+ gives methaemoglobin, which cannot carry oxygen
• Oxygen binding is oxygenation, not oxidation — the iron stays ferrous

Types of Human Haemoglobin

• HbF binds 2,3-BPG poorly, because its γ chains lack the necessary histidine residues → higher oxygen affinity, allowing the fetus to take oxygen from maternal blood

Oxygen Binding and the Dissociation Curve

• Sigmoid curve, reflecting co-operative binding — binding of the first oxygen increases the affinity of the remaining sites
• T (tense, low affinity) and R (relaxed, high affinity) states
• P50 = 27 mmHg — the partial pressure at which haemoglobin is half saturated
• Denaturation abolishes biological activity while leaving the peptide bonds intact, which is why a denatured protein still gives every colour reaction of protein but no longer works
• Most denaturation is irreversible in practice, though small proteins such as ribonuclease can refold spontaneously — the observation that established that the primary sequence determines the final shape
• Heat, extremes of pH, organic solvents, urea, heavy metals and mechanical agitation all denature protein, and each is used deliberately somewhere in the laboratory or the clinic
• Cooking, sterilisation and disinfection all depend on denaturation, as does the coagulation of albumin in the heat test for proteinuria