Haemoglobin synthesis

Haemoglobin performs the main functions of red cells – carrying O2 to the tissues and returning CO2 from the tissues to the lungs. Each normal adult Hb molecule (Hb A) has a molecular weight of 68 000 and consists of two α and two β globin polypeptide chains (α2β2) which have 141 and 146 amino acids respectively. HbA comprises about 97% of the Hb in adults. Two other types, Hb A2 (α2δ2) and Hb F (α2γ2), are found in adults in small amounts (1.5-3.2% and < 1%, respectively).

Haemoglobin synthesis occurs in the mitochondria of the developing red cell. The major rate-limiting step is the conversion of glycine and succinic acid to δ-aminolaevulinic acid (ALA) by ALA synthase. Vitamin B6 is a coenzyme for this reaction, which is inhibited by haem and stimulated by erythropoietin. Two molecules of δ-ALA condense to form a pyrrole ring (porphobilinogen). These rings are then grouped in fours to produce protoporphyrins. Finally, iron is inserted to form haem. Haem is then inserted into the globin chains to form Hb.

The plasma membrane is freely permeable to gases such as O2, CO2 and N2, and to small uncharged molecules such as H2O (not H+ and OH-) and urea. Whilst larger hydrophobic lipid-soluble molecules – like steroids – also pass freely through the membrane, large uncharged molecules (glucose, amino acids and nucleotides) and small charged ions (K+, Na+, Ca2+, Cl-, Mg2+ and HCO3-) cannot pass unless via a specific transport protein embedded in the plasma membrane. Two structural types of transport molecules/complexes exist.

- Channel proteins literally open a channel in the lipid membrane to allow a specific solute to pass through.
- Carrier proteins are slower in action, shuttling the solute across and either facilitating diffusion down a gradient across the membrane, or actively pumping solutes against the gradient using ATP as an energy source.

Active carrier pumps and gated ion channels work together in neural transmission. These carrier proteins pump Na+ and K+ across the neuronal cell membrane to create a differential gradient, but ion channels open in response to stimuli to cause a rapid depolarization, allowing the ions to flow back. At synaptic junctions these ion channels open in response to chemical signals such as the release of glutamate, epinephrine (adrenaline) or acetylcholine.

ATP-dependent transport molecules (ATPases) belong to a superfamily called the ‘ABC transporter superfamily’. These include the multidrug-resistance protein (MDR), which pumps out hydrophobic drugs and is overexpressed by tumour cells, and the chloride ion pump coded by the cystic fibrosis gene. All share a common structure of six transmembrane domains interrupted by a cytoplasmic ATPase domain, followed by a further six transmembrane domains and another cytoplasmic ATPase. The cystic fibrosis chloride ion channel is unusual in that it requires the binding and hydrolysis of both ATP and cAMP for activation.

The major function of the tubule is the selective reabsorption or excretion of water and various cations and anions to keep the volume and electrolyte composition of body fluid normal.

The active reabsorption from the glomerular filtrate of compounds such as glucose and amino acids also takes place. Within the normal range of blood concentrations these substances are completely reabsorbed by the proximal tubule. However, if blood levels are elevated above the normal range, the amount filtered (filtered load = GFR × plasma concentration) may exceed the maximal absorptive capacity of the tubule and the compound ’spills over’ into the urine. Examples of this occur with hyperglycaemia in diabetes mellitus or elevated plasma phenylalanine in phenylketonuria.

Conversely, inherited or acquired defects in tubular function may lead to incomplete absorption of a normal filtered load, with loss of the compound in the urine (a lowered ‘renal threshold’). This is seen in renal glycosuria, in which there is a genetically determined defect in tubular reabsorption of glucose. It is diagnosed by demonstrating glycosuria in the presence of normal blood glucose levels. Inherited or acquired defects in the tubular reabsorption of amino acids, phosphate, sodium, potassium and calcium also occur, either singly or in combination. Examples include cystinuria and the Fanconi syndrome. Tubular defects in the reabsorption of water result in nephrogenic diabetes insipidus. Under normal circumstances, antidiuretic hormone induces an increase in the permeability of water in the collecting ducts by attachment to receptors with subsequent activation of adenyl cyclase. This then activates a protein kinase, which induces preformed cytoplasmic vesicles containing water channels (termed ‘aquaporins’) to move to and insert into the tubular luminal membrane. This allows water entry into tubular cells down a favourable osmotic gradient. Water then crosses the basolateral membrane and enters the bloodstream. When the effect of ADH wears off, water channels return to the cell cytoplasm.