In the red blood cell this reaction plays an important role in CO 2 transport from the
tissues to the lungs. In the kidney, the protons of the H 3 O+are exchanged for Na+ions,
which are reabsorbed, while HCO 3 is decomposed through a shift of the equilibrium to
the left. Carbonic anhydrase therefore plays a crucial role in maintaining the ion and
water balance between the tissues and urine.
When carbonic anhydrase inhibitors block the enzyme in the kidney, H 2 CO 3 formation—
and consequently the availability of H 3 O+(i.e., protons)—decreases. Since the Na+ions
in the filtrate cannot be exchanged, sodium is excreted, together with large amounts of
water, as a result of ion hydration and osmotic effects. The result is diuresis,accompa-
nied by a dramatic increase in urine volume. There is also failure to remove HCO 3 −ions
because there is no H 3 O+to form H 2 CO 3 , which would decompose to CO 2 +H 2 O.
Therefore, the normally slightly acidic urine becomes alkaline. The strong carbonic
anhydrase inhibitors also increase K+excretion, an undesirable effect.
Carbonic anhydrase has also been discovered in the eye and the CNS. Consequently,
its inhibitors have found use in the treatment of glaucoma, in reducing the high intraoc-
ular pressure which can lead to blindness in this disease. They are also effective, in
combination with anticonvulsant drugs, in controlling certain forms of epilepsy.
8.2.4.1 Carbonic Anhydrase: Enzyme Structure
The structure of carbonic anhydrase has been completely elucidated. Both the amino
acid sequence and the three-dimensional structure of the crystalline enzyme are known.
Actually, there are two isozymes, a low- and a high-activity form having been isolated
from human erythrocytes, with the latter designated the C form (HCA-C).
The large molecule consists of a single peptide chain; 35% β-sheet and 20% helical
structure are found in the folded structure. The active site is a 1.2 nm deep conical cavity
in the central pleated sheet, with a Zn^2 +ion located at its bottom. Three histidine residues
hold the Zn^2 +, which also binds an H 2 O molecule. The active-site cavity is divided into
hydrophilic and hydrophobic halves. The inhibitors of the enzyme replace the water on
the Zn^2 +ion and also block the fifth coordination site where CO 2 should bind.
8.2.4.2 Sulfonamide Carbonic Anhydrase Inhibitors
The development of sulfonamide carbonic anhydrase inhibitors was based on the obser-
vation that antibacterial sulfanilamides produce alkaline urine. This discovery led to the
development of acetazolamide (8.29), a thiadiazole derivative. It is not an ideal drug
because it promotes K+excretion and causes a very high urine pH. Since chloride ions
are not excreted simultaneously, systemic acidosis also results. Much more useful are
the chlorothiazide (8.30) derivatives, which are widely used as oral diuretic drugs.
These compounds differ from one another mainly in the nature of the substituent on C3;
ENDOGENOUS MACROMOLECULES 495