Carbonic Anhydrase Activators. Activation of Isozymes I, II, IV, VA, VII, and XIV withL- andD-Histidine and Crystallographic Analysis of Their Adducts with Isoform II: Engineering Proton-Transfer Processes within the Active Site of an Enzyme

Carbonic anhydrase activators: The first activation study of the human secretory isoform VI with amino acids and amines

The secretory isozyme of human carbonic anhydrase (hCA, EC 4.2.1.1), hCA VI, has been cloned, expressed, and purified in a bacterial expression system. The kinetic parameters for the CO(2) hydration reaction proved hCA VI to possess a k(cat) of 3.4 x 10(5)s(-1) and k(cat)/K(M) of 4.9 x 10(7)M(-1)s(-1) (at pH 7.5 and 20 degrees C). hCA VI has a significant catalytic activity for the physiological reaction, of the same order of magnitude as the ubiquitous isoform CA I or the transmembrane, tumor-associated isozyme CA IX. A series of amino acids and amines were shown to act as CA VI activators, with variable efficacies. l-His, l-Trp, and dopamine showed weak CA VI activating effects (K(A)s in the range of 21-42 microM), whereas d-His, d-Phe, l-DOPA, l-Trp, serotonin, and some pyridyl-alkylamines were better activators, with K(A)s in the range of 13-19 microM. The best CA VI activators were l-Phe, d-DOPA, l-Tyr, 4-amino-l-Phe, and histamine, with K(A)s in the range of 1.23-9.31 microM. All these activators enhance k(cat), having no effect on K(M), participating thus in the rate determining step in the catalytic cycle, the proton transfer reactions between the enzyme active site and the environment.

Carbonic anhydrase activators: activation of the human isoforms VII (cytosolic) and XIV (transmembrane) with amino acids and amines

An activation study of the human carbonic anhydrase (hCA, EC 4.2.1.1) isozymes VII and XIV using a small library of natural/non-natural amino acids and aromatic/heterocyclic amines is reported. hCA VII was efficiently activated by L-/D-His, dopamine and serotonin (K(A)s of 0.71-0.93 microM). The best hCA XIV activators were histamine (K(A) of 10 nM), L-Phe, L-/D-His and 4-amino-L-Phe (K(A)s of 0.24-2.90 microM). In view of the significant expression levels of CA VII and CA XIV in the brain, selective activation of these isoforms may be useful when developing pharmacologic agents for the management of major disorders such as epilepsy and Alzheimer's disease.

Phosph(on)ate as a zinc-binding group in metalloenzyme inhibitors: X-ray crystal structure of the antiviral drug foscarnet complexed to human carbonic anhydrase I

Foscarnet (phosphonoformate trisodium salt), an antiviral used for the treatment of HIV and herpes virus infections, also acts as an activator or inhibitor of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). Interaction of the drug with 11 CA isozymes has been investigated kinetically, and the X-ray structure of its adduct with isoform I (hCA I-foscarnet complex) has been resolved. The first CA inhibitor possessing a phosphonate zinc-binding group is thus evidenced, together with the factors governing recognition of such small molecules by a metalloenzyme active site. Foscarnet is also a clear-cut example of modulator of an enzyme activity which can act either as an activator or inhibitor of a CA isozyme.

Carbonic anhydrase activators: l-Adrenaline plugs the active site entrance of isozyme II, activating better isoforms I, IV, VA, VII, and XIV

The activation of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1) with L-adrenaline and histamine has been investigated by kinetic and X-ray crystallographic studies. L-Adrenaline behaves as a potent activator of isozyme CA I (activation constant of 90 nM), being a much weaker activator of isozyme CA II (activation constant of 96 microM). Isoforms CA IV, VA, VII, and XIV were activated by L-adrenaline with K(A)s in the range of 36-63 microM. The X-ray crystal structure of the CA II-L-adrenaline adduct revealed that the activator plugs the entrance of the active site cavity, obstructing it almost completely.

Carbonic anhydrase activators: an activation study of the human mitochondrial isoforms VA and VB with amino acids and amines

The mitochondrial isozymes of human carbonic anhydrase (hCA, EC 4.2.1.1), hCA VA and hCA VB, were investigated for activation with a series of amino acids and amines. D-His, L-DOPA, histamine, dopamine, and 4-(2-aminoethyl)morpholine were excellent hCA VA activators, with KAs in the range of 10-130 nM. Good hCA VB activating effects were identified for L-His, D-Phe, D-DOPA, L-Trp, L-Tyr, serotonin, and 2-(2-aminoethyl)-pyridine, with KAs in the range of 44-110 nM. All these activators enhanced kcat, having no effect on KM, favoring thus the rate-determining step in the catalytic cycle, the proton transfer reactions between the active site and environment. The activation pattern of the two mitochondrial isoforms is very different from each other and as compared to those of the cytosolic isoforms hCA I and II.