Carbonic Anhydrase Activators:  X-ray Crystallographic and Spectroscopic Investigations for the Interaction of Isozymes I and II with Histamine^,

Abstract

The interaction of native and Co(II)-substituted isozymes I and II of carbonic anhydrase (CA) with histamine, a well-known activator, was investigated kinetically, spectroscopically, and X-ray crystallographically. This activator is of the noncompetitive type with 4-nitrophenyl acetate and CO2 as substrates for both HCA I and HCA II. The electronic spectrum of the adduct of Co(II)-HCA II with histamine is similar to the spectrum of the Co(II)-HCA II-phenol adduct, being only slightly different from that of the uncomplexed enzyme. This is the first spectroscopic evidence that the activator molecule binds within the active site, but not directly to the metal ion. X-ray crystallographic data for the adduct of HCA II with histamine showed that the activator molecule is bound at the entrance of the active site cavity in a position where it may actively participate in shuttling protons between the active site and the bulk solvent. The role of the activators and the reported X-ray crystal structure of the HCA II-histamine adduct has prompted us to reexamine the X-ray structures of the different CA isozymes in order to find a structural basis accounting for their large differences in catalytic rate. A tentative explanation is proposed on the basis of possible pathways of proton transfer, which constitute the rate-limiting step in the catalytic reaction.

Carbonic anhydrase inhibitors

“…2), being coordinated by three histidine residues (His 94, His 96, and His 119) and a water molecule/ hydroxide ion. 1,2,7,10 The zincbound water is also engaged in hydrogen bond interactions with the hydroxyl moiety of Thr 199, which in turn is bridged to the carboxylate moiety of Glu 106; these interactions enhance the nucleophilicity of the zinc-bound water molecule, and orient the substrate (CO 2 ) in a favorable location for the nucleophilic attack (Fig. 3).…”

Activation of Two Different Drugs Used in Alzheimer’s Disease Treatment on Human Carbonic Anhydrase Isozymes I and II Activity: an <i>In Vitro</i> Study

“…It was reported to increase the activity of CA and to attend the proton shuttling process. 11 Function of CA activators is to bind at the entrance of the enzyme active site, at the same time to ease the proton transfer processes between active site and solvent system. Histidine, phenylalanine, sildenafil citrate have been shown to be potential activators of different CA isozymes.…”

“…2,[12][13][14][15] Generally it is known that activators bind to different site from the inhibitors within the enzyme active cavity. 11,16 Also, they participate in facilitated the proton transfer processes between active site and solvent system, shuttling protons with groups which have an appropriate pKa such as the carboxylate groups. 17 Memantine is an antagonist of N-methyl-D-aspartate glutamate receptors as uncompetitively.…”

Synthesis and characterization of phenolic Mannich bases and effects of these compounds on human carbonic anhydrase isozymes I and II

“…Since CA inhibitors have been shown to reduce IOP exclusively by lowering the aqueous humour flow and these compounds have been used for the treatment of glaucoma for years 11,12 . The rate-determining step for the CO 2 hydration reaction catalyzed by CAs is the proton transfer reaction from the water bound to the Zn(II) ion to the reaction medium, with generation of the zinc hydroxide species of the enzyme [13][14][15][16][17][18][19][20] . Several anions behave as inhibitors for the CO 2 hydration reaction 21,22 and coordinate directly to Zn(II) ion.…”