Carbonic anhydrases 5-class

Sulfonamides derived from sulfanilamide (p-arninoben2enesulfonainide) are commonly referred to as sulfa dmgs. Although several dmg classes are characterized by the presence of a sulfonamide function, eg, hypoglycemics, carbonic anhydrase inhibitors, saluretics, and tubular transport inhibitors, the antibacterial sulfonamides have become classified as the sulfa dmgs. Therapeutically active derivatives are usually substituted on the N nitrogen the position is generally unsubstituted. These features are illustrated by the stmctures of sulfanilamide (1) and sulfadiazine (2)... 

Iverson, T. M., Alber, B. E., Kisker, C., Ferry, J. G., and Rees, D. C. (2000). A closer look at the active site of gamma-class carbonic anhydrases High-resolution crystallographic studies of the carbonic anhydrase from Methanosarcina thermophila. Biochemistry 39, 9222-9231. 

The carbonic anhydrase (Cam) in M. thermophila cells is elevated several fold when the energy source is shifted to acetate, suggesting a role for this enzyme in the acetate-fermentation pathway. It is proposed that Cam functions outside the cell membrane to convert CO2 to a charged species (reaction A4) thereby facilitating removal of product from the cytoplasm. Cam is the prototype of a new class (y) of carbonic anhydrases, independently evolved from the other two classes (a and P). The crystal structure of Cam reveals a novel left-handed parallel P-helix fold (Kisker et al. 1996). Apart from the histidines ligating zinc, the activesite residues of Cam have no recognizable analogs in the active sites of the a- and P-classes. Kinetic analyses establish that the enzyme has a zinc-hydroxide mechanism similar to that of Cab (Alber et al. 1999). 

Smith KS, Ferry JG. 1999. A plant-type (beta-class) carbonic anhydrase in the thermophilic vaethanoarchaeon Methanobacterium thermoautotrophicum. J Bacteriol 181 6247-53. 

Smith KS, Gosper NJ, Stalhandske C, et al. 2000. Structural and kinetic characterization of an archaeal beta-class carbonic anhydrase. J Bacteriol 182 6605-13. 

Strop P, Smith KS, Iverson TM, et al. 2001. Crystal structure of the cab -type beta class carbonic anhydrase from the archaeon Methanobacterium thermoautotrophicum. J Biol Chem 276 10299-305. 

Tripp BC, Ferry JG. 2000. A structure-function study of a proton transport pathway in a novel gamma-class carbonic anhydrase from Methanosarcina thermophila. Biochemistry 39 9232 0. 

Chemical Class Carbonic anhydrase inhibitor sulfonamide derivative... 

It is noteworthy that sulfanilamide structural modifications have led to other valuable classes of drugs already discussed, including the hypoglycemic sulfonylureas and the diuretic carbonic anhydrase inhibitors. 

Zinc is an essential trace element. More than 300 enzymes that require zinc ions for activity are known. Most catalyze hydrolysis reactions, but zinc-containing representatives of aU enzyme classes are known, such as, for instance, alcohol dehydrogenase (an oxidoreductase), famesyl-Zgeranyl transferase (a transferase), -lactamase (a hydrolase), carbonic anhydrase (a lyase) and phosphomannose isomerase. 

Structure of a-Class Carbonic Anhydrase from Human Erythrocytes (the High Activity form HCA II)... 

Structure of y-Class Carbonic Anhydrase of Methanosarcina thermophila (Cam) A. Proposed C02 Hydration Mechanism for y-Class Carbonic Anhydrase... 

There are several types of -class CAs i.e., a-CA I-VII, reported in the literature, out of which the human carbonic anhydrase II (HCA II), the most extensively studied carbonic anhydrase, has an exceptionally high CO2 hydration rate and a wide tissue distribution 107). The HCA II comprises a single polypeptide chain with a molecular mass of 29.3 kDa and contains one catalytic zinc ion, coordinated to three histidine residues, His 94, His 96, and His 119. A tetrahedral coordination geometry around the metal center is completed with a water molecule, which forms a hydroxide ion with a pK value of 7.0 108). Quigley and co-workers 109,110) reported that the inhibition of the synthesis of HCO3 from CO2 and OH- reduces aqueous humor formation and lowers intra-ocular pressure, which is a major risk factor for primary open-angle glaucoma. 

Crystallographic studies indicated that in the active-site of Zn-Cam, Zn2+ has three residues of the protein and two solvent ligands arranged in a TBP geometry (219). The zinc ion is bound by three histidines (protein residues) of two monomers His-81 and His-122 are contributed by one monomer and His-117 is contributed by an adjacent monomer. The active-site of -class carbonic anhydrase in Cam is located at the base of the cleft between two monomers. The proposed hydrophobic pocket consisting of Met-135b (where b denotes that the residue belongs to the adjacent monomer), Phe-138, Phe-140,... 

A. Proposed C02 Hydration Mechanism for y-Class Carbonic Anhydrase... 

Roberts et al. reported a 27 kDa monomeric carbonic anhydrase, TWCAl, from the marine diatom Thalassiosira weissflogii (221). X-ray absorption spectroscopy indicated that the catalytic zinc is coordinated by three histidines and a water molecule, similar to the active sites of the a- and y-CAs (222). Also, the active site geometry is similar to that of a-CAs. Based on these results the catalytic mechanism is expected to be similar to that of the -class carbonic anhydrases. Tripp et al. (223) proposed that this TWCAl is the prototype of a fourth class carbonic anhydrase designated as 8-class CAs. In the... 

The most important chemical function of Zn2+ in enzymes is probably that of a Lewis acid providing a concentrated center of positive charge at a nucleophilic site on the substrate/ This role for Zn2+ is discussed for carboxypeptidases (Fig.12-16) and thermolysin, alkaline phosphatase (Fig. 12-23),h RNA polymerases, DNA polymerases, carbonic anhydrase (Fig. 13-1),1 class II aldolases (Fig. 13-7), some alcohol dehydrogenases (Fig. 15-5), and superoxide dismutases (Fig.16-22). Zinc ions in enzymes can often be replaced by Mn2+, Co2+, and other ions with substantial retention of catalytic activity/ ... 

Carbonic anhydrase is inhibited by several classes of inhibitor. The sulfonamides, of general formula XS02NH2, are well known, while inhibition by simple anions occurs particularly at higher pH values. Imidazole is a unique example as it is the only compound known at present to act as a competitive inhibitor for the hydration of C02. A great deal of mechanistic information has come from a study of the action of these compounds. 

Diuretics work by increasing the amount of sodium and fluids excreted by the kidneys. Less fluid means less total blood volume and improved circulation and blood pressure. There are five main classes of the drug loop diuretics, thiazide diuretics, potassium-sparing diuretics, osmotic diuretics, and carbonic anhydrase inhibitors. 

Alkaline phosphatases form a well-known class of proteins that perform quite interesting and complicated reactions. As previously reported, Zn enzymes, like carboxypeptidases, thermolysin, and carbonic anhydrases, consist of only one Zn atom per active center. Most of the alkaline phosphatases consist of two 96-kDa subunits, each containing two Zn and one Mg ion. The alkaline phosphatase from E. coli has been crystallized and described in full detail [4], and a mechanism has been proposed. Several enzymes in this category have been mentioned in recent years, some of them also containing different metal ions, such as iron and zinc, as in the purple acid phosphatase [5], It is likely that the detailed structure and mechanism of many more examples of enzymes that remove or add phosphate groups to proteins will become available in the next decade. 

The thieno[2,3-6]furan-2-sulfonamides (11), thieno[2,3-6]thiophene-2-sulfonamides, and thieno-[3,2-6]thiophene-2-sulfonamides were prepared and found to be a new class of topically active ocular hypotensive carbonic anhydrase inhibitors (91JMC1805,92JMC3027). 

A structure-function study of a proton pathway in the y-class carbonic anhydrase from Methanosarcina thermophila was conducted in the work of Tripp and Ferry (2000). Four enzyme glutamate residues were characterized by site-directed mutagenesis. It was shown that Glu 84 and an active site residue, Glu 89, are important for CO2 hydration activity, while external loop residues, Glu 88 and Glu 89 are less important. Glu 84 can be substituted for other ionizable residues with similar pKa values and, therefore, participates in the enzyme catalysis not as a chemical reagent but as a proton shuttle. 

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