Copper-carbonic anhydrase

XH NMR data of copper-carbonic anhydrase (CuCA) complexes in the presence of different anions indicated that water is present in the coordination sphere along with the anions (137). The three histidines, the anion, and the coordinated water molecule arrange themselves to maintain essentially a SQPY. His-94 would be in the apical position of the SQPY and two other histidine residues (His-96 and His-119) along with the anion and the coordinated water are positioned in the basal plane. Most likely the anion is present in the hydrophobic pocket or in the site and the coordinated water molecule is present in the C site or the hydrophilic binding site. 

Theoretical calculations have been carried out on a number of zinc-containing enzymatic systems. For example, calculations on the mechanism of the Cu/Zn enzyme show the importance of the full protein environment to get an accurate description of the copper redox process, i.e., including the electronic effects of the zinc ion.989 Transition structures at the active site of carbonic anhydrase have been the subject of ab initio calculations, in particular [ZnOHC02]+, [ZnHC03H20]+, and [Zn(NH3)3HC03]+.990... 

The identification of different carbonate binding modes in copper(II) and in zinc(II)/2,2 -bipyridine or tris(2-aminoethyl)amine/(bi)carbonate systems, specifically the characterization by X-ray diffraction techniques of both r)1 and r 2 isomers of [Cu(phen)2(HC03)]+ in their respective perchlorate salts, supports theories of the mechanism of action of carbonic anhydrase which invoke intramolecular proton transfer and thus participation by r)1 and by r 2 bicarbonate (55,318). 

Two stable bicarbonato complexes of bis(l,10-phenanthroline) copper(II) were reported for the first time by Mao et al. (44). These are akin to the Lipscomb and Lindskog structures of human carbonic anhydrase (HCA) (45). In the Lipscomb structure the bicarbonate acts as a bidentate ligand while in the Lindskog structure it is essentially coordinated to the metal center in unidentate fashion (Fig. 2). 

Figure 3 Examples of metal cofactors in proteins (a) the zinc center of carbonic anhydrase, (b) the blue-copper center of plastocyanin, (c) the iron center in 2,3-dihydroxybiphenil dioxygenase, (d) the iron binding site of transferrin, and (e) the dinuclear copper site of Cu/ in cytochrome c oxidase.

Figure 4 A zinc metallo-enzyme carbonic anhydrase for the very fast reversible formation of carbonic acid, H2CO3 from CO2 and H2O. Note the complexity of the protein required apparently to secure selectivity and the constrained, S-coordInate, state of the zinc. There is a channel for substrates to the zinc in contrast to the copper site shown in Figure 3.

Are they a form of carbonic anhydrase Carbonic anhydrases (CAs) play an important role in photosynthetic carbon fixation, converting HC03 (aq) in seawater to CO2. It is possible that the copper complexes of these peptides perform this function. Evidence from the studies by van den Brenk et al show that patellamide D (4) forms [PatDH + Cu2 + C02]" and [PatDHa + Cui + COa] complexes in the mass spectrometer lending credence to this proposal. 

Phytoplankton particulate matter (organic and biomineralized) contains many trace elements. The most abundant are magnesium, cadmium, iron, calcium, barium, copper, nickel, zinc, and aluminum (Table 1), which are important constituents of enzymes, pigments, and structural materials. Carbonic anhydrase requires zinc or cadmium (Price and Morel, 1990 Lane and Morel, 2000), nitrate reductase requires iron (Geider and LaRoche, 1994), and chlorophyll contains magnesium. Additionally, elements such as sodium, magnesium, phosphorus, chlorine, potassium, and calcium may be present as ions... 

Zinc is found in more than 80 enzymes. Two of these, carboxypeptidase and carbonic anhydrase, will be discussed here. Copper is also a common metal in enzymes and is present in four different forms. Two of the copper enzymes will also be described. 

Cobalt has recently been used as an ESR active substitute in zinc metalloenzymes. Whilst liquid helium temperatures may be needed and theoretical aspects of the spectra are not yet as well understood, cobalt has two important advantages over copper as a metal substitute, namely that many cobalt derivatives show some enzymic activity (e.g. cobalt in carbonic anhydrase, alkaline phosphatase and superoxide dismutase) and that g values and hyperfine splitting are more sensitive to ligand environment, particularly when low spin. ESR data have been reported for cobalt substituted thermolysin, carboxypeptidase A, procarboxypeptidase A and alkaline phosphatase [51]. These are all high spin complexes. Cobalt carbonic anhydrase has been prepared and reacted with cyanide [52]. In... 

Step 1 A water molecule is attracted to the zinc ion at the active site of carbonic anhydrase. The positively charged zinc ion displaces a proton from the water molecule. The displaced proton finds a new place of residence — the histidine residue. This histidine residue prt>bably aids in the removal of the proton from the water molecule, in concert wdth the action of the zinc ion. Combination of the zinc ion (Zn ) vedth the hydroxyl group does not form a complex with the structure Zn OH. The zinc atom does not change its valence (Its number of charges). Instead, the complex has the structure Zn (OH ). [Calcium ions behave similarly to zinc ions. In contrast, iron and copper ions readily change their valences when they participate in biochemical reactions.)... 

Zinc is used by a great number of enzymes and proteins, whereas copper seems lobe limited to only a few functions. The most thoroughly studied zinc metailoen-zymes of mammals are carbonic anhydrase, carboxypeptidase A and related pep-... 

Use of apoenzymes for the detection of metal ions Generally, apoenzymes of metalloenzymes can be used for the detection of the corresponding metal ion. Restoration of enzyme activity obtained in the presence of the metal ion can be correlated to its concentration. This principle has been demonstrated in the detection of copper while evaluating reconstituted catalytic activities in galactose oxidase and ascorbate oxidase and also in the detection of zinc since this ion is essential for the activity of carbonic anhydrase and alkaline phosphatase [416]. The need of stripping the metal for the preparation of the apoenz5une may demand tedious procedures and a catalytic assay with the addition of the substrate is always required for detection. 

Attempts at identification of the activity of this protein have not been successful thus far. It does not have any activity akin to that of carbonic anhydrase, alcohol dehydrogenase, carboxypeptidase, or dehydropeptidase. In this sense, it can be compared to hepatocuprein, hemocuprein, or the copper protein of Mohamed and Greenberg (1954) (possibly identical with hepatocuprein), all metalloproteins without identified enzymatic activity. Further studies along these lines may reveal the biochemical significance of the presence of this protein. 

Reflecting the lack of in-depth experimental data available at this time, even the classiflcation of metal-dependent enzymes was notably nonsystematic. In an early review, five categories were set forth heme, copper-containing, proteolytic, carbonic anhydrase, and phosphatase . A later classification" was made according to the types of reactions catalyzed electron transfer or redox (Cu, Fe, Mo), group transfer (Mg, Mn), decarboxylations and hydrolyses (Mn, Zn), and binding of pyridine nucleotide cofactors (Zn). 

Half-sandwich copper complexes [CuX(Tpph)] (X = OH-, NJ, NCS-) have been synthesized as model for carbonic anhydrase and the structure of [CuNCS(Tpph)] has been determined by X-ray diffraction analysis. The apparent dehydration rate constant varies linearly with total Cu(II) concentration, and the catalytic activity of the model complexes CuX(Tpph) decreases on going from OH- to N3 and finally to NCS-.124 The EPR-silent species [Cu(L)(Tpph)]+ (HL = 2-hydroxy-3-methylsulfanyl-5-methylbenzaldeyde) exhibits a UV-V IS-NIR spectrum that shows analogies with that of active... 

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