Zinc-containing metalloenzymes

IV. Significance of HSAB Ideas for Zinc-Containing Metalloenzymes... 

The system illustrated by (272) forms the basis of a model for the zinc-containing metalloenzyme, carbonic anhydrase (Tabushi Kuroda, 1984). It contains Zn(n) bound to imidazole groups at the end of a hydrophobic pocket, as well as basic (amine) groups which are favourably placed to interact with a substrate carbon dioxide molecule. These are both features for the natural enzyme whose function is to catalyze the reversible hydration of carbon dioxide. The synthetic system is able to mimic the action of the enzyme (although side reactions also occur). Nevertheless, the formation of bicarbonate is still many orders of magnitude slower than occurs for the enzyme. 

Kinetic and mechanistic studies of the reactivity Zn-Oh ( = 1 or 2) species in small molecule analogs of zinc-containing metalloenzymes, 41, 81 Kinetics and spectroscopy of substituted phenylnitrenes, 36, 255 Kinetics, of organic reactions in water and aqueous mixtures, 14, 203 Kinetics, reaction, polarography and, 5, 1... 

In the case of carboxypeptidase B, Shaklai et al.(2lT> compared the relative contributions to the protein phosphorescence from tyrosine and tryptophan for the apoenzyme, the zinc-containing metalloenzyme in the absence of substrate, the metalloenzyme in the presence of the substrate iV-acetyl-L-arginine, and the metalloenzyme in the presence of the specific inhibitor L-arginine. The tyrosine tryptophan emission ratio of the metalloenzyme was about a factor of four smaller than that of the apoenzyme. Binding of either the substrate or the inhibitor led to an increase in the emission ratio to a value similar to that of the apoenzyme. The change in the tyrosine tryptophan phosphorescence ratio was attributed to an interaction between a tyrosine and the catalytically essential zinc. The emission ratio was also studied as a function of pH. The titration data are difficult to interpret, however, because a Tris buffer was used and the ionization of Tris is strongly temperature dependent. In general, the use of Tris buffers for phosphorescence studies should be avoided. 

Carbonic anhydrase (carbonate dehydratase, EC 4.2.E1) is a small, monomeric zinc-containing metalloenzyme that catalyzes the reversible hydration of C02 to bicarbonate [101][102], In addition to this activity, carbonic anhydrase also catalyzes the hydrolysis of many aromatic esters [103]. 

There are two attributes of a-D-mannosidase (EC 3.2.1.24) that receive particular attention in this article, namely, its behavior as a zinc-containing metalloenzyme, and its action on naturally occurring, D-mannose-containing molecules. However, such more-systematic considerations as the kinetics of action and the distribution of the enzyme in Nature are not overlooked. 

As part of a collection of articles focussing on individual enzymes acting on carbohydrates, this Volume features a contribution by Snaith and Levvy (Aberdeen) on a-D-mannosidase, a zinc-containing metalloenzyme. The life and work of Laszlo Vargha, the Hungarian carbohydrate chemist, is the subject of an obituary article by Kuszmann (Budapest). 

The latest proposed mechanisms1462 for several zinc-containing metalloenzymes combine elements from both types of mechanism by suggesting that the substrate binds to the enzyme through the C—O group, but that in the process the metal-bound water molecule is not displaced, so that the reaction proceeds via a five-coordinate intermediate. This hybrid mechanism is discussed below in greater detail for alcohol dehydrogenase. 

Kinetic and mechanistic studies of the reactivity Zn-Ohn (n = 1 or 2) species in small molecule analogs of zinc-containing metalloenzymes, 41, 81... 

Carbonic anhydrase was the first known example of a zinc-containing metalloenzyme (27). It is present in a large number of tissues in all vertebrates and in many invertebrates. It has also been found in the green tissues of plants and in some bacteria (28). The primary reaction catalyzed by the enzyme... 

The alkaline phosphatase from E. coli is a zinc-containing metalloenzyme comprising two identical subunits. Different groups have claimed that varying numbers of zinc atoms are present in the dimer, but it has recently been demonstrated that there are four, of which only two appear to... 

Biochemical Aspects. The zinc-containing metalloenzyme carbonic anhydrase (carbonic hydro-lyase, EC 4.2.1,1) is specifically inhibited by univalent anions and by aromatic sulphonamides. AfT (25 for the binding reactions between bovine carbonic anhydrase and 3-methyl-2-acetylimino-l,3,4-thiadiazoline-5-sulphonamide (Methazolamide) is — 59.0kJmol and that for 5-phenyl-sulphonamido-l,3,4-thiadiazole-2-sulphonamide (CL 11366) is — 57.7kJmol" The thermodynamic quantities did not correlate with the assumed structural features of the binding process. ... 

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