Peptidase metal-containing

Multinuclear metal centers are present in the active sites of several metalloproteases. A catalytic polymer with peptidase activity containing trinudear active sites has been prepared [51] using 17. Upon treatment of 17 with excess NaH, at least three of the six... 

There are several additional examples of zinc enzymes that contain an active site mononuclear zinc center and catalyze the hydrolysis of peptides bonds. These include carboxypeptidase B83 and neutral protease84 both of which contain a [(NHis)2(OGiu/AsP)Zn-OH2] coordination motif within the enzyme active site. Zinc peptidases that contain a [(NHis)3Zn-OH2] active site metal center include matrix metalloproteinases85,86 and other members of the metzincins superfamily.77,85... 

One particularly interesting area which has not been subjected to detailed study is metal-promoted acyl transfer. Studies on esterases and peptidases have shown that acyl transfer occurs to a nucleophilic group of the enzyme within an enzyme-substrate complex and the acyl group is then hydrolyzed in the second step. Many of these enzymes, e.g. carboxypeptidase, contain zinc(II). 

A metal atom is essential to the catalytic activity of carboxypeptidase A (53). The enzyme, as isolated, contains one gram atom of zinc per molecular weight of 34,600. Removal of the metal atom, either by dialysis at low pH or by treatment with chelating agents, gives a totally inactive apoenzyme. Activity can be restored by readdition of zinc or a number of other divalent metal ions (Table VII). The dual activity of carboxypeptidase towards peptides and esters is quite sensitive to the particular activating metal ion. Thus, the cobalt enzyme has twice the activity of the native zinc enzyme toward peptides but the same activity toward esters. Characteristic peptidase and esterase activities are also observed for the and Mn enzymes as well while the Cd ", Rh ", and Pb " en-... 

Aminopeptidases, enzymes that cleave olf the N-terminal amino acid from a peptide chain, are bismetallo peptidases, a class of metallopeptidase that contain two metals ions in the catalytic site (117,118). These can be inhibited by compounds related to bestatin (60)(Fig. 15.28), which contains the iV-tenni-nal a-hydroxy-j3-amino acid residue, sometimes referred to as norstatine. In leucine amino peptidase, chelation occurs between both the amide carbonyl group and the adjacent hydroxyl and the hydroxyl and the N-terminal amino group (119,120). 

In the C-terminal domain are five helices in a closed bundle. This characteristic fold is typical of thermolysin-like peptidases. Clan MC contains metallocarbox-ypeptidases which belong to only one family (M14) which is divided into the subfamilies A, B and C. Typical for this clan is that one zinc ion is tetrahedrally coordinated by a water molecule, two histidine and one glutamate residues. Clan MF includes aminopeptidases that require cocatalytic zinc ions for their enzymatic activity. The well-known leucyl aminopeptidase has a two-domain structure bearing the active site in the C-terminal domain. Whereas exopeptidases of clan MG require cocatalytic ions of cobalt or manganese, clan MH contains the third group of metallopeptidases that also require cocatalytic metal ions, but here these are all zinc ions. The third clan in which cocatalytic metal ions are necessary is clan MF with zinc or manganese. Only one catalytic zinc ion is required for peptidases of clans MA, MB, MC, MD and ME. 

Enzymes having metals as their components can also be inhibited by a substitution of one of these metal ions by another ion with the same charge and a similar size. For example, the toxic effect of cadmium is due to a substitution for zinc, which is a common component in metalloenzymes. The Zn " " and Cd ions are chemically similar, however, the cadmium-containing enzyme does not function properly. The Cd " ions can result, for instance, in the inhibition of amylase, adenosine triphosphatase, adcohol dehydrogenase, glutamic-oxalacetic transaminase, carbonic anhydrase and peptidase activity in carboxypeptidase [4]. 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

These proteins catalyze the addition or elimination of water to or from a substrate molecule. The carboanhydrases catalyze the hydration of CO2, peptidases and esterases the hydrolysis of carboxylic acid compounds, and phosphatases the cleavage of phosphoric esters. The active centers of hydrolytically active enzymes often contain Zn ions. Other metal ions in some of these enzymes are Mn, Ni ", Ca " and Mg. ... 

Inhibitors are found among food constituents. Proteins which specifically inhibit the activity of certain peptidases (cf. 16.2.3), amylases or 3-fructofuranosidase are examples. Furthermore, food contains substances which nonselectively inhibit a wide spectrum of enzymes. Phenolic constituents of food (cf. 18.1.2.5) and mustard oil (cf. 17.1.2.6.5) belong to this group. In addition, food might be contaminated with pesticides, heavy metal ions and other chemicals from a polluted environment (cf. Chapter 9) which can become inhibitors under some circumstances. These possibilities should be taken into account when enzymatic food analysis is performed. 

These peptidases contain bivalent metal ions or require addition of such ions for full activity. It is assumed that metal complexes (chelates) form and change the electronic configuration or, alternatively, the metal ion reacts with a carbonyl group and loosens up the bond in this way. 

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