Catalytic activity, enzymes proteases

Plant proteolytic enzymes are cysteine proteases, that is, they have a cysteine that is critical for the catalytic activity. Plant proteases are thought to function by a mechanism reminiscent of that shown for chy-motrypsin (fig. 8.11). Propose a structure for the acyl-enzyme intermediate that would exist for plant proteases. 

The first hint that two active-site carboxyl groups—one proto-nated and one ionized—might be involved in the catalytic activity of the aspartic proteases came from studies of the pH dependence of enzymatic activity. If an ionizable group in an enzyme active site is essential for activity, a plot of enzyme activity versus pH may look like one of the plots at right. 

For example, Asuri and his co-workers (Asuri et al., 2007) reported that the proteases attached to the surface of SWCNTs can create self-cleaning surfaces that resist protein adsorption. This kind of SWCNT-enzyme composite exhibited 30 times higher overall catalytic activity than control composites where the proteases were conjugated to a non-nanoscale graphite support. Importantly, the enzymes preserved more than 90% of their initial activity over 30% days in the liquid buffer, with only negligible amounts of enzymes leaching out. The result demonstrates that the nanocomposites of SWCNTs and polymers can act as hosts for enzymes and can prevent protein contamination on the surface of medical devices. We consider that SWCNTs may have the function as biocatalyst to improve the enzyme activity attached to the surface of CNTs. Our previous work also indirectly hints that CNTs could improve the bioactivity of enzymes such as Taq enzyme Cui et al. (2004). However, there are also adverse reports. CNTs can... 

Subsequent to CO2 association in the hydrophobic pocket, the chemistry of turnover requires the intimate participation of zinc. The role of zinc is to promote a water molecule as a potent nucleophile, and this is a role which the zinc of carbonic anhydrase II shares with the metal ion of the zinc proteases (discussed in the next section). In fact, the zinc of carbonic anhydrase II promotes the ionization of its bound water so that the active enzyme is in the zinc-hydroxide form (Coleman, 1967 Lindskog and Coleman, 1973 Silverman and Lindskog, 1988). Studies of small-molecule complexes yield effective models of the carbonic anhydrase active site which are catalytically active in zinc-hydroxide forms (Woolley, 1975). In addition to its role in promoting a nucleophilic water molecule, the zinc of carbonic anhydrase II is a classical electrophilic catalyst that is, it stabilizes the developing negative charge of the transition state and product bicarbonate anion. This role does not require the inner-sphere interaction of zinc with the substrate C=0 in a precatalytic complex. 

Schematic diagrams of the amino acid sequences of chymotrypsin, trypsin, and elastase. Each circle represents one amino acid. Amino acid residues that are identical in all three proteins are in solid color. The three proteins are of different lengths but have been aligned to maximize the correspondence of the amino acid sequences. All of the sequences are numbered according to the sequence in chymotrypsin. Long connections between nonadjacent residues represent disulfide bonds. Locations of the catalytically important histidine, aspartate, and serine residues are marked. The links that are cleaved to transform the inactive zymogens to the active enzymes are indicated by parenthesis marks. After chymotrypsinogen is cut between residues 15 and 16 by trypsin and is thus transformed into an active protease, it proceeds to digest itself at the additional sites that are indicated these secondary cuts have only minor effects on the enzymes s catalytic activity. (Illustration copyright by Irving Geis. Reprinted by permission.)...

Finally, three groups have reported the preparation of artificial enzymes with catalytic activity. Stewart and co-workers [73] incorporated a catalytic triad from the serine proteases into a designed four a-helix protein (80). In their design, they incorporated one of the amino acids involved in the catalytic function at the N-terminal side of the a-helices that are linked together by their C-terminal position (Fig. 29). The authors proposed that the oxyanion hole and the hydrophobic binding pocket are created by the three-dimensional structure formed by the folding of 80. Compared to the spontaneous reaction, impressive... 

From a structural point of view, kallikreins belong to the serine protease family of enzymes. The essential features of serine proteases that are preserved in the kallikreins can be summarized as follows only one serine residue of the protein is catalytically active two residues, a histidine and an... 

Several pharmaceutical enzymes belong to the group of serine-histidine estero-proteolytic enzymes (serine proteases), which display their catalytic activity with the aid of an especially reactive serine residue, whoso p-hydroxyi group forms a covalent bond with the substrate molecule. This reaction takes place by cooperation with the imidazole base of histidine. The specificity of the enzymes is achieved by the characteristic strocture of their substrate-binding centers, which in these proteases are built according to the same principle. They consist of a hydrophobic slit formed by apolar aide chains of amino acids and a dissociated side chain-located carboxyl group of an aspartic add residue at the bottom. 

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