Enzymes groups proteases

The HlV-1 protease is responsible for processing the protein precursors to the enzymes (integrase, protease and reverse transcriptase) and the structural proteins of the HIV-1 virus. Maw and Hall found that topological indices provide rehable QSAR models for the IC50 data of 32 HIV-1 protease inhibitors [29]. The best QSAR model, with r = 0.86, s=0.60 and q = 0.79, was obtained with the shape index Ka, the connechvity index the sum of HE-state indices for ah groups that act as... 

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. 

An example of a type I reaction is the use of enzymes such as proteases or amylases in laundry detergents however, this enzyme reaction has caused some controversy in relation to water pollution. Once in solution, the soluble enzyme may digest (i.e., break down) an insoluble substrate such as a blood stain, A major research effort is currently being directed at type II reactions. By attaching active enzyme groups to solid surfaces, continuous processing units similar to the packed catalytic bed reactor discussed in Chapter 10 can be developed. 

The enzymes for the SIF incubations were, in general, split into two groups proteases and amylase plus lipase. The incubations were performed only on compounds (I)-(VIII), (XI),and (XII) notall the anilines were investigated because of their instability in the stomach. Compounds were incubated in an SIF buffer (pH 7.S-7.9) containing either trypsin, chymotrypsin, carboxypeptidase A B, elastase or a-amylase and lipase. Compounds (I), (V)-(VII), and (XII) showed some degradation in the incubation with proteases which appeared to be enzyme mediated. Compounds (II), (III), (IV), (VII I), and XI were stable in the incubations. All the compounds investigated in the amylase/lipase incubations were stable. 

Proteins are built up of a-aminoacids differing in the presence of reactive groups such as amino, carboxyl, amido, hydroxyl, thiol and imidazole groups. The structure of proteins is an extended chain of aminoacid residues joined by amide linkages which are readily degraded by enzymes, particularly proteases. 

An impressive example of the application of structure-based methods was the design of a inhibitor of the HIV protease by a group of scientists at DuPont Merck [Lam et al. 1994 This enzyme is crucial to the replication of the HIV virus, and inhibitors have bee shown to have therapeutic value as components of anti-AIDS treatment regimes. The star1 ing point for their work was a series of X-ray crystal structures of the enzyme with number of inhibitors boimd. Their objective was to discover potent, novel leads whid were orally available. Many of the previously reported inhibitors of this enzyme possessei substantial peptide character, and so were biologically unstable, poorly absorbed am rapidly metabolised. 

FIGURE 14.11 The pH activity profiles of four different enzymes. Trypsin, an intestinal protease, has a slightly alkaline pH optimnm, whereas pepsin, a gastric protease, acts in the acidic confines of the stomach and has a pH optimmn near 2. Papain, a protease found in papaya, is relatively insensitive to pHs between 4 and 8. Cholinesterase activity is pH-sensitive below pH 7 but not between pH 7 and 10. The cholinesterase pH activity profile suggests that an ionizable group with a pK near 6 is essential to its activity. Might it be a histidine residue within the active site ... 

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. 

Amide hydrolysis is common in biological chemistry. Just as the hydrolysis of esters is the initial step in the digestion of dietary fats, the hydrolysis of amides is the initial step in the digestion of dietary proteins. The reaction is catalyzed by protease enzymes and occurs by a mechanism almost identical to that we just saw for fat hydrolysis. That is, an initial nucleophilic acyl substitution of an alcohol group in the enzyme on an amide linkage in the protein gives an acyl enzyme intermediate that then undergoes hydrolysis. 

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