Serine proteases substrate binding

Peptide Chloromethyl Ketones. Peptide chloromethyl ketone inhibitors have been studied extensively and a fairly detailed picture of the inhibition reaction (see Figure 3) has emerged from numerous chemical and crystallographic studies (30,31). The inhibitor resembles a serine protease substrate with the exception that the scissile peptide bond of the substrate is replaced with a chloromethyl ketone functional group in the inhibitor. The inhibitor binds to the serine protease in the extended substrate binding site and the reactive chloromethyl ketone functional group is placed then in the proper position to alkylate the active-site histidine residue. In addition, the serine OH reacts with the inhibitor carbonyl group to form a hemiketal. 

Figure 2 Standard mechanism of protein serine protease inhibitors bind in a substrate-like manner that completely spans the active site, and act as substrates with a very slow kcat. They interact with both the substrate binding sites (shallow indentation) and the catalytic residues (rectangle) of the serine protease.

FIGURE 11.3 Representation of extended substrate binding site of a serine protease according to Schechter and Berger.17... 

The protease conformation of DegP is still elusive as crystallization of a substratelike inhibitor complex has failed and maintenance of a stably folded protein precludes long-term experimentation at elevated temperatures where it displays protease activity. We propose a profound rearrangement of the LA -L1-L2 loop triad into the canonical conformation of active serine proteases competent for substrate binding. This may be initiated by a collapse of the hydrophobic LA platforms and an enlargement of the hydrophobic contacts caused at high temperature. 

Teplyakov, A. V., van der Laan, J. M., Lammers, A. A., Kelders, H., Kalk, K. H., Misset, O., Mulleners, L.J. Dijkstra, B. W. (1992). Protein engineering of the high-alkaline serine protease PB92 from Bacillus alcalophilus functional and structural consequences of mutation at the S4 substrate binding pocket. Protein Engineering, 5, 413-20. 

Figure 12-15 Schematic drawing of the active site of a cysteine protease of the papain family with a partial structure of an acyl-enzyme intermediate in green. The thiolate-imidazolium pair of Cys 25 His 159 lies deep in the substrate-binding cleft and bridges an interface between two major structural domains, just as the Ser His pair does in serine proteases (Fig. 12-10). This may facilitate small conformational changes during the catalytic cycle. Asn 175 provides a polarizable acceptor for positive charge, helping to stabilize the preformed ion pair, and allows easy transfer of an imidazolium proton to the product of substrate cleavage. The peptide NH of Cys 25 and the side chain of Gin 19 form an oxyanion hole.

Cyclic peptides containing multiple Abz residues adopt bowl-shape conformations the rigid amino acid provides a cavity in the cyclic peptide that may be exploited for molecular recognition experiments. Thus, c[-Xaa-w-Abz-]3 binds phosphomonoestersj25 while serine protease activity is observed in c[-Ser-m-Abz-His-m-Abz-Asp-m-Abz-Ala-m-Abz-] following binding of the substrate in the cavity. 26 ... 

The first crystal structure of a bacterial serine protease to be solved—subtilisin, from Bacillus amyloliquefaciens—revealed an enzyme of apparently totally different construction from the mammalian serine proteases (Figure 1.17). This was not unexpected, since there is no sequence homology between them. But closer examination shows that they are functionally identical in terms of substrate binding and catalysis. Subtilisin has the same catalytic triad, the same system of hydrogen bonds for binding the carbonyl oxygen and the acetamido NH of the substrate, and the same series of subsites for binding the acyl portion of... 

To summarize, the binding sites of lysozyme and the serine proteases are approximately complementary in structure to the structures of the substrates the nonpolar parts of the substrate match up with nonpolar side chains of the amino acids the hydrogen-bonding sites on the substrate bind to the backbone NH and CO groups of the protein and, for lysozyme, to the polar side chains of amino acids. The reactive part of the substrate is firmly held by this binding next to acidic, basic, or nucleophilic groups on the enzyme. 

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