Trypsin-like subunit

The arrangement of S965, H746, and the oxyanion hole suggests that the classical steps of peptide-bond hydrolysis follow the sequence of the trypsin-like serine proteases, namely the formation of the tetrahedral adduct, the acyl-enzyme complex, and hydrolysis. Tricorn has been shown to exhibit both tryptic and chymotryp-tic specificities (Tamura et al. 1996a). The X-ray structure reveals that specificity for basic PI residues is conferred by D936 which is provided by the diad-related subunit (see Figures 10.9 and 10.10). 

In 2007 a series of p-lactam derivatives was designed and synthesized to inhibit the chymotrypsin-like activity of the human 20S proteasome. The most potent compounds of this new structural class of p-subunit exhibit good selectivity over the trypsin-like and post-glutamyl-peptide hydrolytic activities of the enzyme [409],... 

In terms of structure and function, the 265 proteasome is an ATP-dependent multicatalytic enzyme complex comprising one or two 195 regulatory caps and a proteolytic 205 core particle within which protein degradation occurs.18,25,26 The 205 proteasome contains three pairs of proteolytic subunits, (35, (32 and (31, for which chymotrypsin-like (CT-L), trypsin-like (T-L) and caspase-like (C-L) activities have been ascribed, respectively, based on their substrate preferences.27... 

SI pockets [22], The three major activities, the peptidylglutamil-hydrolyzing (PGPH), trypsin-like and chymotrypsin-like activities, have been assigned to the three active subunits of ySl, yS2 and yS5, respectively, based on mutational and crystal structure analyses. Furthermore, it has been claimed, based on studies with model peptides and inactivation by inhibitors, that mammalian proteasomes also contain two other peptidase activities, referred to as branched-chain amino acid-preferring and small neutral amino acid-preferring (SNAAP). 

In the SI pockets of /92, no such large structural changes are noted upon the introduction of IFNy-induced subunits (/92i), as was the case with /91i. Ser-32 and Asp-53 of pi are replaced by glutamine in the pl subunit. It is conceivable that the trypsin-like activity of the pit subunit is enhanced by the acidic Glu residues, which would stabilize the basic substrate residue in the pocket. 

It has been known that the treatment of mammalian [67, 68] or yeast proteasome [69] with larger excesses of the thiol-reagent N-ethylmaleinimide (NEM) leads to selective inhibition of the trypsin-like activity. In the crystal structure of the yeast 20S proteasome the conserved CysllS residue of the P3 subunit protrudes into the S3 sub site of the p2 active site [34], a fact that could explain the inactivation of the trypsin-like activity of proteasomes by its chemical modification with NEM. The par-... 

Because of the importance of ADPGlc synthetase for plant starch synthesis, efforts have been made to determine the structure of the enzyme and to relate catalytic and allosteric function to structure. The spinach leaf enzyme is composed of two subunits of 51 and 54 kd mass (5,6). The molecular mass of the native enzyme is 206,000 and presumably is a tetramer composed of two of each subunit. The subunits are antigenically dissimilar, exhibit different peptide patterns on HPLC after trypsin digestion and their N-terminal amino acid sequences are different (7). Thus it is likely that the peptide subunits are products of different genes. 

The 20S proteasome is a multicatalytic protease containing several active centers. These are located in the hole of the cyhnder and are encoded by the j3-subunits. The 20S proteasome acts independently of ATP or any other factor. It is able to degrade unfolded proteins since those can enter the active centers of the proteasome through the opening formed by the a-subunits. This opening is usually covered by domains of the a-subimits and the active center is therefore only accessible after a certain activation of the proteasome. The 20S proteasome is able to cleave proteins on the carboxyl side ofbasic, hydrophobic, and acidic amino acids, described as a trypsin-hke, chymotrypsin-like, and peptidylgluamyl-peptide hydrolase-like proteolytic activity. 

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