Peptide hydrolase action

Proteases are grossly subdivided into two major groups, such as exopeptidases and endopeptidases, depending on their site of action. Based on the functional group present at the active site, proteases are further classified into four prominent groups, such as serine, aspartic, cysteine, and metal-loproteases (Hartley, 1996). There are a few miscellaneous proteases that do not precisely fit into the standard classification and one of them is ATP-dependent proteases (Menon et al., 1987). The flow sheet for classification of peptide hydrolases is given in Figure 9.3. 

The retrieval mechanism for the M6P receptor resembles the one previously described for the H/KDEL receptor [53]. Optimal binding of M6P receptor to M6P occurs at pH 6.5-6.7, the pH found in the TGN. When transport vesicles arrive at late endosomes, the pH is lowered by the action of H+ pumps. The affinity of the M6P receptor for its ligands is reduced at acid pHs, resulting in M6P receptor releasing the M6P in the late endosome. As a result, transport of lysosomal hydrolases occurs unidirectionally. Once the M6P receptor releases M6P-bearing hydrolases, the receptor can be returned to the TGN for reuse. Transport of the M6P receptor to either TGN or late endosome relies on signal peptides on the cytoplasmic tail region of the M6P receptor. 

The use of enzymes and whole cells as catalysts in organic chemistry is described. Emphasis is put on the chemical reactions and the importance of providing enantiopure synthons. In particular kinetics of resolution is in focus. Among the topics covered are enzyme classification, structure and mechanism of action of enzymes. Examples are given on the use of hydrolytic enzymes such as esterases, proteases, lipases, epoxide hydrolases, acylases and amidases both in aqueous and low-water media. Reductions and oxidations are treated both using whole cells and pure enzymes. Moreover, use of enzymes in sngar chemistiy and to prodnce amino acids and peptides are discnssed. 

Figure 9 Serine hydrolases as targets of ABPP. (a) Catalytic mechanism of peptide substrate cleavage via the action of a catalytic triad, (b) Aryl phosphonate and fluorophosphonate probe scaffolds.

In addition to the freeze-concentration effect, a catalytic role for ice crystals, a favorable orientation of substrate and biocatalyst, the markedly lower dielectric constant of ice compared with water, and the high proton mobility in ice, have been discussed as further factors that possibly influence reactions in frozen systems. In summary, the reverse action of hydrolases provides an attractive alternative to the chemical synthesis of peptides but this approach could also be verified for the synthesis of oligosaccharides and oligonucleotides using glycosidases and ribonu-cleases, respectively11631. 

Pepsinogen is called a zymogen or inactive enzyme precursor, which is activated by the enzymatic action of trace amounts of pepsin already present in the stomach. Activation is achieved by the removal of a small terminal peptide. Pepsin attacks the peptide bonds of amino acids possessing hydrophobic side groups, which reduces proteins to mbttures of smaller peptides. Other enzymatic hydrolases which are found in the small intestine are also secreted as zymogens, and are activated by similar processing. 

The action of 26S proteasome generates several types of products including free peptides, and peptides linked to ubiquitin chains, which are converted to free and reusable Ub by the action of ubiquitin-C-terminal hydrolases or isopeptidases. Cytosolic peptidases further degrade short peptides to free amino acids. 

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