Systems Based on a Hydrolytic Mechanism

As we have already seen zinc-finger peptides are well-studied polypeptide motifs that have found many applications in synthetic systems, mostly because of their abihty to bind metal ions and interact with oligonucleotides. In this context the report by lima and Crooke [44] of the hydrolytic cleavage by a zinc-finger peptide devoid of any metal ion is a surprising. The system they studied, a 30-amino acid sequence, is based on a catalytic mechanism very similar to that discussed above... 

There are many different bioresorbable polymer systems based on different degradation mechanisms and having a range of physical and mechanical properties. However, the scope of our review will be restricted to only fiber-forming polymers. Those that are hydrolytically sensitive are discussed in this chapter, while enzymatically catalyzed bioresorbable polymers are presented in Chap. 6. Table 4.1 shows the classification of fiber-forming hydrolytically sensitive bioresorbable polymers. 

Although, as stated above, we wiU mostly focus on hydrolytic systems it is worth discussing oxidation catalysts briefly [8]. Probably the best known of these systems is exemphfied by the antitumor antibiotics belonging to the family of bleomycins (Fig. 6.1) [9]. These molecules may be included in the hst of peptide-based catalysts because of the presence of a small peptide which is involved both in the coordination to the metal ion (essential co-factor for the catalyst) and as a tether for a bisthiazole moiety that ensures interaction with DNA. It has recently been reported that bleomycins will also cleave RNA [10]. With these antibiotics DNA cleavage is known to be selective, preferentially occurring at 5 -GpC-3 and 5 -GpT-3 sequences, and results from metal-dependent oxidation [11]. Thus it is not a cleavage that occurs at the level of a P-O bond as expected for a non-hydrolytic mechanism. 

Proteasomes of Thermoplasma contain a single type of p subunit but eukaryotic proteasomes contain subunits with at least three distinct substrate preferences.347 M9c They all appear to use the same hydrolytic mechanism but in their substrate specificities they are chymotrypsin-like, peptidylglutamyl-peptide hydrolyzing, branched chain amino acid preferring, and small neutral amino acid preferring based on the P, amino acid residue. In the spleen some of the P subunits of the proteasomes appear to have been replaced by proteins encoded by the major histocompatibility complex of the immune system (Chapter 31).347 This may alter the properties of the proteasome to favor their function in antigen processing. Proteasomes are also ATP- and ubiquitin-dependent, as discussed in Section 6. 

Fungal cellulase enzyme systems capable of efficiently catalyzing the hydrolytic degradation of crystalline cellulose are typically composed of endo-acting cellulases (EGs), exo-acting cellulases (CBHs), and at least one cellobiase (1-6). The CBHs are typically the predominant enzymes, on a mole fraction basis, in such systems (7). Consequently, the CBHs have been the focus of many studies (8). The three-dimensional structure of prototypical CBHs is known (9-12) and their specificities are, in general, well characterized (13,14). However, mechanism-based kinetic analyses of CBH-catalyzed cellulose saccharification are rather limited (15,16). Studies of this latter type are particularly difficult owing to the inherent complexity of native cellulose substrates. 

R ently the formation of an unusual triazene derivative 99 was reported in the coupling reaction of skatole 95 (R=CH3) (42) Based on the change of the UV spectrum of the reaction mixture with time, a mechanism involving primary formation of the azo compound 97 followed by the hydrolytic cleavage of the indolenine system and reclosing of the ring to finally give 99 was proposed (42)... 

Enzyme Models .—Two general mechanisms have been proposed for hydrolytic reactions catalysed by bovine pancreatic carboxypeptidase A the first involves formation of an anhydride intermediate and the second involves the residue Glu-270 as a general base. Work on model systems and the enzyme indicates that the general base mechanism is the correct one and a consistent mechanism (Scheme 4) has been proposed in which both zinc and Arg-145 interact with the substrate. 

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