Hydrolytic modification

It should be mentioned that a solventless method of hydrolytic modification of starch has recently been developed. The method employs solid superacids, per-fluorinated resin-sulfonic acids, which successfully catalyze hydrolysis of starch (and other polysaccharides) and offer the possibility of continuous-process applications in plug-flow reactors.34... 

These nonhydrolytic, post-translational enzymatic modifications of proteins, described in two recent publications (9,10), will not be treated further in this chapter. Hydrolytic modification, in vivo and in vitro, is the single most frequently occurring enzymatic modification of proteins. 

The hydrolytic modifications of proteins catalyzed by enzymes include both generalized reactions, where a relatively large number of peptide bonds are split, and limited reactions where hydrolysis of one or only a few bonds are necessary in order to achieve the desired product (see Table III). Examples of both types of reactions will be presented below. 

Some examples of in vitro hydrolytic modifications of proteins are shown in Table IV. The preparation of cheeses, chillproofing of beer, and the production of protein hydrolysates represent major uses of proteases. With the possible exception of cheese preparation, the application involves a substantial degree of hydrolysis. Therefore rather nonspecific proteases often are used. 

Table IV. In Vitro Enzyme-Catalyzed Hydrolytic Modifications of Food Proteins...

These proteases have been studied extensively because of their importance in the continuous turnover of proteins at the cellular level. The reader is referred to several recent excellent reviews (16,17,18). In this chapter, we shall concentrate on the specificity of protein turnover as an example of the potential that exists for selective hydrolytic modifications of food proteins. 

Translocation of Proteins Across Membranes. The transfer of proteins across biological membranes generally involves a hydrolytic modification step of the precursor form of the mature protein. This processing has been shown clearly to occur during segregation of secretory proteins, transport of proteins into mitochondria, and entry of plant and microbial toxins into cells as shown in Table XII. 

In this section we describe briefly the two models for inserting integral proteins into cell membranes with special emphasis on the protease(s)-catalyzed hydrolytic modification of these proteins associated with the membrane assembly process. These two models are (1) self assembly following translation of the proteins and (2) coupling of translation with insertion of the protein into the membrane. 

Alder route, was converted into the urethan (25) by a series of unexceptional steps. Arndt-Eistert homologation of (25) provided the carboxylic acid (26) which, upon treatment with acetic anhydride, readily gave the lactam (27). Introduction of a one-carbon unit into the aromatic ring was achieved with chloromethyl methyl ether, and further reductive and hydrolytic modification afforded the amino-alcohol (28). Osmium tetroxide oxidation of (28) yielded the stereoisomeric triols (29) (20%) and (30) (22%) which were separated and oxidized to ( + )-clivonine (31) and (+ )-clividine (32), respectively. 

Most ester-forming reactions are reversible. Depending on circumstances, these reactions may be either undesirable side reactions, for example hydrolytic chain scissions occurring during processing, or useful reactions when chemical modification or polymer recycling is considered. 

Phosphonate analogs to phosphate esters, in which the P—0 bond is formally replaced by a P—C bond, have attracted attention due to their stability toward the hydrolytic action of phosphatases, which renders them potential inhibitors or regulators of metabolic processes. Two alternative pathways, in fact, may achieve introduction of the phosphonate moiety by enzyme catalysis. The first employs the bioisosteric methylene phosphonate analog (39), which yields products related to sugar 1-phosphates such as (71)/(72) (Figure 10.28) [102,107]. This strategy is rather effective because of the inherent stability of (39) as a replacement for (25), but depends on the individual tolerance of the aldolase for structural modification close... 

Poly (iminocarbonates) are little known polymers that, in a formal sense, are derived from polycarbonates by the replacement of the carbonyl oxygen by an imino group (Fig. 5). This backbone modification dramatically increases the hydrolytic lability of the backbone, without appreciably affecting the physicomechanical properties of the polymer the mechanical strength and toughness of thin,... 

In mammalian cells, the two most common forms of covalent modification are partial proteolysis and ph osphorylation. Because cells lack the ability to reunite the two portions of a protein produced by hydrolysis of a peptide bond, proteolysis constitutes an irreversible modification. By contrast, phosphorylation is a reversible modification process. The phosphorylation of proteins on seryl, threonyl, or tyrosyl residues, catalyzed by protein kinases, is thermodynamically spontaneous. Equally spontaneous is the hydrolytic removal of these phosphoryl groups by enzymes called protein phosphatases. 

Urethane linkages between amino groups of a protein and PEG provide a stable attachment, more resistant to hydrolytic cleavage (13). In fact, it was demonstrated on radioactively labeled PEG-derivatives that urethane links are completely stable under a variety of physiological conditions (14). The attachment of PEG to a protein via carbamate was obtained (15,16) using carbonyldiimidazole activated PEG. However, the polymer activated in this manner is not very reactive and therefore very long reaction times (48-72 h at pH 8.5) were required to achieve sufficient modifications. 

As secretory vesicles mature, many secretory polypeptides undergo post-translational modifications. Many hormones and neuropeptides as well as hydrolytic enzymes are synthesized as inactive polypeptide precursors that need to undergo proteolysis to become active. This maturation process usually starts in the TGN and continues in the secretory vesicles, but may be completed in the extracellular space soon after exocytosis takes place in some cases. The maturation process for neuropeptides is described in Chapter 18. 

In dentistry, silicones are primarily used as dental-impression materials where chemical- and bioinertness are critical, and, thus, thoroughly evaluated.546 The development of a method for the detection of antibodies to silicones has been reviewed,547 as the search for novel silicone biomaterials continues. Thus, aromatic polyamide-silicone resins have been reviewed as a new class of biomaterials.548 In a short review, the comparison of silicones with their major competitor in biomaterials, polyurethanes, has been conducted.549 But silicones are also used in the modification of polyurethanes and other polymers via co-polymerization, formation of IPNs, blending, or functionalization by grafting, affecting both bulk and surface characteristics of the materials, as discussed in the recent reviews.550-552 A number of papers deal specifically with surface modification of silicones for medical applications, as described in a recent reference.555 The role of silicones in biodegradable polyurethane co-polymers,554 and in other hydrolytically degradable co-polymers,555 was recently studied. 

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