Enzymes enzymatic attack

Specific Diffusion-based Limitations to Decay. If microbial colonization is confined to the surface of materials, the decay rate will inevitably be lower than seen where proximity between substrate and microbial cells is possible because enzymes produced by the cell and soluble products formed by enzymatic attack must diffuse a considerable distance. For example, if closer contact between the starch face and fungus were possible than seen in Figure 2, uptake of starch digestion products would occur at the growing tip and translocation within the mycelium by active transport would be possible. This... 

Microbial vulnerability of polymers is often ascribed to enzyme activity, enzymes being crucial players in the biological biodeterioration process. As enzymes are macromolecular polymers, their attack on the polymer is usually only possible via superficially exposed polymer structures readily accessible via a microporous structure. Alternatively, the enzymatic attack works indirectly via... 

Chain degradation proceeds at the oxidised sites, where two different routes can be discerned. One way uses the p-hydroxy ketone functionalities introduced by the initial enzymatic attack as direct substrates in an aldolase-like scission reaction. The other way proceeds via further oxidation of this structure to a 1,3-diketone, which serves as a target structure for a hydrolase-type splitting enzyme. 

Large substituents often prevent enzymatic attack on a drug, thereby prolonging its useful life. This technique was used to impart resistance to p-lactamase to the semisynthetic penicillins. The need for the proximity of the phenyl group to the lactam is quite interesting phenylbenzyl penicillin (8-26) is inactive as an enzyme inhibitor because the phenyl group no longer hinders access of the enzyme to the lactam bond. 

The categories of substrates which are used for assays of cellulase enzymes are shown in Table I. The use of crystalline, insoluble forms of cellulose as substrates makes assays difficult and has led to such trivial names as Avicelase activity. These assays are useful as indications of the capacity of an enzyme system to degrade native cellulose and indicate the presence of CBH enzyme which cannot be assayed in the presence of endoglucanases or / -glucosidase. The susceptibility to enzymatic attack generally increases with the hydration of the polymer chains that accom-... 

The bacterial D-hydantoinase has been isolated as crystals from cells of Pseudomonas putida (= P. striata) (Table 1) [5]. Because the purified enzyme showed the highest activity and affinity toward dihydrouracil, the enzyme was identified as dihydropyrimidinase (EC. 3.5.2.2). Interestingly, the enzyme also attacked a variety of aliphatic and aromatic D-5-mono-substituted hydantoins, yielding the corresponding D-form of N-carbamoyl-a-amino acids. Thus, the enzyme can be used for the preparation of various D-amino acids. Under the conditions used for the enzymatic hydrolysis of hydantoin at pH 8 to 10, the L-isomers of the remaining hydantoins are racemized through base catalysis. Therefore, the racemic hydantoins can be converted quantitatively into N-carbamoyl-D-amino acids through this step. 

Macdiarmid and Burrell 1992) such as those used to process poultry waste (Kornillowicz-Kowalska 1997a) and actinomycetes have also been discussed (Bockle and Muller 1997 Kunert and Stransky 1988), and fibers incubated for thirty days with an actinomycete lost their luster and looked dull at a gross level (Brady et al. 1990). On detailed examination these fibers had suffered enzyme attack and structural damage. Loss of the protective cuticle exposed the underlying cortex to enzymatic attack, resulting in separation of individual cortical cells. 

The results on the hydrolysis of partially methylated /3-casein by plasmin indicate that proteins radiomethylated to a low level can serve as substrates for trypsin-like enzymes and probably for proteinases in general. Because it is likely that methylation will interfere with enzymatic attack at lysine residues, the complete hydrolysis of /3-casein probably would not be possible. Studies on mastitic milk demonstrate the usefulness of 14C-methyl proteins for qualitative examination of protein hydrolysis in complex multiprotein systems where resolution and characterization of individual protein fragments is difficult. The requirements in such studies are the availability of pure samples of the proteins under investigation and a suitable technique for separating the radio-labeled protein from hydrolytic products. 

By considering these features, the enormous difficulties associated with overcoming the enzymatic barrier to peptide and protein delivery should be apparent. Degradation usually occurs at the site of administration and is possible in every anatomical site en route to the target receptor. Furthermore, protecting a single bond on a peptide or protein drag from a particular type of enzyme is insufficient to confer protection on the entire dmg from enzymatic hydrolysis—other enzymes may attack the protected bond and the other unprotected bonds on the dmg are still vulnerable. 

A Reservation. Inasmuch as the starting material was raodipalmitoyl phosphatidylcholine, the data from the above experimental protocol tells only that the enzyme has stereochemical preference for the sn-3 enantiomer. Even though it is easy to show that there was an equimolar release of free fatty acid and formation of a lysophosphatidylcholine, it is not possible to tell whether the enzymatic attack occurred at the sn- or the sn-2 acyl ester bond. In any event, the stereospecificity of phospholipase A2 has been established by this experimental approach, and with this information a route to proof of specific positioning of fatty acyl substituents on naturally occurring phosphoglyc-erides is accessible. 

In general, acid hydrolysis produces similar degradation. Since acids usually involve a much smaller molecule as the attacking agent, however, acids penetrate the amorphous area of cellulose more easily than enzymes and produce a faster drop in DP. Diffusion into the crystalline areas proceeds more slowly. Because acid hydrolysis is more easily controlled and is faster than enzymatic attack, acid hydrolysis was selected as the second aging system to be examined. 

Chloramphenicol (9) is liable to breakdown by chloramphenicol acetyl-transferases [185]. Fluoro derivatives (57, 58) resist enzymatic attack but little has been heard of these, apparently because of their toxicity [319], Aminoglycoside antibiotics (AGACs) may be chemically modified by AMEs. Some derivatives (e.g. amikacin, 43) are more recalcitrant than others, e.g. kanamycin (42) (see Figure 4.2). Other enzyme-resistant AGACs of low toxicity are needed. 

Non-enzymatic attack In non-enzymatic attack of minerals by microbes, reactive products of microbial metabolism come into play. The microbial enzymes responsible for metabolic product formation are located below the cell envelope, in the cytoplasm of prokaryotes (Bacteria and Archaea) and in cell organelles and/or the cytoplasm of eukaryotes (e.g. fungi, algae, lichens). In these instances of microbial attack, physical contact of the microbial cells with the surface of a mineral being attacked is not essential. The reactive metabolic products are formed intracellularly and are then excreted into the bulk phase where they are able to interact chemically, i.e. non-enzymatically, with a susceptible mineral. Depending on the type of metabolic product and mineral, the interaction with the mineral may result in mineral dissolution or mineral diagenesis by oxidation or reduction or acid or base attack. Mineral dissolution or diagenesis may also be the result of complexation by a microbial metabolic product with that capacity. In some instances mineral attack may involve a combination of some of these reactions. 

In the same study the relative activities of pectinesterases from alfalfa, tomato, orange, and a fungal source were compared using commercial pectin, methyl polygalacturonate (fully esterified), and ethyl polygalacturonate (50% esterified) as substrates. The latter two substrates were prepared by HCl-catalyzed esterification (68, 69). The enzymatic attack rate of the pectinesterases on the methyl ester was approximately 50% of that on pectin (except for the fungal enzyme where the rate was 80% of that on the pectin substrate). Kertesz (70)... 

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