Mechanism for degradation

Figure 10.3 General mechanisms for degradation of polyester in acid and basic solutions.

Mechanism for degrading defective proteins Proteins that are defective or destined for rapid turnover are marked for destruction by the attachment of a small, highly conserved protein called ubiquitin. Proteins marked in this way are rapidly degraded by a cellular component known as the proteasome. 

The conversion of cholesterol to bile acids is quantitatively the most important mechanism for degradation of cholesterol. In a normal human adult approximately 0.5 g of cholesterol is converted to bile acids each day. The regulation of this process operates at the initial biosynthetic step catalyzed by an enzyme in the endoplasmic reticulum, la-hydroxylase (fig. 20.18). The 7a-hydroxylase is one of a group of enzymes called mixed-function oxidases, which are involved in the hydroxylation of the sterol molecule at numerous specific sites. A mixed-function oxidase is an enzyme complex that catalyzes hydroxylation of a substrate with a concomitant production of H20 from a single molecule of 02- The 7a-hydroxylase is one of several enzymes referred to as cytochrome P450. 

Table 18.5 shows indirect photolysis half-lives of azo dyes in natural waters at 40°N at the surface and 1 meter depth using measured rate constants and singlet oxygen concentrations. They apply only if singlet oxygen and/or radical oxidation are the only mechanisms for degradation. 

Figure 6.11 Nef-Isbell-Richards mechanisms for degradation of glucose and fructose and the peeling reaction of 1 -> 3- and 1 ->4-linked polysaccharides. R or R can be another sugar residue, when the mechanism of the peeling reaction is illustrated.

Balakshin, M., Capanema, E., Chen, C. L., Gratzl, J., Kirkman, A., and Gracz, H. (2001) Biobleaching of pulp with dioxygen in the laccase-mediator system Reaction mechanisms for degradation of residual lignin. J. Mol. Catalysis B-Enzymatic 13(1-3), 1-16. 

The intradiol cleaving catechol dioxygenases are bacterial iron-containing enzymes that serve as a component of nature s mechanism for degrading aromatic compounds in the environment. These enzymes, represented by catechol 1,2-dioxygenase (CTD) and protocatechuate(3,4-dihydroxybenzoate) 3,4-dioxygenase (PCD), catalyze the reaction... 

Abiotic mechanisms for degradation of herbicides in soil, discussion, 15 Abiotic transformation processes of pesticide dissipation in soil hydrolytic reactions, 5 role in pesticide degradation, 4-5... 

Distinct from the lysosomal pathway are cytosolic mechanisms for degrading proteins. Chief among these mechanisms is a pathway that includes the chemical modification of a lysine side chain by the addition of ubiquitin, a 76-residue polypeptide, followed by degradation of the ubiquitin-tagged protein by a specialized proteolytic machine. Ubiquitination is a three-step process (Figure 3-13a) ... 

F.R. Tay, D.H. Pashley, Water treeing a potential mechanism for degradation of dentin... 

An alternative mechanism for degradation of leucomentin-3 to the acetate (31) of (-h)-osmunda lactone would involve initial acid catalysed addition of acetic acid to the epoxide followed by intramolecular displacement of the hydroquinone moiety and would lead, significantly. 

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