Covalent modification reactions, table

Metabolic regulation 535 — 581 control elements of 536 sensitivity coefficient 537 Metabolism. See also Specific compounds activation 507, 508 beta oxidation 511, 512 control by covalent modification reactions table 543... 

Protein chemistry is an extensive and highly developed area of organic chemistry that deals with the chemical reactions of proteins. Much of this chemistry concerns reactions that occur in aqueous solution at ambient temperatures and neutral pH, that is, under conditions where proteins are stable. The objective is to modify residues in proteins chemically, either to provide mechanistic information or to produce useful alterations of activity. Some of the more frequent modification reactions are listed in Table 9.1. Spectroscopic probes may be covalently... 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

Two mechanisms that are commonly employed in altering enzyme activity are covalent modification and allosteric regulation. Covalent modification is an enzymatically catalyzed reaction that involves the reversible formation of a covalent bond between a small molecule and a specific amino acid side chain(s) on an enzyme that affects its activity. Allosteric regulation of an enzyme s activity involves noncovalent binding of a small molecule at a site other than the active site that alters the enzyme s activity. Unlike the limited examples of covalent modification that have been discovered (see Table 15-1), a wide variety of small molecules have been found to regulate the activity of particular enzymes allosterically. 

One class of mechanism-based MAO inhibitors includes the unsaturated alkylamines (propargylamine analogs) (Table II). Although the kinetics of enzyme inactivation for these compounds are consistent with a mechanism-based inhibitor, in only a few cases has the chemical mechanism and site of protein modification been determined. Pargyline (iV-benzyl-N-methyl-2-propynylamine) is a classic example. Pargyline reacts stoichiometrically and irreversibly with the MAO of bovine kidney, with protection from inactivation afforded by substrate benzylamine (91). Furthermore, the reaction involves bleaching of the FAD cofactor at 455 nm and the formation of a new absorbing species at 410 nm and a covalent adduct of inactivator with flavin cofactor (92). 

In an attempt to obtain further information on the nature of the metabolite antigen formed in vitro, various modifications of the enzyme generating system were made to see if they altered the antibody activity. Table 3 shows the modifications that were used and compares their effects on the immunological reaction with those obtained by measuring the covalent binding of practolol with liver microsomes (Amos 1978). 

Chemical or Covalent Degradation of Proteins. Proteins are subject to a variety of chemical modification and degradation reactions such as hydrolysis, deamidation, isomerization, disulfide reshuffling, -elimination, and oxidation (Table 1) (13,14). The stability of proteins toward chemical degradation pathways often depends on the protein s folded state. In order for the chemical reactions to occur, the labile residue must be solvent accessible and must have varying degrees of structural freedom of the peptide backbone and/or side chains around the labile residue. Hence, stabilization of the protein s folded state (ie, its compact structure) that minimizes solvent accessibility and rotational freedom can lower the reaction rate of some chemical degradation reactions. 

The reactions shown in the tables above are often just the first step in the functionalization of porSi. Subsequent modification using simple organic chemistry permits limitless options for the covalent attachment of more complex molecules to the smface. Examples of larger molecules grafted to porSi include DNA (Pike et al. 2002), proteins (Li et al. 2010), fullerenes (Dattilo et al. 2006), poly (ethylene glycol) (Schwartz et al. 2005), and anthracyclines (Hartmann et al. 2013). 

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