Protein nucleophilic groups

Several chemical compounds may cause inflammation or constriction of the blood vessel wall (vasoconstriction). Ergot alkaloids at high doses cause constriction and thickening of the vessel wall. Allylamine may also induce constriction of coronary arteries, thickening of their smooth muscle walls, and a disease state that corresponds to coronary heart disease. The culprit is a toxic reactive metabolite of allylamine, acrolein, that binds covalently to nucleophilic groups of proteins and nucleic acids in the cardiac myocytes. 

The Mannich reaction can be used for the immobilization of certain drugs, steroidal compounds, dyes, or other organic molecules that do not possess the typical nucleophilic groups able to participate in traditional coupling reactions (Hermanson et al., 1992). It also can be used to conjugate hapten molecules to carrier proteins when the hapten contains no convenient nucleophile for conjugation (Chapter 19, Section 6.2). In this case, the carrier protein contains the primary amines and the hapten contains at least one sufficiently active hydrogen to participate in the condensation reaction. 

Esters of A - hydro x y s u c c i n i in i d c are also used to carry out chemical modification of peptides, proteins, and other biological molecules by acylation of nucleophilic groups in these molecules. For example, detection of estradiol antibodies can be accomplished using... 

The reactive nitrenium or carbonium ions postulated to be produced will react with nucleophilic groups in nucleic acids, proteins, and sulfydryl compounds such as glutathione (GSH) and methionine. The arylation of DNA by acetylaminofluorene has been demonstrated in vivo and in vitro. The involvement of sulfate conjugation brings other factors into play. Depletion of body sulfate reduces, and supplementation with organic sulfate increases the carcinogenicity of acetylaminofluorene. The production of covalent adducts between acetylaminofluorene and cellular macromolecules in vivo can be shown to be correspondingly decreased and increased by manipulation of body sulfate levels. 

For acid-catalyzed hydrolysis of methyl glucosides53 the kinetic isotope effect observed for the oxygen of the leaving group was /c,6()//c18() = 1.024-1.026. Observation of similar effects for enzymes supports the participation of an acidic group of the protein (Glu 35 of lysozyme) in catalysis but does not eliminate the possibility of concerted involvement of a nucleophilic group, e.g., Asp 52 in lysozyme.81 82... 

To summarize, the binding sites of lysozyme and the serine proteases are approximately complementary in structure to the structures of the substrates the nonpolar parts of the substrate match up with nonpolar side chains of the amino acids the hydrogen-bonding sites on the substrate bind to the backbone NH and CO groups of the protein and, for lysozyme, to the polar side chains of amino acids. The reactive part of the substrate is firmly held by this binding next to acidic, basic, or nucleophilic groups on the enzyme. 

The photoaffinity labels have been useful in mapping out residues at the active sites of enzymes and the binding sites of proteins such as antibodies. The normal affinity labels are more useful for kinetic work, since they are selective for the basic and nucleophilic groups that are so prevalent in catalysis, and since they also give information on the pATfl s of the groups that are modified. 

Figure 6-1. Oxidation of chlorogenic acid (6.4) by polyphenoloxidase (PPO), resulting in chlorogenoquinone (6.5), which can react with nucleophilic groups in proteins (6.6) to give the cross-linked compound 6.7, which can react with another protein molecule to yield 6.8. The quinate residue in structures 6.5, 6.7 and 6.8 is represented by R, whereas Ri and R2 indicate different amino acid residues.

Chlorotetrolic esters are small, highly functionalized, reactive molecules of particular interest is the possibility of using them as reagents for chemical modification of biological macromolecules. Different protein nucleophiles react under mild conditions with methyl 4-chloro-2-butynoate by addition across the triple bond and/or substitution of chlorine while the triple bond and the ester group are involved in the reaction of chlorotetrolic... 

Figure 3 Mechanism of steroid-induced cataract according to the most prominent hypothesis. It involves first the formation of Schiff bases between the steroid C-20 ketone group and nucleophilic groups such as e-amino groups of lysine residues of proteins and then a Heyns rearrangement involving the adjacent C-21 hydroxyl group that results in stable amine-linked adducts.

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