Lysine residues acylation

Several of the proteins of the Golgi transport system are AT-acetylated at either the amino terminal or the e-amino group of a lysine residue. Acylation may be either cotranslational or posttranslational. Amino terminal acylation protects the proteins from degradation, and various acylations are required for the assembly of multisubunit membrane proteins and transport of glycoproteins through the Golgi. 

Cleavage of the oxirane C-0 bond produces a zwitterionic intermediate (Fig. 10.22), which that can undergo chloride shift (Pathway a) to 2,2-dich-loroacetyl chloride (10.90) followed by hydrolysis to 2,2-dichloroacetic acid (10.91). Furthermore, the zwitterionic intermediate reacts with H20 or H30+ (Pathway b) by pH-independent or a H30+-dependent hydrolysis, respectively. The pH-independent pathway only is shown in Fig. 10.22, Pathway b, but the mechanism of the H30+-dependent hydrolysis is comparable. Hydration and loss of Cl, thus, leads to glyoxylyl chloride (10.92), a reactive acyl chloride that is detoxified by H20 to glyoxylic acid (10.93), breaks down to formic acid and carbon monoxide, or reacts with lysine residues to form adducts with proteins and cytochrome P450 [157], There is also evidence for reaction with phosphatidylethanolamine in the membrane. 

Histidine residues are efficient nucleophiles in aqueous solution at pH 7, much more so than lysines, and this is the basis for the site-selective functionalization of lysine residues in folded polypeptides and proteins [24, 25]. p-Nitrophenyl esters react with His residues in a two-step reaction to form an acyl intermediate under the release of p-nitrophenol followed by the reaction of the intermediate with the most potent nucleophile in solution to form the reaction product. In aqueous solution the reaction product is the carboxylic acid since the hydroxide ion is the most efficient nucleophile at pH 7. If there is an alcohol present the reaction product will be an ester and the overall reaction is a transesterification reaction. 

The intermediate can, however, also be trapped by an amine to form an amide although at pH 7 in aqueous solution primary amines are predominantly proto-nated and only poorly reactive. Intramolecularity will, however, improve the poor reactivity of a lysine residue towards an acyl intermediate provided that the His and the Lys residues are close in space. The net reaction under these conditions is therefore an amidation of the lysine side chain by the active ester that is more efficient than the direct acylation of a lysine residue by at least three orders of magnitude (Fig. 10). The lysine residue will also improve the reactivity of the His side chain by electrostatic transition state stabilization and the wasteful reaction with other His residues that gives rise to hydrolysis is therefore suppressed. 

Aside from the Maillard reaction, other covalent modifications of amino acids and proteins are possible within the caries lesion, which merit future investigation. For example, certain oral microorganisms excrete y-glutamyl transferases. These enzymes catalyse the formation of cross-links between glutamic acid and lysine residues of proteins. In addition, N-acyl amino acids are present in plaque, which adsorb to mineral surfaces. 

Block (cap) residual lysine residues with a carboxylic acid by acylation using succinic anhydride prepared in l-methyl-2-pyrrolidinone,... 

The aerobic pathway of metabolism (pathway 1) (Fig. 7.77) produces trifluoroacetyl chloride, a highly reactive acyl chloride, which can react with nucleophiles such as amino groups similar to those on proteins. Alternatively, reaction with water yields trifluoroacetic acid. Trifluoroacetylchloride is the probable reactive metabolite that trifluoroacylates protein, most probably at lysine residues (Fig. 7.77). Removal of the trifluoroacyl moiety from the... 

Homocysteine, HSCH2CH2CH(NH2)C02H, is a toxic amino acid that is thought to acylate lysine residues on proteins via its corresponding thiolactone (72) (Scheme 19). Now, for the first time, the kinetics have been studied of the aminolysis of (72) and of two model compounds, y-thiobutyrolactone (72 H instead of NH2) and N-trimethylamino homocysteine thiolactone (72 Me3N+ instead of NH2). A Brpnsted plot for homocysteine thiolactone (72) gave ft = 0.66. This and other data supported... 

Limiting essential amino acids covalently attached to proteins by using activated amino acid derivatives can improve the nutritional quality and change the functional properties of proteins. The best chemical methods for incorporating amino acids into water-soluble proteins involve using car-bodiimides, N-hydroxysuccinimide esters of acylated amino acids, or N-carboxy-a-amino acid anhydrides. The last two methods can give up to 75% incorporation of the amount of amino acid derivative used. With the anhydride method, as many as 50 residues of methionine have been linked to the 12 lysine residues of casein. The newly formed peptide and isopeptide bonds are hydrolyzed readily by intestinal aminopeptidase, making the added amino acids and the lysine from the protein available nutritionally. 

The glycolytic pathway enzyme fructose-1,6-bisphosphate aldolase forms an acyl-enzyme intermediate with its ketone substrate fructose 1,6-bisphosphate. Given that the enzyme contains a lysine residue that is essential for its activity, what type of covalent intermediate is likely to be formed ... 

Steps 3-4 of Figure 29.5 Carboxytation and Acyl Transfer The third step is a lotul-ing reaction in which acetyl CoA is carboxylated by reaction with HC03 and ATP to yield malonyl CoA plus ADP. This step requires the coenzyme biotin, which is bonded to the lysine residue of acetyl CoA carboxylase and acts as a carrier of CO2. Biotin first reacts with bicarbonate ion to give A carboxybiotin, tvhich then reacts with the enolate ion of acetyl CoA and transfers the CO2 group. Thus, biotin acts as a carrier of CO2, binding it in one step and releasing it in another. 

FIGURE 16.10 Hapten determinants formed by penicillins that contain a p-lactam ring linked to various side chains (R). The primary route of haptenation involves acylation of the E-amino group of lysine residues of serum or cell surface proteins to form a penicilloyl or major antigenic determinant. Isomerization of penicillin leads to the generation of compounds that form disulfide bonds wdth the cysteine sulfhydryl groups of proteins. These epitopes are termed minor determinants. 

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