C-Amino group

Most recently, Baltzer and co-workers have incorporated a lysine-bound nicotinamide into a more complex peptide scaffold [75]. This approach takes advantage of the augmented reactivity of a lysine residue contained in a helix-turn-helix scaffold (as described previously [76]). An adjacent histidine is able to selectively catalyze the formation of an amide bond between activated esters and the lysine c-amino group under aqueous conditions. Thus, reaction of the 42-residue peptide LA-42 withp-nitrophenyl hT-methylnicotinate in an aqueous solution at pH 5.9 yields the nicotinoyl-functionalized peptide (Fig. 27). 

Of all amino acids involved in the browning reaction, lysine with its c-amino group is especially susceptible to side reaction and crosslinking and becomes unavailable. 

Nutritional Effects Due to the Blockage of Lysine. The most important Maillard reaction in food proteins occurs with the c-amino group of lysine. Since lysine is an essential amino acid, nutritional consequences can be expected. These depend on the chemical structure of the lysine derivatives formed. 

Aminoimidazoles do not appear to exist to any great extent in the tautomeric imino forms.448 The C-amino groups have lower basic strength than aniline, and consequently alkylation occurs preferentially at a ring nitrogen. Amino substituents in the 2-position can generally be acylated and diazotized (usually in strongly acid medium), but 4-aminoimidazoles are more unstable and have not been extensively studied. 

Nucleotide binding sites in proteins can be identified by treating the protein-nucleotide complex with periodate, which cleaves ribonucleotides into highly reactive dialdehydes these are able to couple in a Schiff condensation reaction with the c-amino group of any neighbouring lysine residues (Easterbrook-Smith et al. 1976) (Figure 5-7). This coupling is reversible, but it can be made permanent by reduction with NaBH.. ... 

In 1956 our laboratory, in collaboration with Beiser and Lieberman, became interested in preparing steroid-protein conjugates that were to be used to elicit antisteroid antibodies. An examination of the literature at that time showed that the azo coupling techniques of Landsteiner were still dominant. Like him, we chose to use the serum albumins because they were inexpensive and likely to yield soluble conjugates. However, an examination of the amino acid content of bovine serum albumin (BSA) (Fig. 1) convinced us that substitution by such relatively complex haptens as steriods should be attempted by reaction with the more plentiful c-amino groups of the lysine residues rather than by an azo coupling reaction with tyrosine, tryptophan, and imidazole residues. This meant forma-... 

Biotin is also joined in on amide linkage to the c-amino group of a lysine residue of curbumyl phosphate synthetase (CPS) to form biolin-CPS. which participates with two ATPs. HCOf. and glutamine in the synthesis of carbumyl phosphate. This takes place stepwise us follows ... 

In our view, iodine inactivation in the case of rat intestinal alkaline phosphatase causes blocking of the —SH group, since other evidence for the presence of this group is strong. The c-amino group of lysine is considered to be essential not merely because tyrosine can be reasonably excluded, but because rat intestinal alkaline phosphatase is inhibited by the lysine-specific o-methylisourea reagent and because the pZm-pH plot shows a discontinuity at pH 9.6 (G5), the pilf of the e-amino group of lysine (C14). 

Aldolases produce an imine as the initial intermediate from the reaction of a carbonyl group of the substrate with the c-amino group of a lysine residue of the enzyme (Scheme 4). Reactions of this adduct produce enamines that are tautomers of imines. The imine derivatives of the enzyme can be trapped irreversibly (Scheme 5) by reducing agents, such as sodium borohydride, and by nucleophiles, such as cyanide (5). The enamine structure has a zwitterionic resonance contributor with negative charge localized at the carbon a to the carbon attached... 

Probably the amino acid side chains of proteins most often modified are the c-amino group of lysine and the sulfhydryl group of cysteine, or its oxidized product, the disulfide group of cystine. 

Under the basic condition of the -elimination reactions (vide supra) equilibria exist between reactants in the form of the a,/ -unsaturated amino acids generated and nucleophiles present. Shifts in these equilibria are a function of a number of factors, among them the nucleophilicity of groups capable of adding across the double bonds of a,/ -unsaturated amino acids. One of the most prevalent nucleophilic groups in proteins exposed to alkaline conditions is the c-amino group of lysine, the addition of which takes place under circumstances where cystine, for instance,... 

A cyclic AMP independent protein kinase which catalyzes the phosphorylation of histidine in histones has been found in nuclei from rat tissue and in Walker-256 carcinosarcoma cell nuclei (96). Two histone kinases (ATP histone N-phosphotransferase) have been partially purified from the nuclei of the carcinosarcoma cells (78a). One of these enzymes preferentially phosphorylates histone H4 (IV, F2al) at an optimum pH of 9.5, while the other preferentially phosphorylates histone I (FI) at an optimum pH of 6.5. Both enzymes had an absolute requirement for Mg2+, required similar levels of ATP, and were not stimulated by added cyclic AMP or cyclic GMP. The pH 9.5 kinase was strongly inhibited by relatively low concentrations of GTP and CTP, but the pH 6.5 kinase was not affected. The pH 9.5 kinase phosphorylates the only two histidines (His-18 and His-75) in histone H4 to form 3-phosphohistidine. The pH 6.5 kinase phosphorylates the c-amino group of a lysine residue in histone I. Location of this lysine is not known. The phospho groups of both these derivatives are acid labile. 

This is an example of several enzymes in which an essential co-factor is covalently bound to the protein. For example, lipoic acid and biotin are covalently linked to the c-amino group of a specific lysine residue in certain enzymes. In some cases, pyridoxal phosphate is bound to the protein through the formation of a Schiff base involving the carbonyl group of the co-factor and an c-amino group of a lysine residue. In cytochrome c, the heme is attached through two thiol ether linkages to cysteine residues of the protein. 

Groninger and Miller (28) succinylated myofibrillar fish protein and then treated the succinylated proteins with bromelain to obtain an acylated mixture of polypeptides. Both the extents of succinylation and enzymic hydrolysis affected the volume of foam of the whipped protein. Optimum foam volume and stability occurred when 54% of the c-amino groups of lysine were acylated and the succinylated protein was mildly hydrolyzed at pH 7, 25°C for 10 min using a bromelain protein ratio of 1 100 (w/w). 

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