Lysine side-chain methylation

Side-chain methylated amino acids were prepared from N-a-blocked starting materials in the same manner, and in the case of methylated poly lysine, was followed by acid hydrolysis of the peptide bonds. 

In either case, a reservoir of a solution with a higher concentration of precipitant is present in the same compartment. Water evaporates from the samples into the larger reservoir, concentrating the protein and causing its crystallization. Crystals grow slightly better in a spacecraft than on Earth. ° ° Some proteins, notably myosin from muscle, crystallize well only after reductive methylation of all lysine side chains to dimethyllysine with formaldehyde and sodium borohydride (Eq. 3-34). ... 

Structure of the myosin heads. Myosin and myosin fragments can be isolated in large quantities, but they have been difficult to crystallize. However, Payment and coworkers purified SI heads cleaved from chicken myosin by papain and subjected them to reductive methylation (using a dimethylamine-borane complex see also Eq. 3-34). With most of the lysine side chain amino groups converted to dimethylamine groups, high-quality crystals were obtained, and a structure was determined by X-ray diffraction. ... 

Fig. 1 Top Reaction mechanism for the methylation of the Lysine side chain of histone by 5-adenosyhnethionine using HKMT as the catalyst Middle The three cluster models used. Stars indicate fixed atoms in the coordinate-locking scheme. Bottom Potential energy profiles (kcal/mol). Results for various values of the dielectric constant are shown in different colors. Reproduced with permission from [24]...

GVGVP GFGVP GVGVP GVGK[NMeN]P) D-K/IF of Table 5.5, where n is for the aspartic acid residue with the side chain, -CH2-COOH, that accompanies the N-methyl nicotinamide functional group attached to a lysine side chain. Reduction of NMeN to form NMeN shifts the transition zone for protonation of the carboxyl function by 2.5 pH units. This shows electrochemical transduction, the conversion of electrical energy into chemical energy. In particular, the reduction reaction is carried out near pH 9, where the reduced NMeN is most stable. 

In analogy, Ugi et al. reported on a lactam formation by running a one-pot three components reaction the condensation of L-lysine 7, isobutyraldehyde and methyl isocyanide led to the corresponding a-amino-c-caprolactam 9, but the yield was not given. The authors presumed either a nucleophilic substitution of the ester 8 as the primary Ugi product by the amino function of the side chain or, alternatively, the nucleophilic attack of the NH2-group on an intermediately formed 0-acylamide and a subsequent rearrangement (Scheme 1) [4]. 

The way biotin participates in carbon dioxide fixation was established in the early 1960s. In 1961 Kaziro and Ochoa using propionyl CoA carboxylase provided evidence for 14C02 binding in an enzyme-biotin complex. With excess propionyl CoA the 14C label moved into a stable position in methyl malonyl CoA. In the same year Lynen found biotin itself could act as a C02 acceptor in a fixation reaction catalyzed by B-methylcrotonyl CoA carboxylase. The labile C02 adduct was stabilized by esterification with diazomethane and the dimethyl ester shown to be identical with the chemically synthesized molecule. X-ray analysis of the bis-p-bromanilide confirmed the carbon dioxide had been incorporated into the N opposite to the point of attachment of the side chain. Proteolytic digestion and the isolation of biocytin established the biotin was bound to the e-NH2 of lysine. 

Lysine is the most common site for N-methylation, but methylation can also occur on arginine, histidine, glutamine, and asparagine. The enzymes responsible for N-methylation are known as N-methyltransferases aided with SAM as a cosubstrate. All forms of methylation share the same mechanism the nucleophilic amino acid side chain attacks the electrophilic methyl group of SAM and releases S-adenosylhomocysteine (SAH) (Scheme 7). 

Scheme 7 Catalytic mechanism of methylation. The nucleophilic side chain (N, O, S) attacks the methyl group attached to the electrophilic sulfur generating methylated protein and S-adenosylhomocysteine. Lysine can be methylated up to three...

Solution Studies. The solution data for the N -methyl lysine analogs show conformational trends similar to those deduced from the biological data. Evidence pertaining to side chain Md backbone conformation favors similar shapes for 10 and 3a in water. Proton NMR spectra for both compounds show high field lysine Y-methylenes (0.02, 0.33 for 10 0.40, 0.55 for 3a) and single proline B- and Y-methylene protons (0.91> -. l for 10 ... 

As with the cyclo retro isomers, these studies have shown that there may be more than one peptide backbone conformation which can produce similar side chain topography. The N -methyl lysine analogs have also further confirmed that the n -H of lysine has no important role in receptor binding or conformational stabilization. These compounds did not, however, provide any advantage over 3 in biological selectivity or duration of action. 

Our retrosynthetic analysis for lipid I is presented below [Scheme 2], Our protected version of lipid I employed acetate protective groups for the carbohydrate hydroxyls, methyl esters for each of the carboxyl groups in the pentapeptide side chain, and trifluoroacetate for the terminal amino group of the lysine residue. These base-cleavable protective groups could be removed in a single operation in the final step of our synthesis and would not subject the sensitive diphosphate linkage to acidic reagents or reaction conditions. 

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