Prototropic amino acid

An exhaustive series of reports by Grigg et al. (28) outlined two basic methods for the generation of azomethine ylides proceeding via either a 1,2-prototropic shift, or by a decarboxylative approach (29). The decarboxylative route to azomethine ylides can be exemplified by the condensation of benzaldehyde with the cyclic amino acid tetrahydroisoquinoline (108) (30), in DMF at 120 °C, to generate the intermediate awfi-dipole 109, which underwent subsequent cycloaddition with N-methyl maleimide to furnish a 1 1 endo/exo mixture of adducts 110 (R = Ph), in 82% yield (Scheme 3.30). 

Toney, M.D. and Kirsch, J.F. (1993) Lysine 258 in aspartate aminotransferase enforcer of the Circe effect for amino acid substrates and the general-base catalyst for the 1,3-prototropic shift. Biochemistry, 32, 1471. 

Some of the amino acids carry a prototropic side chain e.g., aspartic acid and glutamic acid have an extra carboxyl, histidine has an imidazole, lysine has an amino, and arginine has a guanidino group. The structures of these side chains are shown in Table 3.1, and their pKa values are listed in Table 3.2. 

In some cases the quantities p(/ Ci t), and m yielded by analysis of titration curves may furnish special clues to the structure of particular proteins. For example, if ordinarily dissociable groups participate in formation of intramolecular bonds abnormal values for their intrinsic dissociation constants may result, or their number may even appear to be smaller than that found by amino acid assay. Examples will be furnished in the later sections on stoichiometry and on unreactive prototropic groups. 

Rather than resort to purely empirical selection of suitable values of ni and p(/Cint)< for equation 1 it is more usual to begin by fitting experimental data with values of m chosen to conform with the numbers of prototropic groups determined by several more direct and specific methods of examination of titration data. Even where the theoretical analysis of a titration curve is not attempted and exact values of p(Ki t), for each type of group are therefore lacking, the numbers of groups so determined may furnish valuable clues to the internal structure of the protein, especially when they are compared with the results of amino acid analyses. 

Smaller and less easily interpreted pH drifts have been reported by Kearney and Singer (1951) during inactivation of partially purified i-amino acid oxidases of snake venom. Reference ivill be made later to other proteins that exhibit other less direct indications of the existence of labile masking of certain prototropic groups. 

The crucial feature of the transamination mechanism is a tautomerization reaction, or 1,3 prototropic shift, in which a proton is transferred from the substrate carbon, which is directly bound to the amino group to the C4 of the cofactor (Figure 19). In this process, the external aldimine of the first amino acid substrate is isomerized into a ketimine, which is then hydrolyzed, liberating the related oxo acid product and the enzyme with the cofactor in the PMP form. The second half transamination reaction takes place via a reverse mechanism, starting from the enzyme in the PMP form and the second oxo acid substrate. 

First, derivatives of a-amino acids in which the amino group is part of an amide are considerably more prone to a-epimerization/racemization upon COOH activation than their carbamate congeners. Loss of stereochemical integrity is attributed to cyclization of a generic activated species, 23, to oxazolone 24, which exists in prototropic equilibrium with the aromatic tautomer 25 (Scheme 5). An amide being more nucleophilic than a carbamate, cyclization to an intermediate of type 24 is easier. Second, the tendency to cyclize is even more pronounced if the a-amido... 

The isoelectric point of a prototropic (Greek, tropism means change) amino acid is the average of the values of the protonation transitions on either side of the isoelectric species. For aspartic acid the isoelectric species exists in the pH domain between (2.1) and (3.9). Thus, the pi of aspartic acid is 3.0. 

Model studies (pyridoxal catalyzed conversion of a-amino acid to oxo-acid) indicates that the prototropic shift is in the aldimine <—> ketimine tautomerization, and this step can be greatly accelerated by general acid-base catalysis. Aspartate ( 2-oxoglutarate) aminotransferase (EC 2.6.1.1), which catalyzes transamination between Asp and 2-oxoglutarate (oxaoacetate and Glu), is the most extensively studied representative PLP enzyme. The enzyme is a homodimer containing one PLP molecule per subunit. Experimental observations pertaining to apartate aminotransferase are ... 

D from a deuterio-AL was observed in control experiments wiriiout enzyme or without KG. This indicates that in transamination the interconverrim) of the intermediate SchifPs bases I and II is a typical prototropic change, involving labilization and dissociation of the a-hydn n, followed by electromeric shift in the methylene-azomethine bridge and by uptake of a proton from water in the a-position of the newly formed amino acid residue, as in the formulation (91) ... 

As already mentioned, the amino acid residues in enzymes have prototropic groups which have the potential to act as a general acid or as a general base. Of these, the imidazole ring of histidine is of special interest since it can perform both func-... 

A-(Monosubstituted amino)pyridiniums (972) are in prototropic equilibrium with A-imides (973) and dications (971). For R = H or allyl, the A-imides (973) are very strong bases and cannot usually be isolated if R is an electron-withdrawing group (e.g. acyl, sulfonyl, nitro), then the imide (973) is less basic and more stable. The cations (971) are only obtained in very strongly acid media. 

The prototropic tautomerism of 1,3,5-triazinones, 1,3,5-triazinethiones, and amino-1,3,5-triazines has been discussed in detail . Cyanuric acid was shown to exist as the trioxo structure by UV and x-ray data. l-Methyl-l,3,5-triazine-2,4,6-(l//,3/f,5/f)-trione has the trioxo form as preferred tautomer (see Section 6.12.3.5) <86CB83>. Compounds (19), (20) and (21) were also shown by IR spectroscopy to show the oxo forms as preferred tautomers carbonyl bands were found at 1677-1730 cm-. ... 

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