Amino acid, decarboxylation racemization

The general arguments about the antiquity of cofactors apply to PLP. The nonenzymatic synthesis of pyridoxal under prebiotic conditions is considered possible, whereas the presence of a 5 phosphate group could hint to an ancestral attachment of the cofactor to RNA molecules. " Furthermore, there are specific grounds to assume that PLP arrived on the evolutionary scene before the emergence of proteins. In fact, in current metabolism, PLP-dependent enzymes play a central role in the synthesis and interconversion of amino acids, and thus they are closely related to protein biosynthesis. In an early phase of biotic evolution, free PLP could have played many of the roles now fulfilled by PLP-dependent enzymes, since the cofactor by itself can catalyze (albeit at a low rate) reactions such as amino acid transaminations, racemizations, decarboxylations, and eliminations. " This suggests that the appearance of PLP may have preceded (and somehow eased) the transition from primitive RNA-based life forms to more modern organisms dependent on proteins. 

Figure 11.13 Reactions at a-carbon of a-amino acids catalyzed by pyridoxal enzymes All three substituents at C are subject to labilization in the three types of a-carbon reactions. The hydrogen is labilized in recemization reactions, the amino group is labUized in the transamination and the carboxyl group is labilized in decarboxylation. a-Amino acid condenses with pyridoxal phosphate to yield pyridoxylidene imino acid (an aldimine). The common intermediate, aldimine and distinct ketimines leading to the production of oxo-acid (in transamination), amino acid (in racemization) and amine (in decarboxylation) are shown. The catalytic acid (H-A-) and base (-B ) are symbolic both can be from the same residue such as Lys258 in aspartate aminotransferase.

In addition to amino acid decarboxylation and racemization, PLP is a coenzyme for transamination— the transfer of an amino group from one compound to another. The enzymes that catalyze transaminations are called aminotransferases or transaminases. Many transaminations involve two compounds a-ketoglutaric acid and L-glutamic acid. 

In all reactions, the first stage is formation of SchifTs base a by condensation of PalP and the amino acid. Schiff s bases a and b represent part of transamination, but for the complete mechanism see Transamination. Racemization a- b, followed by b-ia-iamino acid-1-PalP, with addition of the proton in the opposite configuration. Amino acid decarboxylation a -> d- c - amine + PalP. Serine hydroxymethyltransferase (EC 2.1.2.1) X = OH L-serine + PalP a f- g glycine + PalP reversal of these reactions leads to L-serine synthesis from glycine the hydroxymethyl group is carried by te-trahydrofolic acid. Cysteine desulfhydrase (EC 4.4.1.1) X = SH cy eine + PalP a b-> c-y hydro-... 

Pyridoxal phosphate is a necessary coenzyme for a number of different biochemical reactions transamination, amino acid oxidation, amino acid decarboxylation, glycogen breakdown, and racemization of d- and L-amino acids. 

In the preceding sections pyridoxal phosphate has appeared as an essential cofactor for many apparently unrelated processes. It has been possible, however, to demonstrate common features to many different reactions of amino acids decarboxylation, transamination, racemization, substitution, and elimination. The properties of pyridoxal are such that many of the enzymatic reactions have been duplicated nonenzymatically... 

Transaminases have been used to generate p-amino acids from racemic p-amino acids (Scheme 14.3a) or prochiral P-keto acids (Scheme 14.3b) [36]. The latter method has the benefit of potentially converting aU the starting material into a single enantiomer, but because it is stiU relatively early in its development, it suffers from disadvantages such as the propensity for the starting p-keto acids to undergo decarboxylation under very mild conditions. 

Once the amino acid has fonned an imine with PLP, the next step is to break a bond to the a-carbon of the amino acid. Decarboxylation breaks the bond joining the carboxyl group to the a-carbon transamination, racemization, and a,j8-elimination break the bond joining the hydrogen to the a-carbon and C —bond cleavage breaks the bond joining the R group to the a-carbon. 

Another protecting group of amines is 1-isopropylallyloxycarbonyl, which can be deprotected by decarboxylation and a /3-elimination reaction of the (tt-l-isopropylallyl)palladium intermediate under neutral conditions, generating CO2 and 4-methyl-1,3-pentadiene. The method can be applied to the amino acid 674 and peptides without racemization[437]. 

The biologically active form of vitamin Bg is pyridoxal-5-phosphate (PEP), a coenzyme that exists under physiological conditions in two tautomeric forms (Figure 18.25). PLP participates in the catalysis of a wide variety of reactions involving amino acids, including transaminations, a- and /3-decarboxylations, /3- and ") eliminations, racemizations, and aldol reactions (Figure 18.26). Note that these reactions include cleavage of any of the bonds to the amino acid alpha carbon, as well as several bonds in the side chain. The remarkably versatile chemistry of PLP is due to its ability to... 

These enzymes invariably involve a cofactor, pyridoxal phosphate (vitamin B6). In addition, pyridoxal phosphate is also required for most decarboxylations, racemizations, or elimination reactions in which an amino acid is a substrate. Pyridoxal phosphate is not involved in decarboxylations in which the substrate is not an amino acid. So if a question... 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

The lipo-amino acids are generally synthesized as racemic mixtures which are resolved into optically pure a-amino acids by chemical or enzymatic methods.113" 137 133 The synthesis is based on the alkylation of diethyl acetamidomalonate followed by hydrolysis and decarboxylation. 129 138 140 Also, 20% DMF has been used in the hydrolysis step, as it is suitable for industrial scale preparation. 138] Alternatively, lipidated a-amino acids are synthesized by reacting a-bromoalkanoic acid with ammonium hydroxide. 141 ... 

It is involved as a coenzyme (pyridoxal phosphate) in metabolism of tryptophan, in several metabolic transformations of amino acids including transamination, decarboxylation and racemization. 

Pyridoxal phosphate is the coenzyme for the enzymic processes of transamination, racemization and decarboxylation of amino-acids, and for several other processes, such as the dehydration of serine and the synthesis of tryptophan that involve amino-acids (Braunstein, 1960). Pyridoxal itself is one of the three active forms of vitamin B6 (Rosenberg, 1945), and its biochemistry was established by 1939, in considerable part by the work of A. E. Braunstein and coworkers in Moscow (Braunstein and Kritzmann, 1947a,b,c Konikova et al 1947). Further, the requirement for the coenzyme by many of the enzymes of amino-acid metabolism had been confirmed by 1945. In addition, at that time, E. E. Snell demonstrated a model reaction (1) for transamination between pyridoxal [1] and glutamic acid, work which certainly carried with it the implication of mechanism (Snell, 1945). 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

Pyridoxal-5 -phosphate participates in many reactions with a-amino acids, including transaminations, a decarboxylations, racemizations, a,/3 eliminations, j8,y eliminations, aldolizations, and the (3 decarboxylation of aspartic acid. 

Pyridoxal-5 -phosphate promotes decarboxylations, racemizations, transaminations, aldol cleavages, and elimination reactions of amino acid substrates. 

The reactions catalyzed by transaminases are anergonic as they do not require an input of metabolic energy. They are also freely reversible, the direction of the reaction being determined by the relative concentrations of the amino acid-keto acid pairs. Pyridoxal phosphate is not just used as the coenzyme in transamination reactions, but is also the coenzyme for several other reactions involving amino acids including decarboxylations, deaminations, racemizations and aldol cleavages. 

Vitamins, cofactors, and metals have the potential to broaden the scope of antibody catalysis considerably. In addition to hydrolytic and redox reactions, they facilitate many complex functional group interconversions in natural enzymes.131 Pyridoxal, for example, plays a central role in amino acid metabolism. Among the reactions it makes possible are transaminations, decarboxylations, racemizations, and (3,y-eliminations. It is also essential for ethylene biosynthesis. Not surprisingly, then, several groups have sought to incorporate pyridoxal derivatives into antibody combining sites. 

This type of chemistry is well suited for the manipulation of amino acids and peptides which tend to undergo racemization at the a carbon under ionic conditions. First examined was the decarboxylation of A-protected amino acids by using a thiol as hydrogen atom transfer reagent, Scheme 25.71 The power of... 

Other reactions that mimic the enzymic processes that require pyridoxal phosphate also have been realized. Werle and Koch reported the nonenzymic decarboxylation of histine (9). The racemization of alanine occurs in preference to its transamination when aqueous solutions with polyvalent cations are maintained at pH 9.5. Other amino acids are likewise racemized the order of rates is Phe, Met > Ala > Val > lieu. At lower pH, the dominant reaction is transamination, with pH maxima varying from 4.3-8 with the nature of the metal ion used as catalyst. 

Pyridoxal-5-phosphate (PLP) is perhaps one of nature s most versatile coenzymes, capable of modifying a-amino acids by decarboxylation, racemization, transamination (conversion of the a-amino group to an a-keto group) as well as elimination or substitution of a leaving group at the / or even elimination at the y carbon of the a-amino acid133-139. 

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