Amino acid transimination

Step 1 of Figure 29.14 Transimination The first step in transamination is trans-imination—the reaction of the PLP—enzyme imine with an a-amino acid to give a PLP—amino acid imine plus expelled enzyme as the leaving group. The reaction occurs by nucleophilic addition of the amino acid -NH2 group to the C=N bond of the PLP imine, much as an amine adds to the C=0 bond of a ketone or aldehyde in a nucleophilic addition reaction (Section 19.8). The pro-tonated diamine intermediate undergoes a proton transfer and expels the lysine amino group in the enzyme to complete the step. 

The transformation of the cyano group could also introduce a new chiral center under diastereoselective control (Figure 5.13). Grignard-transimination-reduction sequences have been employed in a synthesis of heterocyclic analogues of ephedrine [81]. The preferential formation of erythro-/3-amino alcohols may be explained by preferential hydride attack on the less-hindered face of the intermediate imine [82], and hydrocyanation of the imine would also appear to proceed via the same type of transition state. In the case of a,/3-unsaturated systems, reduction- transimination-reduction may be followed by protection of the /3-amino alcohol to an oxazolidinone, ozonolysis with oxidative workup, and alkali hydrolysis to give a-hydroxy-/3-amino acids [83]. This method has been successfully employed in the synthesis L-threo-sphingosine [84]. 

While aromatic aldimines of amino acid esters are readily accessible from benzaldehyde or related derivatives and amino acid esters with concomitant removal of water,the corresponding ketimines require harsher reaction conditions, e.g. the benzophenone imine of glycine methyl ester is formed in boiling xylene in the presence of boron trifluoride-diethyl ether complex as a Lewis acid in 82% yield.P l These or similar reaction conditions lead also to piperazine-2,5-dione (DKP) formation therefore milder reaction conditions have been developed such as the transimination with benzophenone imine in dichloromethane, which occurs at room temperature overnight in 80-95% yield (Scheme The N -diphen-... 

The first step in each of these reactions is formation of a -imino group or a jS-iminium ion between the aldehyde of the coenzyme and the amino group of the a-amino acid substrates, thereby providing the j3-electron sink required for decarboxylation or ionization of the a-CH. In the resting state of the enzyme the pyridoxal is covalently sequestered to a lysine s-amino group (ENHj) and the amino acid substrate presumably gives initial transimination, prior to decarboxylation. After decarboxylation, there is yet a second transimination, that enables release of the decarboxylated product and regeneration of the pyridoxal for the next turnover. 

PLP becomes attached to its enzyme by forming an imine with the amino group of a lysine side chain. The first step in all PLP-requiring enzymes is a transimination reaction, a reaction in which one imine is converted into another imine. In the transimination reaction, the amino acid substrate reacts with the imine formed by PLP and the enzyme, forming a tetrahedral intermediate. The lysine group of the enzyme is expelled, forming a new imine between PLP and the amino acid. 

If the PLP-catalyzed reaction is a decarboxylation, the carboxyl group is removed from the a-carbon of the amino acid. Electron rearrangement and protonation of the a-carbon of the decarboxylated intermediate by a protonated amino group of a lysine side chain or by some other acid group then reestablishes the aromaticity of the pyridine ring. The last step in all PLP-requiring enzymes is a transimination reaction with a lysine side chain, in order to release the product of the enzyme-catalyzed reaction and regenerate enzyme-bound PLP. 

In the first step of transamination, a proton is removed from the a-carbon of the amino acid bound to PLP. Rearrangement of the electrons and protonation of the carbon attached to the pyridine ring, followed by hydrolysis of the imine, forms the a-keto acid and pyridoxamine. At this point, the amino group has been removed from the amino acid, but pyridoxamine has to be converted back to enzyme-bound PLP before another round of catalysis can occur. Pyridoxamine forms an imine with a-ketoglutarate, the second substrate of the reaction. Removal of a proton from the carbon attached to the pyridine ring, followed by rearrangement of the electrons and donation of a proton to the a-carbon of the substrate forms an imine that, when transiminated with a lysine side chain, releases glutamate and reforms enzyme-bound PLP. 

Thiamine pyrophosphate (TPP) is the coenzyme required by enzymes that catalyze the transfer of a two-carbon fragment. Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon adjacent to a carbonyl group. Pyridoxal phosphate (PLP) is the coenzyme required by enzymes that catalyze certain transformations of amino acids decarboxylation, transamination, racemiza-tion, C —Cp bond cleavage, and a,j8-elimination. In a transimination reaction, one imine is converted into another imine in a transamination reaction, the amino group is removed from a substrate and transferred to another molecule. 

Preexistence of a Schiff base between pyridoxal phosphate and the enzyme may account for the greatly enhanced rates of enzymatic reactions is compared to the rates of the corresponding non-enzymatic reactions. Subsequent reactions of the enzyme with amino acids must involve Schiff base formation via a fast transimination step (see section IV.D). Once the new Schiff base is formed, the e-amino group of the lysine residue that was originally bound to pyridoxal phosphate is free and is in a favorable position to act as a catalyst in subsequent steps of the enzymatic reaction. 

FIGURE 20.3 Mechanism of the transimination reaction of a PLP-enzyme imine with an a-amino acid to give a PLP-amino acid imine plus expelled enzyme. The reaction is analogous to that shown previously in Figure 14.8. 

A transimination reaction with a lysine side chain releases the product of the reaction (the racemized amino acid) and regenerates the imine between the enzyme and PLP. 

The enzymes that catalyze transamination reactions are called aminotransferases. Each amino acid has its own aminotransferase. Transamination allows the amino groups of the various amino acids to be collected into a single amino acid (glutamate) so that excess nitrogen can be easily excreted. (Do not confuse transamination with transimination, discussed previously.)... 

N-Substituted ethanolamines with two stereogenic centers have been prepared in a one-pot synthesis [143,145]. In this procedure the intermediate primary imine is transiminated with a primary amine followed by NaBIU reduction (Scheme 20). Even transimination with amino acid esters [123] and with ethanolamines to form diethanolamines [146] is possible. 

DIBAL Reduction/Transimination/HCN Addition/Hydrolysis p-Hydroxy-a-Amino Acids... 

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