Amino transamination

The reactions that amino acids undergo in living systems include transamination and decarboxylation... 

Fig. 3. (a) Reaction of pytidoxal 5 -phosphate (PLP) witii an amino-temiinal amino group of hemoglobin (Hb). The reagent is in the form of a Schiff s base with tris(hydroxymethyl)aminomethane [77-86-1] (Tris) buffet, and the reaction is a transamination, (b) The resulting unstable Schiff s base is reduced with... 

L-Glutamic acid is not an essential anino acid. It need not be present in the diet because animals can biosynthesize it from sources of a-ketoglutaiic acid. It is, however, a key intennediate in the biosynthesis of other amino acids by a process known as transamination. L-Alanine, for example, is fonned from pyruvic acid by transamination from L-glutamic acid. 

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... 

In the authors opinion (81UK1252), the evidence for such a scheme is the isolation of formamide transamination products, dialkylformamides (in case of l-dialkylaminoalk-l-en-3-ynes). Examples of formamide transamination, e.g., by /3-amino-substituted carbonyl compounds, are known [58CB2832 60CB1402 60HC( 14)272],... 

Amino acids are metabolized by a transamination reaction in which the —NH2 group of the amino acid changes places with the keto group of an a-keto acid. The products are a new amino acid and a new a-keto acid. Show the product from transamination of isoleucine. 

The mechanism of the first part of transamination is shown in Figure 29.14. The process begins with reaction between the a-amino acid and pyridoxal phosphate, which is covalently bonded to the aminotransferase by an iminc linkage between the side-chain -NTI2 group of a lysine residue and the PLP aldehyde group. Deprotonation/reprotonation of the PLP-amino acid imine in steps 2 and 3 effects tautomerization of the imine C=N bond, and hydrolysis of the tautomerized imine in step 4 gives an -keto acid plus pyridoxamine... 

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. 

Figure 29 14 MECHANISM Mechanism of the enzyme-catalyzed, PLP-dependent transamination of an a-amino acid to give an a-keto acid. Individual steps are explained in the text.

Figure 29.15 Mechanism of steps 2-4 of amino acid transamination, the conversion of a PLP-amino acid imine to PMP and an a-keto acid.

What is the structure of the o-keto acid formed by transamination of each of the following amino acids ... 

Transamination (Section 29.9) The exchange of an amino group and a keto group between reactants. 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

Macko, S.A., Estep, M.L.E., Engel, M.H. and Hare, RE. 1986 Kinetic fractionation of stable nitrogen isotopes during amino acid transamination. Geochimica et Cosmochimica Acta 50 2143-2146. 

Figure 11.3 is a flow model representing in extremely simple form the main relevant features of nitrogen metabolism. It is not difficult to propose a sufficient explanation why Bprot is isotopically heavier than the diet. We might expect that the net effect of transamination and deamination of amino acids is to remove isotopically lighter N (Macko et al. 1987). That is to say, we may expect that the equilibrium constant for the reaction ... 

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