Amino group transfer acids

Amino acids for infusion solutions are produced by amino group transfer reactions applying transaminases. Here, a major drawback is the equilibrium conversion of only 50%. Therefore,... 

Although the acidic protons of vinylamine should logically be those of its amino group, whose acidity increases by a charge transfer to the ethylene group, it would... 

Amino group transfer Amino acid Amine... 

Many of the reactions described so far point to the concept of waste nitrogen. What happens to the ammonium ions discharged during catabolism of the various amino acids What happens to the amino groups transferred from various amino adds to oxaloacetic acid, yielding aspartate The ammonium ions and excess amino groups, carried in the form of aspartate, are handled in the following marmer. They are incorporated into a small, water-soluble molecule known as... 

Aspartate aminotransferase catalyzes amino group transfer between acidic amino... 

The amino-group transfer is from L-glutamic acid and involves enzyme-... 

The conversion of pyridoxal to pyridoxamine in heat-sterilized media was confirmed to be due to the reaction of pyridoxal with amino acids (Snell 1945). This was the first discovery of nonenzymatic transamination. The transamination reaction in animal tissues was first discovered in 1937 by Alexander Braunstein and associates as an amino group transfer between glutamate and alanine in pigeon muscle extract (Braunstein 1939). Irwin Gunsalus and associates showed that the tyrosine decarboxylase activity of S. faecalis was... 

Aminotransferase. Aminotransferase is also called transaminase. Aminotransferase belongs to a group of enzymes that require pyridoxal 5 -phos-phate as a coenzyme and catalyse amino group transfer between an amino acid and a 2-oxo acid. The PPAT described in this chapter exceptionally does not require a coenzyme. 

In spite of the failure, to date, to isolate and adequately characterise more than two transaminases, both of which catalyse -amino group transfer, it is now generally accepted that transamination plays an important part in amino acid metabolism. 

FIGURE 28.12 Translation of rnRNA to an amino acid sequence of a protein starts at an rnRNA codon for methionine. Nucleophilic acyl substitution transfers the A/-formylmethionine residue from its tRNA to the amino group of the next amino acid (shown here as alanine). The process converts an ester to an amide. 

One class of enzymes that follow a ping-pong-type mechanism are aminotransferases (previously known as transaminases). These enzymes catalyze the transfer of an amino group from an amino acid to an a-keto acid. The products are a new amino acid and the keto acid corresponding to the carbon skeleton of the amino donor ... 

Several additional points should be made. First, although oxygen esters usually have lower group-transfer potentials than thiol esters, the O—acyl bonds in acylcarnitines have high group-transfer potentials, and the transesterification reactions mediated by the acyl transferases have equilibrium constants close to 1. Second, note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis (Chapter 25), and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate, as well as some amino acids. 

Acidic hydrolysis of 14 occurs via protonation of the nitrogen followed by attack of water on the resulting cationic intermediate. Proton transfer followed by ring-opening affords cation 15, which is trapped by a second equivalent of water. Another proton transfer followed by loss of the amino group affords protonated carboxylic acid 16, which loses to provide the carboxylic acid product. 

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. 

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. 

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

In the second step, the amino acid is bonded to the 2 or 3 -OH function of ribose in the form of an ester, via transfer to the tRNA and the terminal adenosine residue of the tRNA. This ester formation gives a higher-energy bond (or to be more exact, a bond with a high group transfer potential) with a AG°-value of -29 kJ/mol. 

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