Alanine transamination reactions

Problem 21.5 (a) What is the relationship between reactants and products in the transamination reaction (b) Which ketoacid is needed to give (i) alanine (ii) leucine (iii) serine (iv) glutamine (c) Which amino acids cannot be made by transamination 4... 

Pyruvate is first transported from the cytosol into mitochondria or is generated from alanine within mitochondria by transamination, in which the a-amino group is removed from alanine (leaving pyruvate) and added to an a-keto carboxylic acid (transamination reactions are discussed in detail in Chapter 18). Then pyruvate carboxylase, a mitochondrial enzyme that requires the coenzyme biotin, converts the pyruvate to oxaloacetate (Fig. 14-17) ... 

Alanine, aspartate, and glutamate are synthesized by transfer of an amino group to the a-keto acids pyruvate, oxaloacetate, and a-keto-glutarate, respectively. These transamination reactions (Figure 20.12, and see p. 248) are the most direct of the biosynthetic pathways. Glutamate is unusual in that it can also be synthesized by the reverse of oxidative deamination, catalyzed by glutamate dehydrogenase (see p. 249). 

Dunathan suggested that this stereoelectronic requirement explains certain side reactions observed with PLP-requiring enzymes. The idea also received support from experiments with a bacterial a-dialkyl-glycinedecarboxylase.231 232 The enzyme ordinarily catalyzes, as one half-reaction, the combination decarboxylation-transamination reaction shown in Eq. 14-37. It also acts on both d- and L-alanine, decarboxylating the former but catalyzing only removal of the a-H... 

The glucose-alanine cycle. Active muscle functions anaerobically and synthesizes alanine by a transamination reaction between glutamate and pyruvate, The alanine is transported to the liver, where the... 

For two transaminases the remaining unknown stereochemical parameter was determined by demonstrating an internal transfer of tritium (dialkyl amino acid transaminase) [28] or deuterium (pyridoxamine-pyruvate transaminase) [27] from the a-position of the substrate L-alanine to C-4 of the cofactor. Internal hydrogen transfer from the a-position of the substrate amino acid to C-4 of PLP has also been demonstrated for two of the abortive transamination reactions, those catalyzed by tryptophan synthase fi2 protein [32] and by aspartate-/8-decarboxylase [31]. In addition, the same phenomenon must occur in alanine transaminase, as deduced from the observation that the enzyme catalyzes exchange of the /8-hydrogens of... 

Nitrogen is transported from muscle to the liver in two principal transport forms. Glutamate is formed by transamination reactions, but the nitrogen is then transferred to pyruvate to form alanine, which is released into the blood (Figure 23.15). 

Studies on the transamination reaction between f-butyl esters of optically active amino acids and methyl pyruvate were carried out, as shown in Scheme 7. The resulting iminodicarboxylic acid (16) was partially hydrolyzed and then oxidized with f-butyl hypochlorite to form alanine. The oxidation is a generally applicable one, and the optical purity of alanine is high (50-70%). Similar asymmetric transamination between an (S)-amino acid and ketones was carried out. Catalytic hydrogenation of the Schiff s bases prepared from a-keto acid esters and amino acid esters was carried out, and a substituent and temperature effect observed (de 40-70%). ... 

The normal mechanism for the transamination reaction is shown in Fig. 4.24 (R=H) and involves the condensation of alanine and pyridoxal phosphate to give an imine. A proton is lost from the imine to give a dihydropyridine intermediate. This reaction is catalysed by a basic amino acid provided by the enzyme as well as the electron withdrawing effects of the protonated pyridine ring. The dihydropyridine structure now formed is hydrolysed to give the products. 

Ammonia is produced by almost all cells in the body however, only the liver has the enzymatic machinery to convert it to urea. Therefore, extra-hepatic ammonia must be transported to the liver. However, anunonia in the blood is toxic to cells, and therefore the nitrogen from amino acid catabolism is transported in blood either as glutamine or alanine. Glutamine is synthesized from Glu and ammonia in an ATP-requiring reaction that is catalyzed by glutamine synthetase. Alanine is formed from pyruvate in a transamination reaction catalyzed by alanine transaminase (ALT). 

Because transamination reactions are reversible, it is theoretically possible for all amino acids to be synthesized by transamination. However, experimental evidence indicates that there is no net synthesis of an amino acid if its a-keto acid precursor is not independently synthesized by the organism. For example, alanine, aspartate, and glutamate are nonessential for animals because their a-keto acid precursors (i.e., pyruvate, oxaloacetate, and a-ketoglutarate) are readily available metabolic intermediates. Because the reaction pathways for synthesizing molecules such as phenylpyruvate, a-keto-/Thydroxybutyrate, and imidazolepyruvate do not occur in animal cells, phenylalanine, threonine, and histidine must be provided in the diet. (Reaction pathways that synthesize amino acids from metabolic intermediates, not only by transamination, are referred to as de novo pathways.)... 

Transamination reactions have been described as ping-pong reactions. Using the reaction of alanine with a-ketoglutarate, indicate how this ping-pong reaction works. 

Alanine. Recall that the reversible transamination reaction involving alanine and pyruvate is an important component of the alanine cycle discussed previously (Section 8.2). 

Alanine is an allosteric inhibitor of glutamine synthetase, an enzyme with a central role in nitrogen metabolism in the cell. Alanine participates in transamination reactions and in the glucose-alanine cycle. 

A bacterial aminotransferase [46] promotes a decarboxylative transamination reaction with a-aminoisobutyrate in the presence of pyruvate. The reaction occurs via the sequence of Fig. 12 involving an initial cleavage of the Q-COjH bond in the substrate pyridoxal-P Schiff base complex (Fig. 12, 1) followed by reprotonation at C-4 of the coenzyme to give the pyridoxamine-P-enzyme complex (Fig. 12, 4) that participates in the transamination of pyruvate. However, the enzyme will also transform L-alanine at a significant rate by a half-transamination reaction into pyruvate, thereby implying that it is now the C -H bond of the amino acid that is first broken. 

Alanine is produced from pyruvate by a transamination reaction catalyzed by alanine aminotransaminase (ALT) and may be converted back to pyruvate by a reversal of the same reaction (see Fig. 39.4). Alanine is the major gluconeogenic amino acid because it is produced in many tissues for the transport of nitrogen to the liver. 

In 1966 Dunathan (18) proposed that, in PLP-mediated reactions, the bond to be broken in the substrate-cofactor compound should be perpendicular to the plane of the extended conjugated system so that there would be maximum a-n overlap between the breaking bond and the ring-imine n system. Thus 3a, 3b, and 3c represent the conformations of the imine 3 best suited to achieve transamination reactions, decarboxylation reactions, and retroaldol reactions, respectively. The enzyme will be responsible for the orientation of the amino acid-PLP complex and thus dictate the nature of the resultant reaction. An example of an enzyme catalyzing two distinct reactions was found for serine hydroxymethyltransferase (EC 2.1.2.1), which normally catalyzes the retroaldol process outlined in 3c when L-serine or L-threonine are substrates. When D-alanine was used as substrate, however, a slow transamination was observed (19). Comparison of the conformations of the amino acid-PLP complexes, 3c and 17, respectively, for the retroaldol and transamination reactions shows that both the proton removed from the... 

Subsequent to this work, apoaspartate transaminase was used to assay the stereospecificity of a variety of other transaminases, all of which were shown to involve protonation/deprotonation at the C-4 Si face of the cofactor. These enzymes included pyridoxamine-pyruvate transaminase (EC 2.6.1.30) (26) and a-dialkylamino acid transaminase (27). L-Glutamate decarboxylase (EC 4.1.1.15) catalyzes an abortive transamination reaction when oc-methylglutamate is used as substrate, and this too was shown to occur with protonation at the Si face of C-4 in the intermediate 4d (28) as was the abnormal transamination of D-alanine by serine hydroxymethyltransferase (29). 

In nonlegumes, Mo deficiency hampers NOj" reduction and decreases the amounts of most amino acids. Addition of Mo to deficient plants has been found to increase the contents of glutamic acid, glutamine, a-alanine, serine, and aspartic acid in spinach Spinacea oleracea L.), cauliflower, tomato Lycopersicon esculentum Mill.) (Mulder et al., 1959), and maize (Berducou and Mache, 1963). However, decreases in the contents of some amino acids and amides during later stages of growth of Mo-fertilized crops can result from their incorporation into proteins or from subsequent metabolic reactions such as transamination reactions or conversion to amides (Possingham, 1957). 

Recall Draw a transamination reaction between (X-ketoglutarate and alanine. 

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