Transamination condensations reactions

The second amino group now enters from aspartate (generated in mitochondria by transamination and transported into the cytosol) by a condensation reaction between the amino group of aspartate and the ureido... 

It is not yet clear whether biotin is a cofactor no biotin was detected in highly purified preparations of the enzyme. Malate easily converts to oxaloacetate. The formation of oxaloacetate by one of these two pathways is significant, because it provides the initial spark for the citrate cycle, since oxaloacetate is needed as partner for the condensation reaction with acetyl-CoA. Furthermore, oxaloacetate mediates the resynthesis of glucose (see below). It should be pointed out, finally, that pyruvate can be transformed to alanine (Chapt. VIII-10) by transamination so that pyruvate also represents a link to the metabolism of protein. 

A key step in the synthesis of the spiroketal subunit is the convergent union of intermediates 8 and 9 through an Evans asymmetric aldol reaction (see Scheme 2). Coupling of aldehyde 9 with the boron enolate derived from imide 8 through an asymmetric aldol condensation is followed by transamination with an excess of aluminum amide reagent to afford intermediate 38 in an overall yield of 85 % (see Scheme 7). During the course of the asymmetric aldol condensation... 

Alanine also gives rise to a precursor of the vitamin biotin (Eq. 24-39) after a PFP-dependent decar-boxylative condensation with the 7-carbon dicarbox-ylic acid unit of pimeloyl-CoA in a reaction analogous to that of Eq. 14-32.351 The resulting alcohol is reduced to 7-oxo-8-aminopelargonic acid which is converted by transamination, with S-adenosylmethionine as the nitrogen donor,351a to 7,8-diaminopelargonic acid. 

NMR studies have been carried out on Schiff bases derived from pyridoxal phosphate and amino acids, since they have been proposed as intermediates in many important biological reactions such as transamination, decarboxylation, etc.90 The pK.d values of a series of Schiff bases derived from pyridoxal phosphate and a-amino adds, most of which are fluorinated (Figure 11), have been derived from H and19F titration curves.91 The imine N atom was found to be more basic and more sensitive to the electron-withdrawing effect of fluorine than the pyridine N atom. Pyridoxal and its phosphate derivative are shown in Figure 12a. The Schiff base formation by condensation of both with octopamine (Figure 12b) in water or methanol solution was studied by 13C NMR. The enolimine form is favoured in methanol, while the ketoamine form predominates in water.92... 

Pyrazines are formed from transamination reactions, in addition to carbon dioxide and formaldehyde. A requirement is that the carbonyl compound contains a dione and the amino group is alpha to the carboxyl group (16). If the hydrogen on the ct-carbon oI the amino acid is substituted, a ketone is produced. Newell (17) initially proposed a pyrazine formation mechanism between sugar and amino acid precursors. (See Figure 3). The Schiff base cation is formed by addition of the amino acid to the anomeric portion of the aldo-hexose, with subsequent losses of vater and a hydroxyl ion. Decarboxylation forms an imine which can hydrolyze to an aldehyde and a dienamine. Enolization yields a ketoamine, vhich dissociates to amino acetone and glyceraldehyde. 2,5-Dimethylpyrazine is formed by the condensation of the tvo molecules of amino acetone. 

Although the utility of transaminases has been widely examined, one such limitation is the fact that the equilibrium constant for the reaction is near unity. Therefore, a shift in this equilibrium is necessary for the reaction to be synthetically useful. A number of approaches to shift the equilibrium can be found in the literature.53 124135 Another method to shift the equilibrium is a modification of that previously described. Aspartate, when used as the amino donor, is converted into oxaloacetate (32) (Scheme 19.21). Because 32 is unstable, it decomposes to pyruvate (33) and thus favors product formation. However, because pyruvate is itself an a-keto acid, it must be removed, or it will serve as a substrate and be transaminated into alanine, which could potentially cause downstream processing problems. This is accomplished by including the alsS gene encoding for the enzyme acetolactate synthase (E.C. 4.1.3.18), which condenses two moles of pyruvate to form (S)-aceto-lactate (34). The (S)-acetolactate undergoes decarboxylation either spontaneously or by the enzyme acetolactate decarboxylase (E.C. 4.1.1.5) to the final by-product, UU-acetoin (35), which is meta-bolically inert. This process, for example, can be used for the production of both l- and d-2-aminobutyrate (36 and 37, respectively) (Scheme 19.21).8132 136 137... 

A minor pathway of valine catabolism is concerned with its conversion to leucine. Because leucine is an essential amino acid, its synthesis from valine is clearly not sufficiently significant to meet the organism s daily demand for leucine. In this reaction, isobutyryl-CoA (see Figure 20.20) is condensed with a molecule of acetyl-CoA to give /3-ketoisocaproate, which is then transaminated to give (3-leucine. A mutase is then used to convert /3-leucine to leucine. This mutase... 

The interest in the mechanisms of SchifF base hydrolysis stems largely from the fact that the formation and decomposition of SchifF base linkages play an important role in a variety of enzymatic reactions, for example, carbonyl transfers involving pyridoxal phosphate, aldol condensations, /3-decarboxylations and transaminations. The mechanisms for the formation and hydrolysis of biologically important SchifF bases, and imine intermediates, have been discussed by Bruice and Benkovic (1966) and by Jencks (1969). As the consequence of a number of studies (Jencks, 1959 Cordes and Jencks, 1962, 1963 Reeves, 1962 Koehler et al., 1964), the mechanisms for the hydrolysis of comparatively simple SchifF bases are reasonably well understood. From the results of a comprehensive kinetic investigation, the mechanisms for the hydrolysis of m- and p-substituted benzylidine-l,l-dimethylethylamines in the entire pH range (see, for example, the open circles in Fig. 13) have been discussed in terms of equations (23-26) (Cordes and Jencks, 1963) ... 

Transamination of Tris with Methylamine by Inverse Addition. A 2-L, three-neck flask was fitted with a mechanical stirrer and two Claisen adapters. One Claisen adapter was fitted with a reflux condenser and a CH3NH2-sparging tube that could be immersed in the reaction solvent. The other Claisen adapter was fitted with a pressure-equalizing dropping funnel and a thermometer that could be immersed in the reaction solvent. The reflux condenser and addition funnel were also fitted with N2 adapters. The flask was charged with heptane (500 mL) and p-toluenesulfonic acid (p-TSA) monohydrate (6.10 g, 0.032 mol) in a N2 atmosphere. The reaction mixture was heated to reflux temperature, and CH3NH2 was introduced at a continuous rate. 

Transamination of Tris with Methylamine via Standard Addition. Toluene (375 mL) and HSi[N(CH3)2]3 (369.9 g, 2.29 mol) were combined under a N2 atmosphere in an apparatus similar to that described in the previous section. The mixture was stirred and then charged with p-TSA monohydrate (9.0 g, 0.05 mol). The reaction mixture was heated to reflux temperature and then sparged with CH3NH2 continuously. The reaction mixture was maintained at reflux temperature for 4 h, at which time it was determined by GC that no HSi[N(CH3)2]3 remained. The reaction mixture was cooled to room temperature, and the toluene solvent was removed under vacuum. The remaining white resinous product was washed with 200 mL of pentane, and the washing was discarded. The sample was dried further under vacuum at 13 Pa to yield 138.2 g of a white resin that hardened at <40 °C. The product was not distillable under vacuum. The structure of the product mixture was nearly identical to that obtained in the previous experiment (on the basis of H, C, and Si NMR spectroscopic analyses) but was more highly condensed and cross-linked. 

The transformations themselves involved reactions of ketoacids with a pyridoxamine unit, either covalently attached to the polymer or reversibly bound to the hydrophobic core (29), which converted the ketoacids to amino acids, and the pyridoxamine was converted to a pyridoxal unit either covalently attached to the polymer or reversibly dissociated from the polymer. This reaction was modeled directly on the transamination process observed in natural enzymes. However, the second part of a full transamination in nature is the reaction of the pyridoxal with a different amino acid, which runs the transamination backward to form the pyridoxamine again while converting the new amino acid into its corresponding ketoacid. We found that such a process was too slow in our biomimetic system and could not compete with the rapid aldol condensation of the ketoacids with the pyridoxal. 

Pyrophosphate is rapidly hydrolyzed, and so the equivalent of four molecules of ATP are consumed in these reactions to synthesize one molecule of urea. The synthesis of fumarate by the urea cycle is important because it links the urea cycle and the citric acid cycle (Figure 23.17). Fumarate is hydrated to malate, which is in turn oxidized to oxaloacetate. Oxaloacetate has several possible fates (1) transamination to aspartate, (2) conversion into glucose by the gluconeogenic pathway, (3) condensation with acetyl CoA to form citrate, or (4) conversion into pyruvate. 

Amidinium salts can also serve as starting materials for transamination reactions, e.g. Scheme 17 7.14,314 ji g condensation of orthoamides with amidinium salts containing a-CH2 groups af fords a,p-unsaturated amidinium salts, e.g. (144 equation 86). ... 

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. 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

Acdve methylene compounds ranging in acidity from -keto esters, malonates and nitroalkanes pK = 9-13) to ketones (pATa = 16-20) can be used in the Mannich reaction. The lack of examples using simple unactivated esters (p/iTa = 25) appears to be due to their weaker acidity or to transamination and/or hydrolysis side reactions. Enolizable aldehydes have also been used in certain instances however, side products arising from subsequent aldol condensation of the resulting -amino aldehyde often occur. Best results are achieved with a-branched aldehydes, which produce Mannich bases without enolizable protons. 

A number of reactions involving pyridoxal, an amino acid, and a metal ion have been attributed to the formation of a Schiff base chelate in which a hydrogen atom on the a-carbon of the amino acid is labilized, and the Schiff base can now undergo a variety of reactions resulting in a transamination, racemization, aldol condensation, a,/3-elimination, or decarboxylation reactions (188). 

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