Ammonia acid-catalyzed reversion

The chlorito complex [Co(OC10)(NH3)s](N03)2 has been prepared by CIO2 oxidation of Co in the presence of ammonia (Table 70) it is stable for considerable periods when protected from light. The reverse reaction is acid catalyzed. Attempts to prepare a chlorato complex by oxidation were unsuccessful. Reduction using Fe, HSOi", or results in substantial retention of the... 

Basic hydrolysis occurs by nucleophilic addition of OH to the amide carbonyl group, followed by elimination of amide ion ( NH2) and subsequent deprotonation of the initially formed carboxylic acid by ammonia. The steps are reversible, with the equilibrium shifted toward product by the final deprotonation of the carboxylic acid. Basic hydrolysis is substantially more difficult than the analogous acid-catalyzed reaction because amide ion is a very poor leaving group, making the elimination step difficult. 

Applications of ammonia lyases Ammonia lyases catalyze the reversible, regio- and stereoselective addition of ammonia to alkenes. The family of lyases includes aspartate, methyl aspartate, histidine, tyrosine and phenylalanine ammonia lyases. The system consisting of phenylalanine ammonia lyase (PAL, EC 4.3.5.1), cinnamic acid, water and ammonium ions was optimized in terms of pH, temperature, reaction time, concentration and buffer for the synthesis of (2S,3S)- and (25,37 )-[3- H]phenylalanines. 

Many enzymes have absolute specificity for a substrate and will not attack the molecules with common structural features. The enzyme aspartase, found in many plants and bacteria, is such an enzyme [57], It catalyzes the formation of L-aspartate by reversible addition of ammonia to the double bond of fumaric acid. Aspartase, however, does not take part in the addition of ammonia to any other unsaturated acid requiring specific optical and geometrical characteristics. At the other end of the spectrum are enzymes which do not have specificity for a given substrate and act on many molecules with similar structural characteristics. A good example is the enzyme chymotrypsin, which catalyzes hydrolysis of many different peptides or polypeptides as well as amides and esters. 

Lyases are an attractive group of enzymes from a commercial perspective, as demonstrated by then-use in many industrial processes.240 They catalyze the cleavage of C-C, C-N, C-O, and other bonds by means other than hydrolysis, often forming double bonds. For example, two well-studied ammonia lyases, aspartate ammonia lyase (aspartase) (E.C. 4.3.1.1) and phenylalanine ammonia lyase (PAL) (E.C. 4.3.1.5), catalyze the trans-elimination of ammonia from the amino acids, l-aspartate and L-phenylalanine, respectively. Most commonly used in the synthetic mode, the reverse reaction has been used to prepare the L-amino acids at the ton scale (Schemes 19.30 and 19.31).240 242 These reactions are conducted at very high substrate concentrations such that the equilibrium is shifted, resulting in very high conversion to the amino acid products. 

Transamination, the process whereby ammonia is reversibly transferred between amino acids and 2-oxoacids, is catalyzed by aminotransferases, which bind pyridoxal phosphate as a prosthetic group. Pyridoxal phosphate and pyridoxamine phosphate are the coenzyme forms of vitamin B6 (Fig. 15-1). 

Enzymes catalyze the reversible enantioselective addition of ammonia to a./ -unsaturated carboxylic acids to give L-a-amino acids. Immobilized enzymes and whole-cell bioreactor technology competes well with chemical methods31,32. For practical purposes, immobilized whole cells are preferred over immobilized enzyme because of the added cost of enzyme isolation. 

L-Phenylalanine ammonia lyase (PAL EC 4.3.1.5), an enzyme found in a variety of plants, catalyzes both the deamination of L-phenylalanine to (L )-3-phenyl-2-butenoic acid and the reverse reaction33-36. 

Methylaspartase also catalyzes the reversible addition of ammonia to ( )-2-butenedioic, and (Z)-2-chloro- and (Z)-2-bromo-2-butenedioic acid (ammonium salts) 39 40a b. During the bro-mo-2-butenedioate conversion the enzyme became inactive and the aminated product rapidly cyclized to give the aziridine. 

Various commercial routes for the production of L-phenyalanine have been developed because of the utilization of this amino acid in the dipeptide sweetener Aspartame. One route that has been actively pursued is the synthesis of L-phenylalanine from trans-cinnamic acid using the enzyme phenylalanine ammonia lyase (105,106). This enzyme catalyzes the reversible, nonoxidative deamination of L-phenylalanine and can be isolated from various plant and microbial sources (107,108). 

Glutamate dehydrogenase A mitochondrial enzyme present in all tissues that metabolizes amino acids. It catalyzes the oxidative deamination of glutamate to a-ketoglutarate using NAD+ as the electron acceptor to also produce nicotinamide adenine dinucleotide (NADH) and ammonia. The enzyme uses the reducing equivalents of nicotinamide adenine dinucleotide phosphate (NADPH) to perform the reverse reaction. 

The amino acid specificity of glutamate dehydrogenase is known to be broad (e.g.. References 26,27). Since the reaction is reversible, the a-keto acid specificity should also be correspondingly broad. Indeed, Table I shows that glutamate dehydrogenase can catalyze the reductive amination of at least 10 a-keto acids. It should be noted that kinetic data were determined in die presence of a low ammonia concentration (5mM) in order to obtain realistic parameters for the design of optimal... 

The (reversible) transformation of an a-ketocarboxyhc acid in presence of ammonia and one equivalent of NAD(P)H furnishes the corresponding a-amino acid and is catalyzed by amino acid dehydrogenases [EC 1.4.1.X] [962]. Despite major differences in its mechanism, this reaction bears a strong resemblance to carbonyl group reduction and it formally respresents a reductive amination (Scheme 2.133). As deduced for L-Leu-dehydrogenase [963], the a-ketoacid substrate is positioned in the active site between two Lys-residues. Nucleophihc attack by NH3 leads to a hemiaminal intermediate, which eliminates H2O to form an iminium species. The latter is reduced by a hydride from nicotinamide forming the L-amino acid. Since this mechanism is highly tuned for a-keto/a-amino acids, it is clear that a neutral Schiff base cannot be accepted as substrate. 

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