Amines enzyme-catalyzed reactions

As mentioned in the entry on Substrate Purity, ammonia is often a contaminant in many laboratories. It may be necessary to check for ammonia levels on a regular basis as ammonia can act as an inhibitor for many enzyme-catalyzed reactions acting on amine-containing substrates. See Substrate Purity... 

D. M. Roundhill, Chem. Rev. 92, 1-27 (1992) . .Transition Metal and Enzyme Catalyzed Reactions involving Reactions with Ammonia and Amines". 

A number of nucleophilic molecules can lose electrons in enzyme-catalyzed reactions (e.g., peroxidase). Thus hydrazines, amines, hydroquinones, phenothiazines, thiols, and aminophenols can form free radicals by this mechanism. 

The phosphate ester of the aldehyde form of vitamin B6, pyridoxal phosphate (pyridoxal-P or PLP), is required by many enzymes catalyzing reactions of amino acids and amines. The reactions are numerous, and pyridoxal phosphate is surely one of nature s most versatile catalysts. The story begins with biochemical transamination, a process of central importance in nitrogen metabolism. In 1937, Alexander Braunstein and Maria Kritzmann, in Moscow, described the transamination reaction by which amino groups can be transferred from one carbon skeleton to another.139 140 For example, the amino group of glutamate can be transferred to the carbon skeleton of oxaloacetate to form aspartate and 2-oxoglutarate (Eq. 14-24). 

The KIEs for scission of the C—C bond to form the enamine (by measuring 45C02/44C02 ratios) of both the model amine and the acetoacetate decarboxylase catalyzed reactions were studied by O Leary and Baughn125. In the primary amine catalyzed reaction the KIE exhibited pH-dependence consistent with the kinetic data above. In the enzyme catalyzed reaction a pH independent k12/k13 KIE of 1.018 was reported, consistent with the idea that a nucleophilic attack by the lysine amino group and decarboxylation are both partially rate-limiting. 

The electrochemical dealkylation of aliphatic amines is a useful way of mimicking enzymatic dealkylation. This has been effectively used for the synthesis of Af-dealkylated metabolites of drugs with much better efficiencies than the enzyme catalyzed reactions (Scheme 38) [2]. 

The body s goal of these enzyme-catalyzed reactions is not to cause mutations or cancer, but only to modify the heterocyclic amines to forms that are more water soluble and more readily excreted in the urine. Most of the heterocyclic amines that bear acetyl groups or sulfate groups are promptly excreted. However, a minority react with small molecules and macromoleculcs in the cell. 

Sulfation is another common form of conjugation predominately found with phenolic compounds however, sulfate esters can also be formed with alcohols, aryl-amines, and N-hydroxy compounds. Sulfation involves the transfer of S03 from 3 -phosphoadenosine-5 -phosphosulfate (PAPS) to one of the above-mentioned functional groups by an enzyme-catalyzed reaction involving the cytosolic enzymes (sulfotransferases), as illustrated in Fig. 24 [11]. It is common to find phenolic compounds that are metabolized by both sulfation and glucuronidation, as they are often competing pathways. However, sulfation has a significant limitation... 

The predictive importance of the Hammett relationship is impressive. It applies not only to hydrolysis reactions but also to substitution and oxidation reactions of aromatic compounds and even to enzyme-catalyzed reactions like the oxidation of phenols and aromatic amines by peroxidase (Job and Dunford, 1976). According to Exner (1972), data available in 1953 allowed prediction of 42,000 rate or equilibrium constants, of which only 3180 had been measured at the time. 

A number of methyltransferases are able tc methylate small molecules (46,47). Thus, re. actions of methylation fulfill only two of the three criteria defined above, because the methyl group is small compared with the substrate. The main enzyme responsible for O-methylation is catechol 0-methyltransferas (EC 2.1.1.6 COMT), which is mainly cytosolic but also exists in membrane-bound form. Several enzymes catalyze reactions of xenobiotic N-methylation with different substrate specificities, e.g., nicotinamide iV-methyltrans-ferase (EC 2.1.1.1), histamine methyltrans-ferase (EC 2.1.1.8), phenylethanolamine N-methy 1 transferase (noradrenal ine A-meth-yltransferase EC 2.1.1.28), and nonspecific amine N-methyltransferase (arylamine N-methyltransferase, tryptamine JV-methyl-transferase EC 2.1.1.49) of which some isozymes have been characterized. Reactions of xenobiotic S-methylation are mediated by the membrane-bound thiol methyltransferase (EC 2.1.1.9) and the cytosolic thiopurine methyltransferase (EC 2.1.1.67) (3). 

In the previous Section 12.1 the formation of amides from the corresponding nitriles is well addressed. The latter method, the enzyme catalyzed reaction of carboxylic esters or acids with ammonia or amines yielding amides, has only recently been studied in depth. Encouraging results have been described, especially in the field of amidation of esters, a technology now being used by BASF to produce optically pure amines (vide infra). 

Carbon isotope effects for both the amine-catalyzed reaction and the enzyme-catalyzed reaction are significantly different from unity but smaller than those expected if decarboxylation is fully rate determining (99). Thus, in both cases Schiff base formation and decarboxylation must be jointly rate limiting and the enzyme must have accelerated both steps by similar amounts. 

In dehydrogenase-type enzyme-catalyzed reactions, the NAD" " as a cofactor is required for the enzymatic reaction. With substrate oxidation, NAD" " is simultaneously reduced to NADH. NADH contains a tertiary amine and can be acted as the co-reactant of Ru(bpy)3 (Fig. 16A). The ECL intensity is increased in proportion to the concentration of the substrates. However, for the NADH-depleting enzyme, such as the determination of pyruvate using malate dehydrogenase, the determinations... 

The nonlinear optical (NLO) susceptibilities of bioengineered aromatic polymers synthesized by enzyme-catalyzed reactions are given in Tables 2, 3, and 4. Homopolymers and copolymers are synthesized by enzyme-catalyzed reactions from aromatic monomers such as phenols and aromatic amines and their alkyl-substituted derivatives. The third-order nonlinear optical measurements are carried out in solutions at a concentration of 1 mg/mL of the solvent. Unless otherwise indicated, most of the polymers are solubilized in a solvent mixture of dimethyl formamide and methanol (DMF-MeOH) or dimethyl sulfoxide and methanol (DMSO-MeOH), both in a 4 1 ratio. These solvent mixtures are selected on the basis of their optical properties at 532 nm (where all the NLO measurements reported here are carried out), such as low noise and optical absorption, and solubility of the bioengineered polymers in the solvent system selected. To reduce light scattering, the polymer solutions are filtered to remove undissolved materials, the polymer concentrations are corrected for the final x calculations, and x values are extrapolated to the pure sample based on the concentrations of NLO materials in the solvent used. Other details of the experimental setup and calculations used to determine third-order nonlinear susceptibilities were given earlier and described in earlier publications [5,6,9,17-19]. 

Nonlinear optical properties of homopolymers and copolymers synthesized by enzyme-catalyzed reactions in monophasic media are given in Table 2. Aromatic monomers used for the polymerizations are aniline, aniline derivatives, and phenol derivatives. The table also gives x values of a number of monomers (used in the polymerization reactions) and solvent mixtures used to prepare polymer solutions for the measurements. In general, the third-order susceptibilities of all monomers and solvents tested are very low and are in the neighborhood of 10" esu. The values of the aromatic polymers obtained are three to five orders higher than the values of the monomers. Third-order nonlinear susceptibilities of homopolymers synthesized (by enzyme-catalyzed reactions) from aromatic amines such as aniline, benzidine, ethylaniline, propylaniline, butylaniline, and dimethy-... 

Roundhill DM (1992) Transition metal and enzyme catalyzed reactions involving reactions with ammonia and amines. Chem Rev 92(1) 1-27... 

The enzymatic method described above has two disadvantages (1) trapping of CO2 is a cumbersome procedure, and (2) the use of a radioactive substrate requires special precautions for use and disposal of reagents. Measurement of the primary amine formed by decarboxylation of the amino acid can also be exploited to monitor the PLP-dependent, enzyme-catalyzed reaction. This principle has been applied by Allenmark et al. (106), who used L-3,4-dihydroxyphenyl-alanine (L-DOPA) as substrate for tyrosine decarboxylase the dopamine produced by the decarboxylation reaction was determined by HPLC followed by amperometric detection. Both Hamfelt (107) and Lequeu et al. (108) utilized apo-tyrosine decarboxylase with tyrosine as substrate. The tyramine produced by the decarboxylation reaction was separated from the substrate (tyrosine) by HPLC and quantitated by either amperometric (108) or fluorometric (107) detection. The procedures discussed above are still subject to the main disadvantage of enzymatic methods possible interference by other materials present in the PLP containing extract which could either inhibit reconstitution of the holoenzyme or alter the reaction rate of enzyme catalysis. Moreover, HPLC with amperometric detection can hardly be described as less cumbersome than CO2 trapping difficulties in baseline-stabilization encountered with these detectors are well known. 

Amines, compounds in which one or more of the hydrogens of ammonia (NH3) have been replaced by an alkyl group, are among some of the most abundant compounds in the biological world. We will come to appreciate their biological importance as we explore the stmctures and properties of amino acids and proteins in Chapter 22 as we study how enzymes catalyze reactions in Chapter 23 as we investigate the ways in which coenzymes—compounds derived from vitamins—help enzymes catalyze reactions in Chapter 24 and as we learn about nucleic acids (DNA and RNA) in Chapter 26. 

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