Transamination catalysts

In addition, DNA and RNA may be modified with hydrazide-containing fluorophores by a transamination reaction of their cytosine residues using bisulfite as a catalyst (Chapter 27, Section 2.1) (Draper and Gold, 1980). 

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). 

Transesterification or transamination are metal-directed reactions which are commonly encountered. We have discussed transamination processes in Section 5.5.2 and also in Chapter 3. The key step involves the attack of a co-ordinated alcohol or alkoxy group upon the carbonyl of a co-ordinated ester or amide. Many Lewis acidic metal ions have been shown to be effective catalysts for transesterification reactions for example, heating diethyl picolinate with copper(n) salts in methanol results in rapid and clean transesterification (Fig. 5-86). In the absence of the metal ion, the rate of reaction is vanishingly slow. 

Murakami et al. studied alternative pyridoxamine-surfactant systems [23]. These authors synthesized hydrophobic pyridoxamine derivatives (30 and 31) and peptide lipid molecules (32-35). Catalyst 30 or 31 and the peptide lipids formed bilayer membranes in water, which showed transamination reactivity in the presence of metal ions such as Cu(ii). It was proposed that the pyridoxamine moiety was placed in the so-called hydrogen-belt domain interposed between the polar surface region and the hydrophobic domain that is composed of double-chain segments within the bilayer assembly. The basic group (such as imidazole) in the peptide lipid molecules could catalyze the proton transfer involved in the transamination reaction. In addition, marked substrate discrimination by these bilayer membrane systems was performed through hydrophobic interactions between substrates and the catalytic site. 

Murakami et al. also found that the transamination reaction between hydrophobic pyridoxals (36 and 37) and a-amino acids, to produce a-keto acids, was extremely slow for neutral pyridoxals even in the presence of Cu(n) ions [24]. Detailed kinetic analysis of the reactions carried out in the vesicular system indicated that the transformation of the Cu(n) -quinonoid chelate into the Cu(n) -ketimine chelate was kinetically unfavorable compared with the competing formation of the Cu(n)-aldimine chelate from the same quinonoid species. This problem was solved to a certain extent by quaternization of the pyridyl nitrogen in pyridoxal, as Murakami et al. successfully accomplished transamination between catalyst 36 and L-phenylalanine to produce phenylpyruvic acid. 

Thus, to attain the racemase reactivity it is important to learn how to block the transamination process. Our idea was to place a catalytic group that can protonate only the a-carbon of the amino acid unit but not the C4 of the pyridoxal moiety in the quinonoid intermediate (Scheme 2.5). Thus we synthesized catalyst 38, which carries a rigid pyridine side chain [39]. Catalyst 39, which lacks the double bond, was also synthesized as a less rigid control. Both catalysts catalyzed loss of optical activity from the aldimine equally well - about twice as fast as simple pyridoxal. However, 39 could catalyze the transamination reaction 2.5 times faster than 38. Therefore, 38 showed a small preference for racemization over the transamination reaction as compared with 39 by a... 

Comparison with similar parameters obtained from reactions with free pyridoxamine indicated that IFABP-PX60 catalyzed transamination some 200 times more efficiently. Analysis of the specific kinetic constants kcat and KM indicated that the observed rate acceleration was due mostly to an increase in substrate binding (50-fold), with a smaller effect on the maximal rate (4-fold). While this is an impressive result, the absolute magnitude of kcat/Ku (0.02 s 1 m 1) makes it clear that this catalyst is still quite primitive compared to natural enzyme systems that occasionally operate with catalytic efficiencies near the diffusion limit. 

Figure 5.10 Abbreviated mechanism for transamination reactions promoted by pyridoxal-based catalysts.

Transamination. Transamination with pyridoxal or analogs requires a metal catalyst such as Cu(II). However, transamination can be effected with DPL in combination with hexadecyltrimethylammonium chloride. By using these two reagents phenylglycine undergoes transamination with 2-oxoglutaric acid (equation I). 

Other reactions that mimic the enzymic processes that require pyridoxal phosphate also have been realized. Werle and Koch reported the nonenzymic decarboxylation of histine (9). The racemization of alanine occurs in preference to its transamination when aqueous solutions with polyvalent cations are maintained at pH 9.5. Other amino acids are likewise racemized the order of rates is Phe, Met > Ala > Val > lieu. At lower pH, the dominant reaction is transamination, with pH maxima varying from 4.3-8 with the nature of the metal ion used as catalyst. 

Harada and Matsumoto studied the effect of solvents on the % ee of the amino acids formed in asymmetric transamination.56 Generally, the optical activity of alanine prepared from benzyl pyruvate and (S )-)-)-1-phenylethylamine decreased with increasing polarity of the solvents used. From the finding that sodium a-phenylglycinate was hydrogenolyzed easily to ammonia and phenylacetate over palladium catalyst, Harada prepared optically active alanine, butyrine, glutamic acid, and aspartic acid in 40-60% optical purities from the corresponding a-oxo... 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

The transamination reactions of Tris with typical secondary amines were extremely slow at best in the absence of catalyst. Even when catalyzed, these reactions remained sluggish and generally incomplete ... 

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