Other Aminomutases

Some other aminomutases, for example /(-lysine 5,6-aminomutase (EC 5.4.3.3), D-lysine 5,6-aminomutase (EC 5.4.3.4), and D-ornithine 4,5-aminomutase (EC 

are not involved in the shift of the amino group from C2 to C3 in amino acids. The should, nevertheless, be mentioned briefly, because / -amino acids are partially substrates for this class of enzyme. In addition, they were the first examples of the action of B12-dcpcndcnt aminomutases [66, 67]. 

Reaction Mechanism Although the /Mysine 5,6-aminomutase requires cobalamin as cofactor instead of SAM its reaction mechanism seems to be similar to that of the lysine 2,3-aminomutase [73], Experiments with tritium-labeled lysine and B12 showed that Bn is directly involved in the hydrogen shift from position 5 in d- 

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Biosynthesis of (S)-/ -Tyrosine in Bacillus brevis Vm4 //-Tyrosine 43 is a constituent of the peptide antibiotics edeine A and B [60] obtained from cultures of BaciUus brevis Vm4. //-Tyrosine is derived from a-tyrosine 42 by use of a tyrosine 2,3-aminomutase [61]. The purified enzyme has properties fundamentally different from those of all other aminomutases so far mentioned. It requires ATP and Mg2+ ions, but no other cofactors. 

Recently, a new SAM dependent lysine 2,3-aminomutase was detected and characterized in Bacillus subtilis. Unlike the enzyme from C. subterminah SB4, the enzyme in B. subtilis apparently consists of four identical subunits each with a molecular mass of 54 kDa [30]. A PLP binding motif was identified in this amino-mutase that is also highly conserved in other lysine 2,3-aminomutases [31]. 

So far, two types of aminomutase have been investigated in detail. Lysine 2,3-aminomutase from Clostridium subterminale SB4 is the example par excellence for the SAM-dependent type of aminomutase. Several other enzymes belonging to the same family are known. Examples are biotin synthase [82], pyruvate formate lyase [83, 84], and anaerobic ribonucleotide reductase [85]. 

In 1980 Poston (PI) proposed that vitamin B12 was required for the conversion of the branched-chain amino acid p-leucine to leucine. He found circulating P-leucine levels elevated in patients with vitamin B12 deficiency. The concentration of leucine on the other hand was found to be much lower. He suggested that 2,3-aminomutase, which catalyzes the interconversion of P-leucine and leucine, is a vitamin B12-dependent enzyme which is consequently reduced in patients with pernicious anemia. The enzyme has been found in the liver of several animals and in human leucocytes, and in vitro experiments have shown it to be adenosylcobalamin dependent (P2). 

Leucine 2,3-aminomutase interconverts the a and /3 forms of leucine. Similar mutases are known for other amino acids including lysine, tyrosine, and serine. The resolving power of HPLC is useful in separating and quantitating the a and /3 derivatives of these amino acids. 

Evidence for the [4Fe S] cluster as the active form of lysine aminomutase was obtained by Frey and co-workers, who showed by a combination of EPR spectroscopy and enzyme assays that the [4Fe-4S] -LAM generated in the presence of AdoMet was catalytically active. Unlike aRNR-AE, however, LAM catalyzes a reversible reductive cleavage of AdoMet, and thus methionine production and cluster oxidation could not be monitored as evidence of turnover. It is of interest to note that in the case of LAM, the presence of AdoMet facilitates reduction to the [4Fe-4S] state very little [4Fe-4S]" cluster is produced by the reduction of LAM with dithionite in the absence of AdoMet, while the presence of AdoMet or its analogue S-adenosylhomocysteine dramatically increases the quantity of [dFe-dS] " produced. It is not clear whether the presence of AdoMet affects the redox potential of the cluster or whether some other effect, such as accessibility of the cluster by the reductant, is at work. 

On the other hand, the biocatalytic enantioselective addition of hydrazine, hydroxylamine, methoxylamine, and methylamine to fiimarate has been described employing aspartate ammonia lyase [97]. Other enzyme-catalyzed procedures include the addition of ammonia to substituted cinnamic acids employing phenylalanine aminomutase [98, 99]. 

In others, amino groups are transferred from one position of the carbon chain to another. Such reactions are catalyzed by aminomutases. At least three B12 coenzyme-dependent aminomutases are known Lj- -lysine, Di-lysine, and ornithine aminomutase. Coenzyme Bi2 also catalyzed in the presence of the appropriate apoenzyme the conversion of ethylene glycol and 1,2-propanediol to acetaldehyde and propionalde-hyde, respectively. The apoenzyme involved in these reactions is a diodehydrase (see below). 

In 2010, Janssen and co-workers reported that the kinetic resolution of p-phenylalanine catalysed by a tandem biocatalytic system composed of phenylalanine aminomutase (PAM) and phenylalanine ammonia lyase (PAL) yielded the corresponding enantiopure (5)-p-phenylalanine in good yield (48%) and excellent enantiomeric excess of >99% ee (Scheme 4.13). The process was based upon the PAM-catalysed, reversible, enantioselective transformation of (I )-p-phenylalanine to (S)-a-phenylalanine. The latter one was transformed in a PAL-catalysed regioselective process into ( )-cinnamic acid, with liberation of ammonia. This constituted an example of a tandem biocatalytic, kinetic resolution in which one enzyme catalysed the equilibration between the substrate and reaction intermediate, while the other shifted this equilibrium between the substrate towards the final product... 

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