Racemases aspartate racemase

To prevent the formation of byproducts like L-malic add and D-alanine, die cells undergo a pH-treatment to inactive fumarase and alanine racemase. Several reactor conformations have been investigated, but a two reactor system was found to be the most effective. The flow sheet of this two reactor system is given in Figure A8.15. In the first reactor L-aspartic add is formed, which reacts in die second reactor to L-alanine. 

Nimura N, Fujiwara T, Watanabe A, Sekine M, Furuchi T, et al. 2003. A novel chiral thiol reagent for automated precolumn derivatization and high-performance liquid chromatographic enantioseparation of amino acids and its application to the aspartate racemase assay. Anal Bio-chem 315 262-269. 

After formation of the aldimine, numerous factors in the enzyme facilitate deprotonation of the a-carbon (Fig. 3, Step II). The lysine liberated by transimi-nation is utilized as a general base and is properly oriented for effective deprotonation [11]. Furthermore, the inductive effects of the ring system are tuned to increase the stabilization of the quinoid intermediate. For example, the aspartate group that interacts with the pyridyl nitrogen of the co enzyme promotes proto-nation to allow the ring to act as a more effective electron sink. In contrast, in alanine racemase, a less basic arginine residue in place of the aspartic acid is believed to favor racemization over transamination [12]. 

ASPARTYLCLUCOSAMINIDASE ASPARTATE AMINOTRANSFERASE ASPARTATE AMMONIA-LYASE ASPARTATE CARBAMOYLTRANSFERASE ASPARTATE a-DECARBOXYLASE ASPARTATE /3-DECARBOXYLASE ASPARTATE KINASE d-ASPARTATE OXIDASE ASPARTATE RACEMASE... 

Panizzutti, R., Miranda, J. de, Ribeiro, C. S., Engelender, S., Wolosker, H. A new strategy to decrease N-methyl-D-aspartate (NMDA) receptor coactivation inhibition of D-serine synthesis by converting serine racemase into an eliminase, Proc. Natl. Acad. Sci. USA 2001, 98, 5294-5299. 

A number of other racemases and epimerases may function by similar mechanisms. While some amino acid racemases depend upon pyridoxal phosphate (Chapter 14), several others function without this coenzyme. These include racemases for aspartate,113 glutamate,114-1153 proline, phenylalanine,116 and diamino-pimelate epimerase.117 Some spiders are able to interconvert d and l forms of amino acid residues in intact polypeptide chains.118119... 

The carboxyl group of an amino acid can also activate the a-hydrogen. This may be the basis for an aspartate racemase and other racemases that are not dependent upon PLP.156-158 See also Chapter 13, Section B,4. 

Aspartate a-decarboxylase 753, 755 Aspartate p-decarboxylase 746 Aspartate racemase 741 Aspartic acid (Asp, D) 52, 53s biosynthesis 517 pXa value of 293, 487 Aspartic proteases 621-625 Aspartyl aminopeptidase 621 p-Aspartyl phosphate 539, 540s Assays of enzyme activity 456 Assembly core of virus shell 365 Assembly pathway... 

Figure 12.11 Superposition of active sites from the different proteins. The active site of SmeHyuA model (gray) is compared with that of (a) aspartate racemase from Pyrococcus horikoshii (IF)LA, black) and that of (b) glutamate racemase from Aquifex pyrophilus (1B73A, black).

The procedure reported in Scheme 13.11 describes deracemization of an amino acid involving oxidation with an L-specific enzyme and transamination with a D-amino transferase using D-aspartate 10, which is generated from L-aspartate 11 by aspartate racemase, as the amino donor. The oxidative enzyme is defined as an L-amino acid deaminase, a flavoprotein from Proteus myxofadens [34]. The transamination reaction is shifted towards the product since the oxalacetate 12 formed decarboxylates spontaneously to give pyruvate and carbon dioxide. 

Amino acid racemases have long been known to be important in bacterial metabolism, because several u-amino acids are required for the synthesis of cell wall mucopolysaccharides. u-Serine is found in relatively large amounts in mammalian brain, where it acts as an agonist of the N-methyl-n-aspartate (NMDA) glutamate receptor. Serine racemase has been purified from rat brain and cloned fromhuman brain (Wolosker et al., 1999 De Miranda et al., 2000). 

Snyder, S.H. Wolosker, H. Blackshaw, S. Serine racemase a glial enzyme synthesizing D-serine to regulate glutamate-V-methyl-D-aspartate neuro-transmission. Proc. Natl. Acad. Sci. USA 1999, 96, 13,409-13,414. 

D-Amino acid transaminases have been less well characterized but proceed by a similar catalytic mechanism and show similar potential as effective biocatalysts. Further strain development has incorporated amino acid racemases, enabling complete utilization of racemic amino donors. Thus, L-aspartic acid can be used as the n-amino acid donor through use of aspartate racemase within the system. 

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