Methylaspartate, formation

PAL/TAL both belong to the L-amino acid ammonia lyase family, which catalyzes the formation of various a,/3-unsaturated acids by elimination of ammonia (ammonium ion) from the corresponding L-ct-amino acids. This family of proteins includes aspartate ammonia lyase (AAL), methylaspartate ammonia lyase (MAL), HAL,... 

While the 2-oxobutyrate needed for isoleucine formation is shown as originating from threonine in Eig. 24-17, bacteria can often make it in other ways, e.g., from glutamate via p-methylaspartate (Eig. 24-8) and transamination to the corresponding 2-oxoacid. It can also be made from pyruvate by chain elongation using acetyl-CoA (Eig. 17-18) citramalate and mesa-conate (Eig. 24-8) are intermediates. This latter pathway is used by some methanogens as are other alternative routes. The first step unique to the biosynthetic pathway to leucine is the reaction of the... 

Fig. 18 Glutamate mutase (GM) interconverts (S)-glutamate and (2S,3S)-3-methylaspartate. Proposed reaction mechanism of the carhon skeleton rearrangement, catalyzed by GM involving H-atom abstraction (step a), radical rearrangement (step b) and back transfer of H-atom (step c). (The experimentally supported substrate triggered formation of the 5 -deoxy-5 -adenos)d radical and of cob(ll)alamin (23, Bi2r) by homolysis of protein bound AdoCbl (2) is omitted here, see Fig. 16 [173,175,176])...

So, the reversible formation of alkenes currently appears limited to a handful of ammonia lyases that include the already noted aspartate ammonia-lyase that produces fumarate (Equation 12.2), methylaspartate ammonia-lyase that yields mesaconate (EC 4.3.1.2) (Equation 12.3), the urocanate-producing histidine... 

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