| Popis: |
Naturally occurring heterocyclic β -substituted alanines such as l -quisqualic acid and β -(pyrazol-1-yl)- l -alanine are enzymatically synthesized from O -acetyl- l -serine (OAS) and suitable precursors by cysteine synthases (CSase, O -acetylserine(thiol)lyase [EC 4.2.99.8]). By contrast, l -willardiine or the isomeric l -isowillardiine are synthesized by uracilylalanine synthases, in the same manner as the formation of l -cysteine as a primary metabolite. CSase isoenzymes also catalyse the formation of the neurotoxic amino acid gb -cyano- l -alanine from OAS and cyanide as an additional activity. This activity is different from the biosynthesis of β -cyano- l -alanine with l -cysteine and cyanide as a substrate, catalysed by β -cyanoalanine synthase (BCASase, [EC 4.4.1.9]) isolated from some higher plants. The properties of the purified CSase isoenzymes suggest that all CSases in higher plants are multifunctional enzymes involved in the biosynthesis of some β -substituted alanines, and that these enzymes may have different substrate specificities. Several properties, including the amino acid compositions and the immunological characterization of the purified CSases, are given. Furthermore the physiological role of these enzymes in higher plants is described. The biosynthesis in vitro of γ -substituted α -aminobutyric acids, which are the higher homologues of heterocyclic β -substituted alanines with one more carbon in the side chain, and the biomimetic synthesis of some heterocyclic β -substituted alanines catalysed by pyridoxal 5′-phosphate and metal ions, are also described. |
