Alkaloids amino acid decarboxylase

The enzyme, i.e. lysine decarboxylase, that is required for the conversion of lysine into cadaverine, and thus the first step of alkaloid biosynthesis, has been isolated from chloroplasts of L. polyphyllus,28 Like the majority of amino-acid decarboxylases, this enzyme is dependent on pyridoxal 5 phosphate. Its activity was found not to be affected by the presence or absence of quinolizidine alkaloids. Control of the enzyme by simple product feedback inhibition therefore seems unlikely. The operational parameters of this enzyme resemble those of the 17-oxosparteine synthase. Co-operation between the two enzymes would explain why cadaverine is almost undetectable in vivo. 

DDC catalyzes the conversion of L-3,4-dihydroxyphenylalanine (l-DOPA) into dopamine (Figure 10), a neurotransmitter found in the nervous system and peripheral tissues of both vertebrates and invertebrates and also in plants where it is implicated in the biosynthesis of benzylisoquinoline alkaloids. " DDC also catalyzes the decarboxylation of tryptophan, phenylalanine, and tyrosine and of 5-hydroxy-L-tryptophan to give 5-hydroxytryptamine (serotonin), and, therefore, is also referred to as aromatic amino acid decarboxylase. Inhibitors of DDC, for example, carbiDOPA and benserazide, are currently used in the treatment of Parkinson s disease to increase the amount of l-DOPA in the brain. 

In Fig. 9, the principal enzymatic steps are schematically outlined, which lead to the formation of piperidine alkaloids, most of which modulate acetylcholine receptors (Table 3 and 4). In the first step, amino acid decarboxylases, which are present in every plant, must be modified... 

Most alkaloids are derived from amino acids (Fig. 3) and the first reaction in the otherwise independent pathways is the decarboxylation of the respective amino acid by an amino acid decarboxylase (AADC) (Figs. 1 and 3) this step is often under complex regulation. Plant and animal AADCs share high amino acid identity, with significant similarities in subunit structure and kinetic characteristics. In contrast to their mammahan and insect counterparts, plant AADCs exhibit high specificity for their respective substrates. The reaction is pyridoxal-5 -phosphate (PLP)-dependent. 

Plant aromatic L-amino acid decarboxylases (AADCs) catalyze the initial reactions in the formation of terpenoid indole alkaloids (TIAs) such as quinine and strychnine, and benzyliso-quiuoline alkaloids (BIAs) such as morphine and codeine (Fig. 3). L-tryptophan decarboxylase (TDC) initiates TIA synthesis with the formation of tryptamine. TDC is encoded by two genes in Cola accuminata TDCl is expressed as part of a developmentally regulated chemical defense system, whereas TDC2 is induced after elicitation with yeast extract or methyl jas-monate (MI). 

The benzylisoquinolines are formed from two molecules of fhe aromafic amino acid, tyrosine. In the past ten years, this pathway has been probed at the enzyme and gene level. The recent linking of the phloem-specific expression of tyrosine/Dopa decarboxylase (TYDC) genes with the bios)mthesis of the isoquinoline alkaloids in the opium poppy, Papaver somniferum (Facchini and De Luca, 1994, 1995, 2008 Liscombe and Facchini, 2008), and the association with alkaloid accumulation as part of the plant defence mechanism (Wink, 1993 Facchini et al, 1996) are of particular interest in furthering our knowledge of the location of alkaloid biosynthesis. 

Alkaloid metabolism in lupine was proved by Wink and Hartmann to be associated with chloroplasts (34). A series of enzymes involved in the biosynthesis of lupine alkaloids were localized in chloroplasts isolated from leaves of Lupinus polyphylls and seedlings of L. albus by differential centrifugation. They proposed a pathway for the biosynthesis of lupanine via conversion of exogenous 17-oxosparteine to lupanine with intact chloroplasts. The biosynthetic pathway of lupinine was also studied by Wink and Hartmann (35). Two enzymes involved in the biosynthesis of alkaloids, namely, lysine decarboxylase and 17-oxosparteine synthetase, were found in the chloroplast stoma. The activities of the two enzymes were as low as one-thousandth that of diaminopimelate decarboxylase, an enzyme involved in the biosynthetic pathway from lysine to diaminopimelate. It was suggested that these differences are not caused by substrate availability (e,g., lysine concentration) as a critical factor in the synthesis of alkaloids. Feedback inhibition would play a major role in the regulation of amino acid biosynthesis but not in the control of alkaloid formation. 

Tropane alkaloid biosynthesis has been studied at the biochemical level, and several enzymes from the biosynthetic pathway have been isolated and cloned, although the pathway has not been elncidated completely at the genetic level (Fig. 3b) (138). L-arginine is converted to the nonproteogenic amino acid L-omithine by the nrease enzyme arginase. Ornithine decarboxylase then decarboxylates ornithine to yield the diamine pntrescine. In Hyoscyamus, Duboisia, and Atropa, putrescine serves as the common precnrsor for the tropane alkaloids. 

Papaver Alkaloids.—Biosynthesis of benzylisoquinoline alkaloids involves the amino-acid dopa, which is in part implicated through decarboxylation to dopamine. The presence of L-dopa decarboxylase in Papaver orientale latex has recently been demonstrated. ... 

In general, alkaloids derive from the metabohsm of amino acids such as phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), omitine (Om), or lysine (Lys). Quinolizidine alkaloids derived from L-lysine. Its decarboxylation by means of the enzyme lysine decarboxylase gives cadaverine (Cad), the first detectable intermediate of this biosynthetic pathway (Scheme 14.1). 

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