- 作者： Eric E. Conn
- 作者服務機構： 美國加州大學戴維斯分校生物化學及生物物理學系
- 中文摘要：
生氰糖?（cyanogenic glycosides）係由一些蛋白質氨基酸（proteinaceous amino acid）而來，其合成途徑有一部份與芥子
油糖?（mustard oil glycoside）和脂肪硝基化合物（aliphatic nitro compounds）的生合成途徑相同。同樣地，在植物數百種的
苯基類丙烷化合物（phenylpropanoid compounds）的生合成上，前面一些反應相同，則後來再經由不同的分枝途徑產生許
多的酚酸（phenolic acids），類黃酮（flavonoids），與異類黃酮（isoflavonoids）。在生氰糖?生合成過程中，以代謝管道（meta-
bolic channel）將不安定的代謝中間產物逐一送往次一反應的酵素上，可以避免發生無用的副作用。而在一些生物鹼(alkal-
oid)的生合成上已經證實有代謝管道，許多天然物生合成也可能有這一個機制。糖?隔離在缺少糖?酵素（glucosidase）的組
織內或細胞某個部份，可以解釋在完整組織中糖?的穩定性，而當細胞破裂時會發生水解的現象。這些天然產物可能和有
機酸，如丙二酸（malonic），奎寧酸（guinic），酚酸（phenolic）或無機酸，如硫酸，磷酸等形成酯鍵結合。植物體可以產生這
種鍵結的方式，排除由生理代謝所產生的有毒化合物（如氰酸），為一種保護的代謝機制。
由於包括生氰糖?在內的天然物是由蛋白質氨基酸衍生而來的，就出現了這些氨基酸供應的調控問題。其中尤以黃樟
酸（shikimic acid）代謝途徑的調控特別引起興趣，因為其中苯基丙氨酸（phenylalanine），酪氨酸（tyrosine）和色氨酸（trypto-
phan）為數千種植物天然產物的先驅物。將就此討論植物苯基丙氨酸和酪氨酸的生合成及調控上的最新進展。
y - 英文摘要： Studies on cyanogenesis in plants are directly applicable to other natural products, especially thosethat occur as glycosides. Arising as they do from certain protemaceous amino acids, cyanogenic glycosidesshare a biosynthetic pathway common, in part, with the mustard oil glycosides (glucosinolates) and aliphaticnitro compounds. Similarly, hundreds of phenylpropanoid compounds share several reactions in commonbefore branched pathways account for the diversity of phenolic acids, flavonoids, isoflavonoids found inplants. Metabolic channeling during biosynthesis of cyanogenic glycosides provides a mechanism wherebylabile/reactive intermediates are channeled along the pathway rather than undergo wasteful side reactions.Again, metabolic channeling has been demonstrated during the biosynthesis of certain alkaloids and isindicated for many natural product biosyntheses. Physical compartmentation of glycosides in tissues orsub-cellular compartments lacking the β-glucosidase explains the stability of glycosides in intact tissue andtheir hydrolysis upon cellular disruption. Natural products may also occurs as conjugates (esters) of organic(e.g. malonic, quinic, phenolic) and inorganic (e.g. sulfuric, phosphoric) acids. In addition, cyanogenicplants posses a protective metabolic mechanism for disposing of toxic compounds (e.g. HCN) if such areproduced in vivo. Since many natural products including cyanogenic glycosides are derived from proteinaceous aminoacids, there arises the question of how the supply of these precursor amino acids is regulated. Especiallyof interest is the regulation of the shikimic acid pathway leading to phenylalanine, tyrosine and tryptophan,since these amino acids (e.g. phenylalanine) are precursors of thousands of plant natural products. Recentdiscoveries on the biosynthesis of phenylalanine and tyrosine in plants and its regulation is discussed inthis context.
- 中文關鍵字： cyanogenic glycosides; biosynthesis; compartmentation; chemical conjugation; β-cyanoalanine; β-glucosidase; arogenate; phenylalanine; tyrosine;shikimic acid.
- 英文關鍵字： --
