Chapter 5. Plant Secondary Metabolites (Natural Products)

§1. Introduction 1 .Concepts: Plants produce a vast and diverse assortment of organic compounds,the great majority of which do not appear to participate directly in growth and development. These substances are traditionally referred to as secondary metabolites. The primary metabolites, in contrast, such as phytosterols, acyl lipids, nucleotides, amino acids, and organic acids, are found in all plants and perform metabolic roles that are essential and usually evident.

2. Properties of Secondary Metabolites Complexity of their chemical structures and biosynthetic pathways Insignificant physiological functions Natural products have primary ecological functions. Species-related distribution Mainly distributed in vacuoles and cell walls The boundary between primary and secondary metabolism is blurred. Functional definition: primary products participating in nutrition and essential metabolic processes inside the plant, and natural (secondary) products influence ecological interactions between the plant and its environment.

The boundary between primary and secondary metabolism is blurred

3. Functions of secondary products Phytoalexins(植保素) and their role in plant pathogen resistance. Some are involved in the symbiosis between host and microbes Salicylic acid(水杨酸) and Jasmonate acid (茉莉酸) are involved in signal transmission in system acquired resistance under pathogen stress Some are involved in allelopathy response Many are functional ingredients in Chinese herbs and natural seasonings.

§2 terpenoids (类萜, terpene萜烯) I. Synthesis of terpernoids All terpernoids are derived from 5-C units based on isopentane skeleton。 　 　 CH3 Head CH2＝C－CH＝CH2 Tail

1. Classification monoterpene(单萜)： 2 isoprene sesquiterpene(倍半萜)：3 diterpene (双萜)： 4 triterpene(三萜)： 6 tertraterpene(四萜)： 8 ………… multiterpene (多萜)：>20

2.Synthesis of isoprene units isoprene units is synthesized from acetyl CoA， 3 acetyl CoA→ → → → 1 IPP or DMPP IPP and DMPP are acitive 5-C units。

Synthesis of IPP

Synthesis of terpenoids from IPP

二. Common terpernoids resin：10~30 C rubber：1500~15000 5-C Hormones and vitamin： carotenoids——tertraterpene GB——diterpene ABA——sesquiterpene

§2 phenolic compound(酚类) A group of chemical compounds containing phenolic group 1. Synthesis of phenolic compound A: Shikimic acid(莽草酸)pathway - more important in plants B: malonic acid(丙二酸)pathway - more important in fungi

A: Shikimic Acid Pathway Converts intermediates from glycolysis (phosphoenol-pyruvate) and the PPP (erythrose-4-PO4) into the aromatic amino acids: phenylalanine, tryptophan, and tyrosine

PEP + E-4-P 庚酮糖-7-P 脱氢奎尼酸 没食子酸 脱氢莽草酸 苯丙氨酸 5-P-莽草酸 酪氨酸 预苯酸 分支酸 L-Arogenic acid 酪氨酸 预苯酸 分支酸 3-烯醇式丙酮基-莽草酸-5Pi 色氨酸 Shikimic acid pathway：Synthesis of aromatic amino acids -Phe,Tyr,Trp from simple carbohydrates derived from Pentose phosphate pathway

Aromatic AA for synthesizing phenolic compounds

B: Malonic acid pathway Acyl CoA 3× malonyl CoA 多酮酸 间苯三酚衍生物（黄酮衍生物） Malonic acid pathway

2 Some common phenolic compounds 1)　simple phenolic compound lignin (木质素) Flavinoids(类黄酮) Tannin（丹宁）

1）Simple Phenolics Allelopathic compounds Defense cmpds against insect herbivores and fungi Certain phenolics (furanocoumarins) are phototoxic - not toxic till activated by sunlight (near UV-A) Activated furanocoumarins can insert into DNA and bind to pyrimidine bases(嘧啶碱基) - blocks transcription and leads to cell death

Coumarins（香豆素） Large group – 1300 coumarins have been identified Basic coumarin is a 2 ring structure, phenol and pyrone Many anticoagulants including Warfarin

2）Lignin a complex phenolic Lignin monomers Polymer of three phenolic alochols Different species have different ratios of these monomers Lignin in beech has 100:70:7 of coniferyl(松柏醇): sinapyl（芥子醇）: para-coumaryl（香豆醇）

3)Flavinoids （类黄酮） One of the largest classes of phenolics Carbon skeleton has 15 carbons with two benzene rings connected by a 3-C bridge -C3-

Flavinoids Flavinoids classified into different groups based on the degree of oxidation of the 3-C bridge Anthocyanins（花青素） Flavones（黄酮） Flavonols（黄酮醇） Isoflavones（异黄酮） Majority of flavinoids exist as glycosides

Anthocyanins（花青素）

Tannins(丹宁) Tannin a general name for a large group of complex phenolic substances that are capable of tanning animal hides into leather by binding collagen

§3 Nitrogen-containing compounds Three categories: 1. Alkaloids(生物碱) 2. cyanogenetic glycoside （生氰糖苷） 3. Non-protein amino acids(非蛋白氨基酸)

1. Alkaloids

2 Non-protein amino acids Structure same to normal amino acids, toxic to insects and animals for false insert into proteins。 3　 cyanogenetic glycoside Composed of cyanhydrin(氰醇) and glycoside, decomposed into glycoside and cyanhydrin under the catalysis of glycosidase(糖苷酶) , then cyanhydrin turn out into HCN which is toxic to insects and herbivorous animals. In normal plants , cyanogenetic glycoside and glycodidase is isolated in different organelles.