Let us make an in-depth study of the plant phenolics. After reading this article you will learn about 1. Biological Functions of Plant Phenolics 2. Classification of Plant Phenolics and 3. Biosynthesis of Plant Phenolics.
Thousands of organic compounds which contain one or more phenolic residue i.e., a functional hydroxyl group on a benzene ring are produced by plants and are called as phenolic compounds or plant phenolics. They include such well known substances as coumarins, lignins, flavonoids and tannins etc.
Phenolic compounds are extremely diverse in chemical structure. Some of them are soluble in organic solvents. Some occur as carboxylic acids and glycosides and are soluble in water while others may be large complex polymers that are insoluble.
Biological Functions of Plant Phenolics:
Large number of phenolic compounds occur in plants as secondary metabolites which perform the following functions:
(i) Some of them act as chemical deterrents against herbivores and pathogens.
(ii) Plant phenolics such as lignins provide mechanical strength to the plants and have significant proective functions in them.
(iii) Some phenolics play important role in plants in attracting pollinators and fruits & seeds dispersers.
(iv) Some plant phenolics play important role in allelopathy (Greek, allelon = of one another; pathos = diseases). Allelopathy is the influence of chemicals released by one plant species on another plant or animal with resulting benefits to the species which contains them.
Classification of Plant Phenolics:
Plant phenolics may be classified into major categories on the basis of the no. of C-atoms and basic arrangement of carbon skeletons in their structure as shown in Table 24.2.
The phenolics containing 6-10 C-atoms and basic carbon skeletons C6, C6 – C1, C6 – C2, C6 – C3 and C6 – C4 as mentioned in the above table are sometimes grouped together as simple phenolic compounds or simple phenolics. Elaborations of basic carbon skeleton produce large array of phenolic compounds.
Biosynthesis of Plant Phenolics:
Except flavonoids, all other plant phenolics are biosynthesized in plants from a common biosynthetic intermediate, phenylalanine or its close precursor shikimic acid through shikimic acid pathway (Fig. 24.14). The starting metabolites in this pathway are erythose-4-phosphate and phosphoenol pyruvate which are intermediates of pentose phosphate pathway and glycolysis respectively.
In case of flavonoids which have C6 – C3 – C6 carbon sketon, one aromatic ring and its side chain arises from phenyl alanine while the other aromatic ring arises from acetyl-CoA via malonic acid pathway (Fig. 24.15). The first step in the synthesis of phenolic compounds from phenylalaine in plants is deamination of phenylalanine by the enzyme phenylalanine ammonia lyase (PAL) Fig. 24.15. The latter is most extensively studied enzyme of secondary metabolisim in plants.
Malonyl-CoA can be produced from acetyl-CoA in the presence of the enzyme acetyl-CoA carboxylase as follows: