Biosynthetic Pathways

Biosynthetic Pathways:Biogenesis of plant secondary metabolites

Amandeep Singh BhatiaAsst. Prof.M.Pharm.(Pharmacognosy)

Introduction: All organisms need to transform and interconvert a vast number of organic compounds to enable them to live, grow, and reproduce. For this

They need to provide themselves with energy in the form of ATP, and a supply of building blocks to construct their own tissues. How they will do this?

An integrated network of enzyme mediated and carefully regulated chemical reactions is used for this purpose, collectively referred to as intermediary metabolism, and the pathways involved are termed metabolic pathways.

Organisms vary widely in their capacity to synthesize and transform chemicals.For instance, plants are very efficient at synthesizing organic compounds via photosynthesis from inorganic materials found in the environment, whilst other organisms such as animals and microorganisms rely on obtaining their raw materials in their diet, e.g. by consuming plants.

Despite the extremely varied characteristics of living organisms, the pathways for generally modifying and synthesizing carbohydrates, proteins, fats, and nucleic acids are found to be essentially the same in all organisms, apart from minor variations. These processes demonstrate the fundamental unity of all living matter, and are collectively described as primary metabolism, with the compounds involved in the pathways being termed primary metabolites.

In contrast to these primary metabolic pathways, which synthesize, degrade, and generally interconvert compounds commonly encountered in all organisms, there also exists an area of metabolism concerned with compounds which have a much more limited distribution in nature. Such compounds, called secondary metabolites, are found in only specific organisms, or groups of organisms, and are an expression of the individuality of species.

Building blocks; Essential Requirement of secondary metabolite

Building blocks are the starting material or precursors of secondary metabolites which are generated by the combinations of various intermediates produced during the primary metabolism. Eg. C5 isoprene unit that is used as a precursor/ building block for the biosyntheis of terpenoids are made from the mevalonate.

Processes of primary metabolism Glycolysis, Pentose Phosphate cycle (PP cycle), Photosynthesis Kreb’s Cycle

Important intermediates of Primary metabolism Acetyl Co-A Shikhimic Acid Mevalonic Acid 1-Deoxyxylulose-5-Phosphate

All these intermediates are used in secondary metabolic pathway for the synthesis of different plant constituents or secondary metabolites hence the name was given Acetate , Shikhimate , Mevalonate and Deoxyxylulose-5-Phosphate pathways.

In addition to acetyl-CoA, shikimic acid, mevalonic acid, and deoxyxylulose phosphate, other building blocks based on amino acids are frequently employed in natural product synthesis. Alkaloids and many antibiotics are derived from amino acids. Intermediates from the glycolytic pathway and the Krebs cycle are used in constructing many of them, but the aromatic amino acids phenylalanine, tyrosine, and tryptophan are themselves products from the shikimate pathway. Ornithine, a non-protein amino acid, along with its homologue lysine, are important alkaloid precursors having their origins in Krebs cycle intermediates.

Generation of building blocks for secondary metabolites from the intermediates of primary metabolism

Biosynthetic Pathways in plants

Biosynthesis of Alkaloids

Alkaloids are the Nitrogen containing compound. The nitrogen atoms in alkaloids is originated from an amino acid (Act as a precursor/building blocks generated from the acetate, shikimate, or deoxyxylulose phosphate pathways). The principal amino acid being used are ornithine, lysine, nicotinic acid, tyrosine, tryptophan, anthranilic acid, and histidine.

Alkaloids are often classified according to the nature of the nitrogen-containing structure, e.g. pyrrolidine, piperidine, quinoline, isoquinoline, indole, tropane, phenylethylamine etc.

The nature of amino acid affects the nature of nitrogen group and the fundamental portion or chemical skeletal of alkaloids .

1) Ornithine gives rise to pyrrolidine and tropane alkaloids 2) Lysine to piperidine, quinolizidine, and indolizidine alkaloids3) Nicotinic acid to pyridine alkaloids4) Tyrosine produces phenylethylamines and simple

tetrahydroisoquinoline alkaloids 5) Anthranilic acid acts as a precursor to quinazoline, quinoline and acridine

alkaloids, 6) Histidine gives imidazole derivatives.

Biosynthesis of Alkaloids from Ornithine

Ornithine gives rise to pyrrolidine and tropane alkaloids.L-Ornithine is a non-protein amino acid generated from the urea cycle in animals, whereas in plants it is formed mainly from L-glutamate.1) Formation of Ornithine from Glutamate

2) Formation of Pyrrolium ion from Ornithine.

Tropane Alkaloids (Atropine & Hyoscyamine) derived from ornithine

Biosynthesis of Alkaloids from Tyrosine

Tyrosine gives rise to Phenylethylamine & tetrahydroisoquinoline alkaloids.

Phenylethylamine alkaloids are ephedrine, epinephrine, adrenaline etc.

Tetrahydroisoquinoline alkaloids are Papaverine and Morphine.

Biosynthesis of Papaverine

Biosynthesis of Morphine