BP504T Unit-I: Metabolic Pathways in Higher Plants: Shikimic Acid, Acetate & Amino Acid Pathways | B.Pharm Notes

B.PHARM 5TH SEMESTER

Pharmacognosy and Phytochemistry-II

BP504T – Unit I: Metabolic Pathways in Higher Plants and Their Determination

Pharmacognosy & Phytochemistry-II (BP504T) deals with the formation, isolation, identification and industrial production of phytoconstituents. Unit-I introduces the major metabolic pathways involved in the formation of secondary metabolites and the use of isotope tracer techniques in biogenetic studies. 

Learning Objectives

After completing this unit, the student should be able to:

1. Explain primary and secondary metabolism in higher plants.
2. Describe the major biosynthetic pathways involved in secondary metabolite formation.
3. Explain the Shikimic acid pathway.
4. Explain the Acetate pathway.
5. Describe the Amino acid pathway.
6. Identify important phytoconstituents produced through these pathways.
7. Explain the principle and applications of radioactive isotope tracer studies.

1. Plant Metabolism

Metabolism is the sum of all biochemical reactions occurring within a living organism. Plant metabolism can broadly be classified into primary metabolism and secondary metabolism.

Primary Metabolism

Primary metabolism consists of biochemical processes essential for growth, development and survival.

• Carbohydrates
• Proteins
• Lipids
• Amino acids
• Nucleic acids
• Organic acids

Secondary Metabolism

Secondary metabolism produces specialized metabolites that contribute to plant defense, signaling, adaptation and ecological interactions.

• Alkaloids
• Flavonoids
• Tannins
• Terpenoids
• Phenolics
• Coumarins

2. Formation of Secondary Metabolites

Primary Metabolism → Precursors → Biosynthetic Pathways → Secondary Metabolites

Primary Metabolites → Metabolic Intermediates → Biosynthetic Pathways → Secondary Metabolites

3. Major Biosynthetic Pathways

01. Shikimic Acid Pathway

Important for the biosynthesis of aromatic amino acids and many aromatic secondary metabolites.

Aromatic compounds Phenolics Flavonoids

02. Acetate Pathway

Important pathway for the formation of polyketides and several aromatic natural products.

Polyketides Anthraquinones

03. Amino Acid Pathway

Amino acids act as precursors for many nitrogen-containing secondary metabolites, especially alkaloids.

Alkaloids Nitrogenous compounds

4. Shikimic Acid Pathway

The shikimic acid pathway is an important biosynthetic pathway involved in the formation of aromatic compounds. It connects carbohydrate metabolism with the biosynthesis of aromatic amino acids.

Starting Materials

Phosphoenolpyruvate
(PEP) + Erythrose-4-phosphate
(E4P) → DAHP

Shikimate Pathway – Flow Diagram

PEP + Erythrose-4-phosphate

↓

3-Deoxy-D-arabino-heptulosonate-7-phosphate (DAHP)

↓

3-Dehydroquinic acid

↓

3-Dehydroshikimic acid

↓

Shikimic acid

↓

Shikimate-3-phosphate

↓

5-Enolpyruvylshikimate-3-phosphate (EPSP)

↓

Chorismic acid (Chorismate)

Formation of Aromatic Amino Acids

Chorismate → Phenylalanine + Tyrosine + Tryptophan

Shikimate → Phenylpropanoid Connection

Phenylalanine → Cinnamic Acid → p-Coumaric Acid → Phenolics / Flavonoids / Coumarins / Lignans

High-Yield Point:
Shikimate pathway is strongly associated with the formation of aromatic amino acids and aromatic secondary metabolites.

5. Acetate Pathway

The acetate pathway, also known as the acetate–malonate pathway, is an important pathway for the biosynthesis of polyketides and several natural products.

Basic Pathway

Acetyl-CoA

↓

Malonyl-CoA

↓

Repeated Condensation

↓

Polyketide Chain

↓

Cyclization & Modification

↓

Polyketides / Aromatic Natural Products

Anthraquinone Formation – Conceptual Flow

Acetyl-CoA → Malonyl-CoA → Polyketide → Cyclization → Anthraquinone Skeleton

Anthraquinone Drugs

Important crude drugs containing anthraquinone derivatives include:

• Senna
• Aloe
• Rhubarb

Key Concept

Repeated condensation of acetate-derived units generates a polyketide chain, which can undergo cyclization and further modification.

High-Yield Point:
Acetyl-CoA → Malonyl-CoA → Polyketide → Cyclization → Natural Products

6. Amino Acid Pathway

Amino acids act as important precursors for the biosynthesis of several nitrogen-containing secondary metabolites, particularly alkaloids.

Important Amino Acid Precursors

Amino Acid	Major Product Class	Examples
Ornithine	Tropane alkaloids	Atropine, Hyoscyamine, Scopolamine
Lysine	Piperidine and Quinolizidine alkaloids	Piperidine and quinolizidine derivatives
Tyrosine	Isoquinoline alkaloids	Morphine, Codeine, Papaverine, Berberine
Tryptophan	Indole alkaloids	Reserpine, Vincristine, Vinblastine, Strychnine
Histidine	Imidazole alkaloids	Imidazole derivatives

Ornithine → Tropane Alkaloids

Ornithine → Putrescine → N-Methylputrescine → Tropane Skeleton → Atropine / Hyoscyamine / Scopolamine

Tyrosine → Isoquinoline Alkaloids

Tyrosine → DOPA → Dopamine → Benzylisoquinoline Intermediates → Morphine / Codeine / Papaverine / Berberine

Tryptophan → Indole Alkaloids

Tryptophan → Tryptamine → Indole Alkaloid Intermediates → Reserpine / Vincristine / Vinblastine / Strychnine

7. Comparison of Major Biosynthetic Pathways

Pathway	Main Precursors	Major Products	Key Concept
Shikimic Acid	PEP + E4P	Aromatic amino acids, phenolics, flavonoids, coumarins and lignans	Aromatic compounds
Acetate	Acetyl-CoA / Malonyl-CoA	Polyketides, anthraquinones and related products	Polyketide formation
Amino Acid	Amino acids	Alkaloids and nitrogen-containing secondary metabolites	Alkaloid biosynthesis

8. Biogenesis and Biosynthesis

Biosynthesis

The enzyme-mediated formation of a natural product from precursor molecules within a living organism.

Biogenesis

The origin and formation of natural products in living organisms, including the origin of their structural components.

9. Radioactive Isotopes in Biogenetic Studies

Radioactive isotope studies help determine the origin and fate of atoms during the biosynthesis of natural products.

Principle

Labeled Precursor → Administration → Metabolic Conversion → Product Isolation → Detection of Label

Common Isotopes

Isotope	Application
14C	Tracing carbon atoms in biosynthetic pathways
3H	Tracing hydrogen-containing positions
32P	Studies involving phosphorus-containing metabolites
35S	Studies involving sulfur-containing compounds
15N	Stable-isotope tracing of nitrogen metabolism

Important: ¹⁵N is a stable isotope, not a radioactive isotope. It is nevertheless widely used as an isotope tracer in modern biosynthetic studies.

Radioactive Tracer Experiment

Selection of Suspected Precursor

↓

Radioactive Labeling

↓

Administration to Plant

↓

Metabolic Conversion

↓

Isolation of Secondary Metabolite

↓

Detection of Radioactivity

↓

Interpretation of Biosynthetic Pathway

Example of a Tracer Study

¹⁴C-Phenylalanine → Plant Tissue → Phenylpropanoid Pathway → Labeled Phenolic Product

Detection of Radioactivity

Autoradiography

Used to visualize the distribution of radioactively labeled compounds.

Geiger–Müller Counter

Used for detection and measurement of ionizing radiation.

Scintillation Counting

A sensitive method for measuring radioactivity in samples.

10. Advantages of Isotope Tracer Studies

• Identification of biosynthetic precursors.
• Determination of the origin of atoms in natural products.
• Investigation of metabolic sequences.
• Confirmation of proposed biosynthetic pathways.
• Study of precursor incorporation.
• Investigation of complex natural-product biosynthesis.

11. Limitations

• Radioisotopes require specialized facilities.
• Radiation-safety precautions are necessary.
• Radioactive waste requires controlled disposal.
• Metabolic redistribution of the label can complicate interpretation.
• Precursor incorporation alone does not always prove that the compound is the direct precursor.

12. Modern Approaches to Biogenetic Studies

Stable Isotope Labeling → LC-MS / GC-MS → NMR → Metabolomics → Biosynthetic Pathway Elucidation

13. Quick Revision

Three Pathways – Three Key Associations

SHIKIMATE → AROMATIC COMPOUNDS

ACETATE → POLYKETIDES

AMINO ACIDS → ALKALOIDS

ISOTOPE → TRACE BIOSYNTHESIS

14. Important Examination Questions

Long Answer Questions

1. Describe the shikimic acid pathway and discuss its importance in secondary metabolite biosynthesis.
2. Explain the acetate pathway with suitable examples.
3. Describe the role of amino acids in the biosynthesis of alkaloids.
4. Discuss the utilization of radioactive isotopes in biogenetic studies.
5. Explain the major metabolic pathways involved in the formation of secondary metabolites in higher plants.

Short Answer Questions

1. Define primary metabolism.
2. Define secondary metabolism.
3. What is the shikimic acid pathway?
4. Name the starting materials of the shikimate pathway.
5. What is chorismate?
6. What are polyketides?
7. Name two products of the acetate pathway.
8. Name four amino acids involved in alkaloid biosynthesis.
9. What is a radioactive tracer?
10. Write the principle of isotope tracer studies.

15. One-Minute Revision

SHIKIMATE → AROMATIC COMPOUNDS

ACETATE → POLYKETIDES

AMINO ACIDS → ALKALOIDS

ISOTOPE → TRACE THE PATHWAY

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B.Pharm 5th Semester • BP504T Pharmacognosy & Phytochemistry-II
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