Unit 1: Foundations of Natural Products & Biosynthetic Pathways

Natural products are organic molecules synthesized by living organisms that typically possess intricate stereochemical architectures and specialized biological activities. Historically spanning herbal pharmacopeias to modern structural biology, natural products represent the foundational crucible of organic chemistry and medicinal discovery.

Historical Evolution

The formal discipline of natural products chemistry originated in the early 19th century with the isolation of pure crystalline active principles from botanical matrices:

1. Morphine Isolation (1804–1817): Friedrich Sertürner isolated morphine from opium (*Papaver somniferum*), recognizing it as an alkaline, nitrogenous base ('alkaloid'), which overturned the dogma that plant constituents were exclusively acidic or neutral.
2. Quinine, Caffeine, and Strychnine (1820s): Pierre-Joseph Pelletier and Joseph-Bienaimé Caventou systematically isolated quinine (*Cinchona* bark), strychnine (*Strychnos nux-vomica*), and caffeine (*Coffea arabica*), establishing alkaloid extraction protocols.
3. Synthesis and Structural Proof: The late 19th and early 20th centuries were defined by degradative structural proofs (Baeyer, Emil Fischer, Robert Robinson, Richard Willstätter) and milestone total syntheses, proving that natural molecules obey the universal laws of covalent bonding and thermodynamics.

Ecological and Evolutionary Dimensions

Unlike primary metabolic machinery, which is ubiquitous and invariant across life, secondary metabolites exhibit restricted taxonomic distributions. Their evolutionary emergence is driven by ecological selection pressures:

• Allelopathy and Plant Competition: Secretion of phytotoxic compounds into the rhizosphere (e.g., juglone from *Juglans nigra*) suppresses seedling germination of competing flora.
• Chemical Defense against Herbivory and Pathogens: Phytoalexins (induced upon microbial assault), bitter sesquiterpene lactones, and neurotoxic alkaloids act as deterrents or toxins against herbivores.
• Mutualistic Signaling: Floral monoterpenes and anthocyanin pigments mediate pollinator attraction, while legume flavonoids activate bacterial nodulation genes (*nod* genes) in nitrogen-fixing *Rhizobium* symbioses.

Benchmark Table: Major Classes of Natural Products & Biosynthetic Origins

Natural products are categorized according to primary precursors, pathway routing, and signature structural hallmarks:

Living organisms operate dual metabolic networks: primary metabolism, sustaining universal cellular vitality, and secondary (specialized) metabolism, facilitating organismal survival within complex ecological niches.

Fundamental Comparative Criteria

The divergence between primary and secondary metabolism can be formalized along structural, energetic, and genetic axes:

Bioenergetic Costs and Trade-Offs

Specialized metabolites impose substantial bioenergetic burdens on host cells:

Plants and microbes balance these metabolic expenditures through strictly regulated transcriptional cascades, compartmentalization within specialized organelles (e.g., glandular trichomes, resin ducts, vacuolar storage), and the synthesis of non-toxic pro-drugs (e.g., cyanogenic glycosides) activated only upon mechanical tissue disruption.

Advanced Research Monograph: Synthetic Biology & Heterologous Terpene Flux in *S. cerevisiae*

In natural host plants, secondary metabolites are frequently produced in trace quantities ($<0.01\%\text{ dry weight}$), presenting severe supply bottlenecks. Modern metabolic engineering reconstructs entire biosynthetic cascades in industrial microbial chassis:

[Primary Carbon Source: Glucose / Ethanol]
              │
              ▼ (Glycolysis & Pyruvate Dehydrogenase)
         Acetyl-CoA
              │
              ▼ (Overexpressed tHMGR, ERG Kinases)
             IPP  <==== Isomerase ====>  DMAPP
              │                            │
              └──────────────┬─────────────┘
                             ▼ (Heterologous GPPS / FPPS)
                        GPP / FPP
                             │
                             ▼ (Target Plant Cyclase: Amorphadiene Synthase)
                       Sesquiterpene Scaffold
                             │
                             ▼ (Cytochrome P450: CYP71AV1 + CPR + CYB5)
                    Artemisinic Acid (Precursor to Artemisinin)

1. Overexpression of Truncated HMG-CoA Reductase (tHMGR):

In native Saccharomyces cerevisiae, HMGR is subject to sterol-mediated feedback ubiquitination and degradation. Removing the $N$-terminal membrane-anchoring domain leaves a constitutively active, cytosolic truncated catalytic fragment (tHMGR), boosting mevalonate pool flux by $>40$-fold.

1. Plastidial Translocation vs Cytosolic Routing:

Plastidial enzymes (such as monoterpene and diterpene synthases) possess $N$-terminal transit peptides. Engineering codon-optimized constructs lacking targeting sequences allows functional cytosolic expression while co-expressing dual-function geranyl diphosphate synthases (GPPS).

1. Cytochrome P450 Engineering (The Keasling Artemisinin Breakthrough):

Jay Keasling and coworkers reconstructed the biosynthesis of the antimalarial artemisinin in yeast by expressing amorphadiene synthase (ADS) along with plant cytochrome P450 CYP71AV1, NADPH:cytochrome P450 reductase (CPR), and cytochrome $b_5$ (CYB5). Fermentation titers exceeded $25\text{ g/L}$ of artemisinic acid, which is photochemically converted into artemisinin using singlet oxygen ($^1\text{O}_2$) without agricultural land cultivation.

The structural vastness of natural products is categorized systematically based on the fundamental biosynthetic pathways and primary metabolic precursors from which their carbon skeletons originate.

Major Biosynthetic Classes

1. Terpenoids (Isoprenoids): Derived from the assembly of five-carbon ($C_5$) isoprene units: isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). Class covers monoterpenes ($C_{10}$), sesquiterpenes ($C_{15}$), diterpenes ($C_{20}$), triterpenes ($C_{30}$), and tetraterpenes ($C_{40}$).
2. Phenylpropanoids and Shikimate Metabolites: Formed from phosphoenolpyruvate (PEP) and D-erythrose 4-phosphate via chorismic acid. Includes cinnamic acids, coumarins, lignans, flavonoids, and aromatic amino acids.
3. Polyketides: Assembled by iterative decarboxylative condensation of acyl-CoA thioesters (acetyl-CoA, malonyl-CoA, methylmalonyl-CoA) mediated by polyketide synthases (PKS). Includes macrolides, anthraquinones, tetracyclines, and polyethers.
4. Alkaloids: Nitrogen-containing secondary metabolites, predominantly heterocyclic, derived from proteinogenic amino acids (L-ornithine, L-lysine, L-tyrosine, L-tryptophan, L-histidine) or purines.
5. Carbohydrates: Polyhydroxy aldehydes or ketones and their condensed oligomeric and polymeric forms, functioning in structural scaffolding (cellulose) and metabolic storage (starch, glycogen).
6. Lipids and Steroids: Hydrophobic molecules derived from fatty acyl thioesters or the triterpenoid cyclization of squalene, featuring fused sterane (cyclopentanoperhydrophenanthrene) frameworks.

The isolation of natural products requires rigorous extraction techniques tailored to the thermolability, polarity, and chemical matrix of the raw biomass.

Solid-Liquid Solvent Fractionation

Classical isolation employs a polarity gradient extraction scheme. Dried plant material is extracted successively with solvents of increasing dielectric constant:

Steam Distillation Thermodynamics

For volatile, water-insoluble essential oils (monoterpenes and sesquiterpenes), steam distillation enables codistillation below the thermal decomposition temperatures of the constituents. According to Dalton's Law of Partial Pressures:

Boiling occurs when $P_{\text{total}} = P_{\text{atm}} = 101.325\text{ kPa}$. Because $P_w^\circ(T) < P_{\text{atm}}$, boiling occurs strictly at $T_b < 100^\circ\text{C}$. The mass ratio of organic distillate ($m_o$) to condensed water ($m_w$) in the receiver is governed by their vapor pressures and molar masses ($M_o, M_w$):

Supercritical Fluid Extraction ($sc\text{CO}_2$)

Supercritical carbon dioxide ($T_c = 31.1^\circ\text{C}$, $P_c = 7.38\text{ MPa}$) provides an inert, non-toxic, and tunable extraction medium. By modulating pressure and temperature, the fluid density $\rho(P, T)$ is varied continuously between gas-like ($0.2\text{ g/cm}^3$) and liquid-like ($0.9\text{ g/cm}^3$) values, tuning the solubility parameter $\delta$ according to Giddings' relation:

Small additions of polar cosolvents (modifiers such as ethanol, 1–5 mol%) dramatically enhance the extraction recovery of moderately polar polyphenols and alkaloids without leaving toxic residues.

The Mevalonic Acid (MVA) pathway operates in the eukaryotic cytoplasm, archaea, and plant cytosol, synthesizing the fundamental $C_5$ terpene building blocks: isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP).

Stepwise Enzymatic Cascade

1. Acetoacetyl-CoA Thiolase Condensation: Two molecules of acetyl-CoA undergo a Claisen condensation to form acetoacetyl-CoA:

1. HMG-CoA Synthase Condensation: Acetoacetyl-CoA condenses with a third acetyl-CoA via stereospecific aldol addition to generate (3S)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA):

1. HMG-CoA Reductase (HMGR) — Rate-Determining Step: HMGR catalyzes the irreversible, two-step, four-electron reduction of the thioester to a primary alcohol, utilizing two molecules of NADPH:

This step is the major pharmacological target for statin cholesterol-lowering drugs (e.g., atorvastatin).

1. Consecutive Phosphorylations: Mevalonate is phosphorylated by mevalonate kinase (yielding mevalonate 5-phosphate) and phosphomevalonate kinase (yielding mevalonate 5-pyrophosphate), consuming 2 ATP:

1. Decarboxylation to IPP: Mevalonate 5-diphosphate decarboxylase catalyzes an ATP-dependent trans-elimination with decarboxylation, affording isopentenyl pyrophosphate (IPP):

1. Isomerization to DMAPP: Isopentenyl pyrophosphate isomerase establishes a reversible equilibrium via stereospecific protonation-deprotonation, yielding dimethylallyl pyrophosphate (DMAPP):

The 2-C-Methyl-D-erythritol 4-Phosphate (MEP) pathway, also termed the 1-deoxy-D-xylulose 5-phosphate (DOXP) pathway, functions in plant plastids, cyanobacteria, and major human pathogens (e.g., Plasmodium falciparum, Mycobacterium tuberculosis). It represents an alternative route to IPP and DMAPP distinct from the cytosolic MVA pathway.

Stepwise Catalytic Mechanism

1. DOXP Synthase (DXS): Condensation of pyruvate and D-glyceraldehyde 3-phosphate (GAP) mediated by thiamine pyrophosphate (TPP) with concurrent decarboxylation:

1. DOXP Reductoisomerase (IspC / Dxr): DOXP undergoes an intramolecular retro-aldol / aldol rearrangement followed by NADPH reduction to form 2-C-methyl-D-erythritol 4-phosphate (MEP). This enzyme is specifically inhibited by fosmidomycin:

1. CDP Conjugation (IspD): MEP reacts with cytidine triphosphate (CTP) to form 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDP-ME):

1. Phosphorylation and Cyclization (IspE, IspF): Phosphorylation of the C2 hydroxyl by IspE (ATP $\to$ ADP) followed by cyclization with elimination of CMP yields 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MEcPP).
2. Reductive Ring Opening and Cleavage (IspG, IspH): Iron-sulfur cluster $[4\text{Fe}-4\text{S}]$ enzymes catalyze sequential two-electron reductions:

Unlike the MVA pathway where IPP is synthesized first, the MEP pathway produces both IPP and DMAPP simultaneously in a ~5:1 ratio directly from (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP).

The shikimate pathway operates in plants, fungi, and bacteria (absent in mammals), channeling carbohydrates into aromatic secondary metabolites and the essential aromatic amino acids: L-phenylalanine, L-tyrosine, and L-tryptophan.

Stepwise Pathway Architecture

1. DAHP Synthase Condensation: Phosphoenolpyruvate (PEP) and D-erythrose 4-phosphate (E4P) condense stereospecifically:

where DAHP is 3-deoxy-D-arabino-heptulosonate 7-phosphate.

1. Dehydroquinate Synthase (DHQS): Intramolecular cyclization of DAHP yields 3-dehydroquinate (DHQ), consuming and regenerating $\text{NAD}^+$ catalytically.
2. Dehydration to 3-Dehydroshikimate: Dehydroquinase dehydrates DHQ to introduce a double bond:

1. Shikimate Dehydrogenase: NADPH-dependent reduction of the C3 ketone yields shikimate:

1. Phosphorylation and EPSP Formation: Shikimate kinase phosphorylates the C3 hydroxyl (forming shikimate 3-phosphate). Subsequently, 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase condenses it with a second molecule of PEP:

EPSP synthase is the exclusive cellular target of the broad-spectrum herbicide glyphosate (Roundup).

1. Chorismate Synthase: Eliminates phosphate to generate chorismate, the pivotal branch-point intermediate of aromatic secondary metabolism.
2. Downstream Divergence: Chorismate mutase catalyzes an intramolecular Claisen rearrangement to prephenate (leading to Phe and Tyr), whereas anthranilate synthase incorporates glutamine nitrogen to yield anthranilate (leading to Trp and indole alkaloids).

The polyketide synthase (PKS) pathway assembles one of the most structurally diverse and pharmacologically potent superfamilies of natural products, including macrolide antibiotics (erythromycin), immunosuppressants (rapamycin, FK506), and polyketide polyethers (monensin).

Mechanistic Principles: Decarboxylative Claisen Condensation

Polyketides are biosynthesized by iterative condensations of simple carboxylic acid thioesters, mimicking fatty acid synthesis:

1. Starter and Extender Units:

• Starter units: Acetyl-CoA, propionyl-CoA, benzoyl-CoA.
• Extender units: Malonyl-CoA, methylmalonyl-CoA, ethylmalonyl-CoA.

1. Chain Extension Cycle:

The ketoacyl synthase (KS) domain catalyzes a decarboxylative Claisen condensation between the growing polyketide chain tethered to the acyl carrier protein (ACP) and an incoming extender unit:

Architecture of Modular Type I PKS Enzymes

Type I modular PKSs (e.g., 6-deoxyerythronolide B synthase, DEBS) operate as gigantic, multifunctional molecular assembly lines where each successive elongation cycle is carried out by a dedicated, distinct catalytic module:

• Minimal Core: Every elongating module contains at least three mandatory domains:
• KS (Ketosynthase): Catalyzes C-C bond formation.
• AT (Acyltransferase): Selects and loads the specific extender unit onto the ACP.
• ACP (Acyl Carrier Protein): Transports the growing intermediate via a phosphopantetheine swinging arm ($20\text{ \AA}$ long).
• Reductive Modification Loop: The $\beta$-keto group formed by condensation can undergo programmed, variable reduction:
• If no reductive domains: The $\beta$-ketone ($\text{C}=\text{O}$) is retained.
• KR (Ketoreductase): Reduces ketone to a $\beta$-hydroxyl group ($-\text{CH(OH)}-$).
• DH (Dehydratase): Dehydrates $\beta$-hydroxyacyl to an $\alpha,\beta$-unsaturated trans-alkene ($-\text{CH}=\text{CH}-$).
• ER (Enoylreductase): Reduces the double bond with NADPH to a fully saturated methylene ($-\text{CH}_2-\text{CH}_2-$).
• Termination and Cyclization:

The final module terminates with a Thioesterase (TE) domain, which catalyzes intramolecular nucleophilic attack of a distal hydroxyl group onto the terminal thioester, releasing the polyketide as a macrocyclic lactone (macrolide).