Review of organic mechanisms used in construction of 2 o metabolites I.Reactions used in building and modifying carbon skeletons 1. Alkylations using SAM to produce a methoxy, N-methyl or methyl group 2 examples of how C becomes nucleophilic and forms bond with CH 3 group:

2. Prenylations: Addition of an isoprene group Ex: C-alkylation with DMAPP Vit K skeletons

3. C-C bond formation through electrophilic addition Assembly of building blocks and cyclization requires generation of a carbocation center that reacts with nearby pi electrons

Gain and loss of a carbocation can occur in different ways

4. Rearrangements: once the pieces are put together, parts may migrate due to hydride and alkyl shifts:

Example: Skeletons of terpenes like these volatiles and essential oils vary greatly in structure due to rearrangements

Precursor to cholesterol is formed from squalene through cyclizations and shifts

5. Aldol & Claisen condensations often occur to facilitate C-C bond formation when reactants are carbonyl compounds

Claisen condensation of thioesters is used in chain-building: malonyl CoA + acetyl CoA are condensed to form a 4 C unit in the acetate pathway Another example: aldol-type condensations are used to assemble a ring from PEP and D-E-4-P at the beginning of the shikimate pathway

6. C-C bond formation through Mannich reaction: Iminium nitrogen activates the imine C towards nucleophilic attack Example: cyclization to form a tropane ring structure

7.Phenolic oxidative coupling: Formation of radicals happens readily with phenols, enabling two aromatic rings to attach through homolytic bond formation

8. Glycosylation: nature’s way of increasing water-solubility by attaching one or more sugars to the organic part or “aglycone” moiety to an organic product Example: quercetin-3-galactoside, and other glycosides of quercetin are found in fruit and leaves of cranberry Quercetin is a flavonol Antioxidant with mild anti-tumor activity it also may protect the plant from pests galactose or other sugars

Digitoxin a “cardiac glycoside” from foxglove, Digitalis purpurea Digitoxin competes with K + for a binding site on enzyme potassium-ATPase (potassium blocker/antagonist) inhibiting Na-K ATPase pump. By blocking this activity, it causes Ca 2+ to be transported out more slowly; so heart muscle is exposed to Ca 2+ for a longer period of time. As a result, the heart contracts more forcefully (“inotropic effect”) It also increases cholinergic stimulation to the heart, which slows it down (steadier, stronger heartbeat with more rest between beats) Too high a dose of Digitalis can be toxic, but the therapeutic dose is about 50% of the toxic dose so it must be taken with care. Similar compounds have been used as arrow poisons!

9. Decarboxylations Loss of COOH group as CO 2 occurs in amino acids and other acids Often assisted by coenzymes or cofactors (Vit B 6 & B 1 )

II. Mechanisms of functional group transformation 1.Gain or loss of N Often occurs through formation or hydrolysis of imine (Schiff base)

1a. Amination reactions: Gain of N by a molecule Reductive amination: N comes from ammonia Transamination: NH 2 group is transferred from an amino acid

1b. Loss of N: Deamination of an amino acid by Vitamin B 6 involves imine formation and hydrolysis

2. Gain or loss of bonds to oxygen: Oxidations and Reductions Catalyzed by enzymes together with cofactors NAD + /NADP + (works with dehydrogenases) --Oxidation of hydroxyl group to carbonyl or aldehyde to carboxylic acid --Oxidation of amine to imine NADH/NADPH --Reduction of carbonyls to alcohols --Reduction of oxygen in reactions with substrates that gain a single O: 2a. Mono-oxygenases catalyze the addition of a single O atom from O 2 to the substrate (substrate is oxidized), The role of NADH is to reduce the second oxygen to form water (next slide)

Some monooxygenases are cytochrome P-450-dependent (e.g. hydroxylases, important in detoxification, drug metabolism) Produce alcohols & phenols Aromatic hydroxylation involves formation of epoxide intermediate + H 2 O CYP450 dependent monooxygenases can also form methylenedioxy bridges (ex: podophyllotoxin, Fig. 2.3 antimitotic)

2b. Dioxygenases catalyze oxidative cleavage reactions of C=C yielding ketones, aldehydes or acids Common with alkenes Even aromatic C=C can be cleaved this way...pi bonds adjacent to OH are particularly reactive

2c. Oxidases & peroxidases catalyze carbonyl formation. Substrate OH group loses H, and oxygen or H 2 O 2 acts as acceptor Example: Oxidation of phenols to quinones 2d. Amine oxidases swap nitrogen for an oxygen, usually transform amines to aldehydes Key role of MAO: breakdown of neurotransmitters in the brain (serotonin, epinephrine, etc)…too much MAO can cause depression, thus MAO inhibitors are a class of anti-depressant