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

3. 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

4. Gain and loss of a carbocation can occur in different ways

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

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

7. Precursor to cholesterol is formed from squalene through cyclizations and shifts

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

9. 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

10. 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

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

12. 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

13. 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!

14. 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 )

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

16. 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

17. 1b. Loss of N: Deamination of an amino acid by Vitamin B 6 involves imine formation and hydrolysis

18. 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)

19. 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)

20. 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

21. 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