1. Name the three groups of compounds in which secondary metabolism can be divided. Give for each group two examples.
Terpenoids: derived from IPP
- Menthol
- Stigmasterol
Alkaloids: contain one or several N, derived from amino acids
- Morphine
- Reserpine
Phenylpropanoids: derived from shikimate or malonate/acetate pathway
- Coumarins like warfarin
- Flavones like quercetin
2. Terpenoids derive from IPP. Describe the two alternative pathways for IPP formation and name two compounds each that are preferentially formed by either one of the pathways. (5 points)
Mevalonate/ pathway (cytosol/ER): ABA, strigolactons
DOXP pathway (plastids): Sterols, brassinosterol, sesquiterpenes, gibberellins
Both starts with pyruvate
Plastids: directly used, DOXP as an intermediate
Cytosol: cleaved into acetyl-CoA first
3. Describe the formation of a monoterpene by head-to-tail addition. (10 points)
General prerequisite: IPP has two mesomeric forms both of which are necessary for the reaction.
Head-to-tail addition: cleavage of pyrophosphate, formation of a stabilized allyl cation, nucleophilic addition of the donor IPP (important: remember that PP is cleaved from the substance with the double bond located in alpha position to the PP)
Head-to-head addition:
4. Name and explain the four steps of terpene synthesis. (4-5 point)
Synthesis of IPP
- Either via the mevalonate pathway or the DOXP pathway
Prenyl diphosphate homolog formation (terpene formation)
- Via repetitive addition of IPP to form IPP oligomers
- Prenyl transferases à methyl branches, no conjugated double bonds
- Formation can take place either head-to-tail, head-to-head or head-to-middle
Terpenoid skeleton formation
Specific terpene synthases are involved.
Similar reaction to prenyl transferases but intramolecularly, leading to cyclizations
Depending on where the charge of the allylic cation is localized, different products are produced
Enzymes have a small percentage of side products
Alteration of the active center can result in the formation of different product composition
Example: limonene synthase
Secondary modifications
Redox reactions, isomerization reactions, conjugation reactions
Often catalyzed by NADP+-dependent cytochrome P450 oxidases
5. Describe the core features of an alkaloid and give examples.
Alkaloids are heterocyclic (mostly), nitrogen-containing and basic
Generally, they are very diverse and cannot be classified easily
They are devided into three groups:
True Alcaloids, e.g. nicotin
Contain nitrogen in a heterocycle
Are derived from amino acids
Protoalkaloids, e.g. meskalin
Contain nitrogen but not in the heterocycle
Pseudoalkaloids (not derived from amino acids but, e.g., from acetate), e.g. coniin
6. What are the major groups of phenylpropanoids? Give one example for three different groups.
Catechins: Epigallocatechin gallate
Flavones: Quercetin
Isoflavones: Genistein
Stilbenes: Resveratrol
Coumarins: Warfarin
Lignans: Podophyllotoxin
Polyketides: Tetrahydrocannabinol
7. Name three end products of phenylpropanoid biosynthesis and explain their biosynthesis.
Lignans (H/S/G lignin), lagnans (pinoresinol), and “volatiles” (?) (benzaldehyde).
Phenylpropanoid biosynthesis always starts from phenylalanine. Phenylalanine ammonia lyase (PAL) converts it to trans-cinnamic acid which is then converted to coumaric acid via P450 monooxygenase (cinnamic acid 4-hydroxylase). Other monooxygenases and methyl transferases produce ferulic acid and sinapic acid. Those four molecules build the core structures of phenylpropanoid biosynthesis.
8. What is the difference between lignans and lignins? (Small bonus question in the first exam)
Lignins: oligomers
Lignans: dimers
9. Name three hormones that belong to the group of terpenoids and briefly describe their biosynthesis.
Gibberellins (GA)
DOXP pathway —> IPP
Prenyl transferase elongation reactions until geranylgeranyl diphosphate —> ent-kaurine
All steps until ent-kaurene take place in the plastid
Transport of ent-kaurine to the ER, oxidative modifications —> gibberellines
(fungi have a different pathway to be able to manipulate the plant pathway without impairing their own)
Abscisic acid (ABA)
DOXP pathway, prenyl transferase elongation until C40 (tetraterpene)
Modifications to form zeaxanthin, then:
Strigolactones
DOXP pathway, prenyl transferase elongation until C30, carotene precursor is formed
Also: brassinosterols derived from squalene, cyclization, oxidation of the b-ring, modification
10. Give two examples and explain the differences where fungi and plants have evolved different biosynthetic pathways for the same hormone.
Gibberellines: start as well with ent-kaurine but with different enzymes, different steps for oxidative modifications
Abscisic acid: fungal pathway is completely independent of the plant pathway (and much shorter, starting with C15)
Auxin: agrobacterium has a different pathway than plants
(Also: coronatin (specialized metabolite synthesized by bacteria))
11. Name two general principles, each by which hormones are activated or inactivated.
Gibberellins
Activation via β3-hydroxylation:
Inactivation by β2-hydroxylation:
Salicylic acid
Inactivation by hydroxylation and conjugation
Oxidative modification
Conjugation
With examples!
NHP (?)
12. What is the difference between the C40 and the C15 pathway in ABA biosynthesis?
C40 – in plants: precursor zeaxanthin (C40) in the plastid, broken down until xanthoxin is produced, export to cytoplasm, oxidations
C15 – in fungi: precursor farnesyl diphosphate (C15), much shorter, independent from plant pathway
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