#biosynthesis

Transgenic Plant -2

Ethylene synthesis pathway involves which intermediate?

Ethylene synthesis pathway involves ACC as obligate intermediate bridging methionine metabolism and hormonal action. Methionine sulfur is conserved via Yang cycle, while ethylene formation proceeds via S-adenosylmethionine intermediate SAM, then ACC. ACC, chemically 1-aminocyclopropane-1-carboxylic acid, is a strained cyclic nonprotein amino acid accumulating transiently in cytosol and vacuole. It is synthesized in cytosol, can be conjugated to N-malonyl-ACC by ACC N-malonyltransferase for vacuolar storage or to γ-glutamyl-ACC, or transported long distance from flooded roots to shoots via xylem sap causing leaf epinasty. ACC oxidase localized in apoplast and cytosol catalyzes oxidative cleavage of cyclopropane ring, producing ethylene, carbon dioxide and hydrogen cyanide detoxified by beta-cyanoalanine synthase to asparagine. Use of ACC as intermediate explains physiology: flooding induces ACC synthase in roots but ACC oxidase is oxygen-dependent, so ACC transported to aerobic shoots converted to ethylene causing adaptive responses. Identification of ACC solidified targeting strategies using ACC synthase inhibitors for ripening control and flower senescence delay.

Ref: Buchanan Biochemistry Molecular Biology Plants Ch 17 ACC pathway; PMID 11536542 Ethylene biosynthesis.

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Transgenic Plant -2

ACC synthase catalyzes conversion of SAM to:

Plant ethylene biosynthesis, termed Yang cycle, converts sulfur amino acid methionine to gaseous hormone ethylene through two committed enzymatic steps plus recycling. Methionine is first activated by SAM synthetase encoded by MAT genes using ATP to form S-adenosylmethionine, universal methyl donor and precursor for polyamines and ethylene. The rate-limiting step is catalyzed by ACC synthase, a pyridoxal 5'-phosphate dependent aminotransferase encoded by multigene family ACS1-ACS12 differentially regulated by auxin, cytokinin, wounding, flooding and fruit ripening cues. ACS cleaves SAM into 1-aminocyclopropane-1-carboxylic acid plus 5'-methylthioadenosine, which is recycled to methionine via salvage pathway involving MTA nucleosidase, MTI kinase, conserving sulfur. ACC accumulation regulated by conjugation to malonyl and glutamyl derivatives and xylem transport. Transcriptional activation of ACS by ripening transcription factors RIN and NOR, and post-translational phosphorylation by MPK6 stabilizing protein and ETO1 mediated degradation, underlies autocatalytic ethylene burst in climacteric fruits. Inhibition via antisense or aminoethoxyvinylglycine blocks ACC pool and downstream signaling cascade.

Ref: Taiz & Zeiger Plant Physiology 6th ed Ch 22 Ethylene; Yang Cycle review Annu Rev Plant Physiol 1984.

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Plasma Membrane

Which of the following molecules is the main precursor for sphingolipids?

Building sphingolipids requires creation of sphingoid long-chain base de novo in endoplasmic reticulum membrane through highly regulated steps essential for barrier formation. Serine palmitoyltransferase, heterodimer of SPTLC1 and SPTLC2 regulated by ORMDL feedback inhibition and small subunits ssSPTa/b that determine acyl chain preference, condenses L-serine and palmitoyl-CoA producing 3-ketodihydrosphingosine, reduced by 3-ketodihydrosphingosine reductase KDSR to dihydrosphingosine, also called sphinganine. Dihydrosphingosine is N-acylated by six ceramide synthases CerS isoforms with fatty-acyl specificity ranging C14 to C26 to dihydroceramide and desaturated by dihydroceramide desaturase DES1 to ceramide whose backbone is sphingosine, 2-amino-4-octadecene-1,3-diol. This amino alcohol thus represents defining precursor whose fatty acid chain can be remodeled via salvage pathway where sphingosine generated from sphingomyelin hydrolysis by acid sphingomyelinase is rapidly reacylated. Subsequent phosphorylation by sphingosine kinases 1 and 2 yields sphingosine-1-phosphate high-affinity ligand for S1PR1-5 GPCRs controlling lymphocyte egress from lymph nodes, angiogenesis, and vascular barrier integrity and immune regulation in mammals critically.

Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 21: Sphingosine Precursor and S1P Signaling.

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Gene Dissection and Complementation

Arrangement of pathway intermediates is done by

Ordering pathway intermediates relies on systematic tabulation of mutant growth versus supplemented metabolites in matrix format. Each column corresponds to specific compound added to minimal medium, each row to independent mutant isolate. Positive growth scored as one indicates rescue and restoration of prototrophy. Columns rescuing largest number of mutants represent earliest intermediates because many blocks lie upstream of them, while columns rescuing fewest represent latest steps. Sorting columns by descending count and mutants by rescue breadth arranges metabolites linearly as A→B→C→D revealing enzymatic sequence without biochemical assay.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 14: Ordering Intermediates by Growth Matrix Analysis

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PSS- Sec G- Sec metabolites

Methionine serves as precursor for biosynthesis of:

Pick B: Glucosinolates. Thinking through PSS- Sec G- Sec metabolites step by step rules out the lookalikes and leaves Glucosinolates. Skip these: A) Alkaloids; C) Phenolics; D) Terpenoids. For hormones and genes, match the molecule or gene to its real role, not a neighboring pathway.