Methionine chain elongation

Kroymann, J., Textor, S., Tokuhisa, J.G., Falk K.L., Bartram, S., Gershenzon, J. and Mitchell-Olds, Z. (2001) A gene controlling variation in Arabidopsis glucosinolate composition is part of the methionine chain elongation pathway. Plant Phi/sioL, 127, 1077-88. 

GRASER, G., SCHNEIDER, B., OLDHAM, N.J., GERSHENZON, J., The methionine chain elongation pathway in the biosynthesis of glucosinolates in Eruca. saliva (Brassicaceae)., Xrc/i. Biochem. Biophys., 2000,378, 411-419. 

Glucosinolates are derived from amino acids. The precursor amino acids comprise seven protein amino acids (alanine, valine, leucine, isoleucine, phenylalanine, tyrosine, and tryptophan), a number of chain-elongated methionines (CH3-S-(CH2)2-(CH2)n-CH(NH2)-COOH, n = 1-9), and chain-elongated... 

For the synthesis of methionine-containing peptides the Fmoc group is recommendable for temporary protection in the chain elongation steps as shown in Scheme 15. 

Monensin A 41 is derived from five acetate, seven propionate and one butyrate units, with an extra 0-methyl group derived from L-methionine [76, 80]. The NAC thioester of a postulated triketide chain elongation intermediate has been incorporated intact into monensin A by Patzelt and Robinson. This experiment required the extremely careful use of several p-oxidation inhibitors and other additives to minimise precursor degradation [77]. 

So once the formyl-methionine initiator is in place, attached to the m-RNA on the A site of the ribosome, chain elongation... 

Following peptide bond synthesis, the ribosome Is translocated along the mRNA a distance equal to one codon. This translocation step is promoted by hydrolysis of the GTP in eukaryotic EF2-GTP. As a result of translocation, tRNAj , now without its activated methionine, is moved to the E (exit) site on the ribosome concurrently, the second tRNA, now covalently bound to a dIpeptIde (a peptIdyl-tRNA), Is moved to the P site (Figure 4-26, step U). Translocation thus returns the ribosome conformation to a state in which the A site Is open and able to accept another amlnoacylated tRNA complexed with EFlct-GTP, beginning another cycle of chain elongation. 

Figure 2.5 Condensation Reactions of the Chain Elongation Pathway for the Shortest and Longest 2-Oxo Acid Derivatives of Methionine in Arabidopds.

TEXTOR S., BARTRAM, S., KROYMANN, J., FALK, K.L., HICK, A., PICKETT, J.A., GERSHENZON, J., Biosynthesis of methionine-derived glucosinolates in Arabidopsis thaliana Recombinant expression and charaeterization of methylthioalkylmalate synthase, the condensing enzyme of the chain elongation cycle., Planta, 2003, in press. 

Chain elongation continues until a codon is reached that does not code for any amino acid, i.e. UAA, UAG or UGA. It then ceases and the formed peptide chain is liberated by hydrolysis, which requires the rupture of one high-energy phosphate bond in GTP. The methionine residue is then removed enzymatically. 

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