Amino acids methionine synthetase

Methionyl tRNA synthetase, (a) The enzyme specifically recognizes the amino acid methionine in one region of the active site and the methionyl tRNA in another, (b) The acylation reaction that results in a covalent linkage of the amino acid to the tRNA. 

The information contained in the DNA (i.e., the order of the nucleotides) is first transcribed into RNA. The messenger RNA thus formed interacts with the amino-acid-charged tRNA molecules at specific cell organelles, the ribosomes. The loading of the tRNA with the necessary amino acids is carried out with the help of aminoacyl-tRNA synthetases (see Sect. 5.3.2). Each separate amino acid has its own tRNA species, i.e., there must be at least 20 different tRNA molecules in the cells. The tRNAs contain a nucleotide triplet (the anticodon), which interacts with the codon of the mRNA in a Watson-Crick manner. It is clear from the genetic code that the different amino acids have different numbers of codons thus, serine, leucine and arginine each have 6 codewords, while methionine and tryptophan are defined by only one single nucleotide triplet. 

The distinction between an initiating (5 )AUG and an internal one is straightforward. In bacteria, the two types of tRNA specific for methionine are designated tRNAMet and tRNAfMet. The amino acid incorporated in response to the (5 )AUG initiation codon is A7-formyl-methionine (fMet). It arrives at the ribosome as A7-formylmethionyl-tRNAfMet (fMet-tRNAfMet), which is formed in two successive reactions. First, methionine is attached to tRNAfMet by the Met-tRNA synthetase (which in E. coli aminoacylates both tRNAfMet and tRNAMet) ... 

The results of these efforts show that no method of tRNA recognition is universal.2443 In some cases, e.g., for methionine- or valine-specific tRNAs, the synthetase does not aminoacylate a modified tRNA if the anticodon structure is incorrect. Although the anticodon is 7.5 ran away from the CCA end of the tRNA, the synthetases are large enzymes. Many of them are able to accommodate this large distance between a recognition site and the active site (Fig. 29-9A). For some other tRNAs the anticodon is not involved in recognition 245 For yeast tRNAphe residues in the stem of the dihydrouridine loop and at the upper end of the amino acid acceptor stem seem to be critical.241... 

Vitamin B12 is required by only two enzymes in human metabolism methionine synthetase and L-methylmalonyl-CoA mutase. Methionine synthetase has an absolute requirement for methylcobalamin and catalyzes the conversion of homocysteine to methionine (Fig. 28-5). 5-Methyltetrahydrofolate is converted to tetrahydrofolate (THF) in this reaction. This vitamin B12-catalyzed reaction is the only means by which THF can be regenerated from 5-methyltetrahydrofolate in humans. Therefore, in vitamin B12 deficiency, folic acid can become trapped in the 5-methyltetrahydrofolate form, and THF is then unavailable for conversion to other coenzyme forms required for purine, pyrimidine, and amino acid synthesis (Fig. 28-6). All folate-dependent reactions are impaired in vitamin B12 deficiency, resulting in indistinguishable hematological abnormalities in both folate and vitamin B12 deficiencies. 

Kiick KL, Tirrell DA. Protein engineering by in vivo incorporation of non-natural amino acids Control of incorporation of methionine analogues by methionyl-tRNA synthetase. Tetrahedron 2000 56 9487-9493. 

L. Serre, G. Verdon, T. Choinowski, N. Hervouet, J.L. Risler, and C. Zelwer. 2001. Ho v methionyl-tRNA synthetase creates its amino acid recognition pocket upon 1-methionine binding J. Mol. Biol. 306 863-876. (PubMed)... 

Serre, L.. Verdon, G.. Choinowski, T., Hervouct, N., Risler, J. L, and Zelwer, C. 2001. How methionyl-tRNA. synthetase creates its amino acid recognition pocket upon l,-methionine binding./. Mol Biol 306 863-876. 

Glutamylcysteine synthetase, cysteine, or methionine was 100 times more reactive to hypochlorous acid in comparison with amino acids that did not contain thiol groups (Folkes et al., 1995). Sublethal exposures to HOCl decreased GSH levels in several cell types (Vissers and Winterboum, 1995 Pullar et al., 1999). In a study by Pullar et al. (1999) using human umbilical vein endothelial cells, doses of 25 nmol of HOCl and less were sublethal when the exposure was done over 10 min, there was a concentration-dependent loss of intracellular GSH. Tissue exposure to HOCl resulted in a reduction of GSH. The metabolite of the HOCl interaction with GSH was an unexpected cyclic sulfonamide that was exported from the cell. The expected metabolites of glutathione disulfide (GSSH) and GSH sulfonic acid were actually minimal (Pullar et al., 2001). Inactivation of acetylcholinesterase by HOCl could be a contributory cause of airway hyperreactivity (den Hartog et al., 2002). 

As for the biosynthesis of cyclosporin A, four head-to-tail acetate units are involved in the formation of the 8-carbon chain of olefinic amino acid, and N-methyl groups of cyclosporin A originate from methionine (Kobel et al., 1983 Zocher et al., 1984). The enzyme named cyclosporin synthetase has been shown to be responsible for cyclosporin biosynthesis (Billich and Zocher, 1987 Lawen et al., 1989). This enzyme was subsequently purified and characterized (Lawen and Zocher, 1990). 

The replacement of sulfur-containing amino acids in proteins by their selenium analogs has received some attention. For example, a variant of Escherichia coli was shown to indiscriminately incorporate selenium into its proteins l. In fact, the /8-galactosidase isolated from this system, which had 70-75% of its methionine residues replaced by the selenium analog, exhibited a virtually unaltered catalytic activity. Similarly, the indiscriminate incorporation of selenomethionine in proteins of the rat as well as E. coli suggests that the enzymes methionyl-tRNA synthetase and amino acid polymerase can accept both methionine and selenomethionine and their corresponding tRNA derivatives. There is some evidence which suggests that a similar phenomenon occurs with cysteine and selenocysteine ... 

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