Amino acids Methionine Met

Each sequence of three bases in DNA may be translated into one of the 20 protein amino acids in an amino acid chain (Box 2.2). Each chain begins with the start code ATG corresponding to the amino acid methionine (Met). The information stored in DNA is used to encode the synthesis of all of the... 

Although cysteine (Cys, C) is considered nonessential in animals, it is biosynthesized from the essential amino acid methionine (Met, M), which, as it is essential, must be ingested. 

Aspartate 4-semialdehyde, seen, for example, in Scheme 12.13, which provided a pathway for the biosynthesis of the essential amino acid methionine (Met, M) and in Scheme 12.14, which holds a representation of the biosynthesis of threonine (Thr, T), is also a place to begin to describe a pathway to lysine (Lys, K). As shown in Scheme 12.19, aspartate 4-semialdehyde undergoes an aldol-type reaction with pyruvate (CHsCOCO ") in the presence of dihydropicoUnate synthase (EC 4.2.1.52) to produce a series of intermediates that, it is presumed, lead to (5)-23-dihydropyridine-2,6-dicarboxylate. Then, dihydrodipicolinate reductase (EC 1.3.1.26) working with NADPH produces the tetrahydropyridine, (S)-2,3,4,5-tetrahydropyridine-2,6-dicarboxylate.This heterocycle, in the presence of glutamate (Glu, E) and water, is capable of transamination directly to 2-oxoglutarate and (2S, 6S)-2,3-diaminopimelate in the presence of LL-diaminopimelate aminotransferase (EC 2.6.1.83), while the latter, in the presence of the pyridoxal dependent racemase... 

Figure 12.14. The RNA codons for mRNA read from DNA and are installed on the ribosome to be read by tRNA bringing the amino acids to the ribosome for peptide (protein) synthesis. Note that AUG is both the start codon (and thus requires the tRNA anticodon of UAC for the first amino acid, methionine [Met, M]) as well as the codon for methionine (Met, M) that might be found elsewhere, and that UAA, UGA, and UAG are stop codons. Note, too, that this 4 X 16 array, with 64 combinations, is redundant for the 20 amino acids, and thus some amino acids are specified in more than one way, while the start codon is unique. Information in this table was obtained from Nirenberg, M Leder, R Bernfield, M. Brimacombe, R. Trupin, I Rottman, F. O Neal, C. Nat. Acad. Sd. U.S., 1965,53,1161 and subsequent work.

Protein content of dehulled Trapper field peas is negatively correlated with the amino acids threonine, cystine, glycine, alanine, methionine, and lysine and positively correlated with glutamic acid and arginine (8). Holt and Sosulski (19) obtained similar correlations with Century field peas for all amino acids except glutamic acid. Other investigators (20) also found that sulfur amino acids (cys, met) are negatively correlated with protein content. 

For a long time, it was considered that, unlike the other vitamins, vitamin E had no specific functions rather it was the major Upid-soluble, radicaltrapping antioxidant in membranes. Many of its functions can be met by synthetic antioxidants however, some of the effects of vitamin E deficiency in experimental animals, including testicular atrophy and necrotizing myopathy, do not respond to synthetic antioxidants. The antioxidant roles of vitamin E and the trace element selenium are closely related and, to a great extent, either can compensate for a deficiency of the other. The sulfur amino acids (methionine and cysteine) also have a vitamin E-sparing effect. 

Chain initiation Initiation requires the smaller ribosomal subunit, an initiation tRNA (with a 5 -CAU-3 anticodon), mRNA with its initiator codon (S -AUG-3 ), and several initiation factors, all of which form the ribosomal initiation complex. The initiation complex is near completion when a tRNA carrying the amino acid methionine hydrogen bonds to the AUG codon on the mRNA. When these components are in place, the larger ribosomal subunit joins the complex in such a way that the initiator met-tRNAjjj j is localized in the P- or peptidyl site (Figure 9-1). The chain initiation phase ends and a second amino acid can be inserted. 

Optical resolution of racemic amino acids (methionine, phenylalanine, tryptophan, valine) by the action of L-specific amino acylase from Aspergillus oryzae (Tanabe Seiyaku Co., Ltd.). The theoretical productivity of 1000 liter immobilized amino acylase columns ranges from 214 kg per day for L-Ala to 715 kg per day for L-Met. 

Volatile sulfur compounds are found in most cheeses and can be important flavor constituents. The origin of sulfur-containing compounds is generally thought to be the sulfur-containing amino acids methionine and cysteine (Law, 1987). As Cys is rare in the caseins (occurring at low levels only in Os2- and K-caseins, which are not extensively hydrolyzed in cheese), the origin of sulfur compounds must be primarily Met. Sulfur compounds formed from Met include H2S, dimethylsulfide, and methanethiol. The importance of methanethiol and related compounds in cheese aroma is discussed by Law (1987). 

Initiation and chain elongation Rihosome Met-tRNA, mRNA amino acyl-tRNAs A start codon binds the first tRNA carrying amino acid methionine to the mRNA. Successive tRNAs bind to and detach from the ribosome as each amino acid adds to the protein. 

Evidence soon emerged that the endogenous opioids were peptides rather than simple morphine-like molecules (9). The first direct evidence for endogenous opioids in brain extracts was provided in 1975 when two pentapeptides were purified that differed only in the carboxyl terminal amino acids (10) (Table 1). These peptides were called methionine- (Met-) and leucine- (Leu-) enkephalin, from the Greek term meaning "in the head."... 

A certain polypeptide is shown by arid hydrolysis to contain only three amino acids serine(Ser), alanine(Ala), and methionine(Met) with mole fractions of j, and respectively. Enzymatic hydrolysis yields the following fragments Ser-Ala Ser-Met Ala-Ser. Deduce the primary structure of the polypeptide. 

On complete hydrolysis, a polypeptide gives two alanine, one leucine, one methionine, one phenylalanine, and one valine residue. Partial hydrolysis gives the following fragments Ala-Phe, Leu-Met, Val-Ala, Phe-Leu. It is known that the first amino acid in the sequence is valine and the last one is methionine. What is the complete sequence of amino acids ... 

HL-60 cellular proteins were labeled uniformly with [35S]methionine and then oxidized with H202. The proteins were then assayed for the presence of Met(O) using either the CNBr assay or amino acid analysis. 

The terms first, second, and third nucleotide refer to the individual nucleotides of a triplet codon. U, uridine nucleotide C, cytosine nucleotide A, adenine nucleotide G, guanine nucleotide Term, chain terminator codon. AUG, which codes for Met, serves as the initiator codon in mammalian cells and encodes for internal methionines in a protein. (Abbreviations of amino acids are explained in Chapter 3.)... 

The N-terminal methionine is posttranslationally acetylated [86], Similar N-term-inal sequences were found in several other proteins that contain N-acetylmethionine as the N-terminal amino acid, suggesting that the signal for acetylation may be the Met-Asp or Met-Glu sequence [71],... 

As noted above, the presence of Met(O) in proteins would go undetected after acid hydrolysis and subsequent amino acid analysis. Thus, since this method of hydrolysis is most commonly used, it is impossible to ascertain from the literature the abundance of Met(O) residues normally present in proteins. However, a number of studies have reported the presence of Met(O) residues in various proteins using one of the appropriate procedures described above. It has been found that Met(O) residues comprise 30% of the total Met in proteins isolated from bovine glomerular basement membranes and anterior lens . Other investigators have reported that the levels of Met(O) in proteins of the trabecular meshwork of human eyes increased with the age of the donor . The amount of Met(O) detected ranged from 15% (10 years old) to 55% (79 years old) of the total methionine content found in the tissue samples. Other studies have shown that in certain species of clams the proteins of the hinge ligament contain only Met(0) residues and no Met . In addition, it has also been reported that as much as 18% of the Met residues in pea seed proteins is in the form of Met(O) . Lastly, Met(O) residues have been found in... 

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