Synthetases tryptophan synthetase

Ferredoxln, Anthranllate synthetase. Tryptophan synthetase. Cytochrome p-A50, Cytochrome cl, Phosphoglucomutase, Fructose-1,6-dlphosphatase... 

HO- 0 H Tryptophan synthetase Tryptophan synthetase jSj protein Tryptophanase Retention [69] Retention [69] Retention [104]... 

Applications of tryptophan synthetase Tryptophan synthetase (EC 4.2.1.20) is a pyridoxal phosphate-dependent enzyme that, in the cell, catalyzes the a,/3-elimination of water from serine to form a pyridoxyl-bound a-aminoacrylate, which undergoes Michael addition of indole to form the named amino acid. This type of reaction has been used to prepare (5)-tryptophan isotopomers with a variety of labeling patterns by use of different labeled indoles and (5)-serines in yields of up to 98% based on indole and 92% based on (5)-serine. 

Various minor hematological effects have been noted in animals. Rats exposed to 50-800 ppm of trichloroethylene continuously for 48 or 240 hours showed time- and dose-related depression of delta-aminolevulinate dehydratase activity in liver, bone marrow, and erythrocytes (Fujita et al. 1984 Koizumi et al. 1984). Related effects included increased delta-aminolevulinic acid (ALA) synthetase activity, reduced heme saturation of tryptophan pyrrolase and reduced cytochrome P-450 levels in the liver and increased urinary excretion of... 

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. 

Freedberg, W.B., and Hardman, J.K. (1971) Structural and functional roles of the cysteine residues in the a-subunit of the Escherichia coli tryptophan synthetase./. Biol. Chem. 246, 1439. 

Heilmann, H.D., and Holzner, M. (1981) The spatial organization of the active sites of the bifunctional oligomeric enzyme tryptophan synthetase Cross-linking by a novel method. Biochem. Biophys. Res. Comm. 99, 1146. 

Folding to native-like structure has been demonstrated with fragments of jS-galactosidase, lysozyme, serum albumin, penicillinase, and tryptophan synthetase. The capability of protein fragments for independent formation of structure therefore has substantial experimental basis. This generalization also makes plausible the idea that, in general, protein folding occurs by parts, that is, in a modular fashion. 

Most of the proteins on which fragment folding studies have been carried out are extracellular. Of the nine discussed in this article, only tryptophan synthetase and /3-galactosidase are not secreted. Many secreted proteins are synthesized with 20 or so additional amino acid residues at the N-terminus of the peptide chain (Blobel... 

Tryptophan synthetase from Escherichia coli is a simple example of a multienzyme complex It contains two types of subunits, and /3, that have molecular weights of 29,500 and 54,000, respectively.83-84 The fully associated enzyme has the composition a2/3285 and catalyzes the reaction... 

The coenzyme pyridoxal 5 -phosphate is required for the reactions in both (9) and (11). The sum of (10) and (11) gives (9), so that tryptophan synthetase is indeed a multienzyme complex catalyzing a sequence of reactions. The intermediate indole cannot be detected when the overall reaction is carried out, although the native enzyme will catalyze the partial reactions [(10) and (11)] 50 to 100 times more efficiently than the isolated subunits.86 88... 

Numerous equilibrium and kinetic studies have been made with tryptophan synthetase and its subunits, and considerable information has been obtained about the reaction pathway and reaction intermediates (cf. Refs. 89-92). For the purposes of this review, the principal conclusion reached is that the interaction of the a and j8 subunits appears to restrict the conformations of the a and /3 subunits to those that bind the substrates tightly and catalyze the reaction efficiently. The basic mechanism is not altered by the subunit interactions instead stabilization of particular conformations and binding sites is the important advantage gained in formation of the multienzyme complex. 

The pyruvate dehydrogenase complex from Escherichia coli is considerably more complex than tryptophan synthetase. It has a molecular weight of approximately 4.6 millon and contains three enzymes pyruvate dehydrogenase (Et), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).82 The overall reaction catalyzed by the complex is... 

Discrimination between some pairs of tRNAs depends entirely on the anticodon sequence. For example, tRNAMet contains the anticodon CAU. That for a minor tRNAIle is the same except that the cytosine has been posttranscriptionally modified by covalent linkage of a molecule of lysine via its e-amino group to C2 of the cytosine. The latter base (Iysidine) is correctly recognized by E. coli isoleucyl-tRNA synthetase but, if the cytosine is unmodified, it is aminoacylated by methionyl-tRNA synthetase.192 In most instances the acceptor specificity, or tRNA identity, is not determined solely by the anticodon sequence. Thus, when a methionine initiator tRNA was modified to contain a tryptophan anticodon, it was only partially charged with tryptophan in vivo. However, when A73 of the methionine tRNA was also converted to G73, only tryptophan was inserted.193 Nucleotide 73 (Fig. 

As any elementary textbook on molecular biology will relate, the sequences of proteins are stored in DNA in the form of a triplet code. Each amino acid is encoded by one or more triplet combinations of the four bases A, T, G, and C. For example, tryptophan is coded by the sequence TGG. The sequence of triplets is converted into a protein by a process in which DNA is first transcribed into mRNA. This message is then translated into protein on the ribosomes in conjunction with tRNA and the aminoacyl-tRNA synthetases. In prokaryotes, there is a one-to-one relationship between the sequence of triplets in the DNA. and the sequence of amino acids in the protein. In eukaryotes, the DNA often contains stretches of intervening sequences or introns which are excised from the mRNA after transcription (Chapter 1). 

In oligomeric proteins composed of nonidentical subunits, it is possible to determine the functional role of each subunit, if dissociation and isolation of subunits are attained in their native states. Examples are tryptophan synthase, lactose synthetase, and aspartate transcarbamoylase.25 ... 

Schneider, W. P., Nichols, B. P., and Yanofsky, C. (1981). Procedure for production of hybrid genes and proteins and its use in assessing significance of amino acid differences in homologous tryptophan synthetase alpha polypeptides. Proc. Natl. Acad. Set. USA, 78, 2169-2174. 

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