Cysteine thiamin biosynthesis

In plants, little is known about the basic building blocks and the reactions involved in thiamine biosynthesis. An early study with chloroplasts of spinach indicated that 1-deoxy-D-xylulose 5-phosphate, tyrosine, and cysteine act as precursors of the thiazole moiety in analogy to the pathway in E. coli. More recently, it has been shown that a homolog of the THIC protein that converts 5-aminoimidazole ribotide into 38 is essential (25). These results suggest that the plant pathway is similar to the pathway in prokaryotes but not to that in yeast. 

Although ubiquitin has not been found in prokaryotes, ubiquitin s molecular ancestors were recently identified in prokaryotes. Remarkably, these proteins take part not in protein modification but in biosynthesis of the coenzyme thiamine (p. 423). A key enzyme in thiamine biosynthesis is ThiF, which activates the protein ThiS as an acyl adenylate and then adds a sulfide ion derived from cysteine (Figure 23.9). ThiF is homologous to human Ft, which includes two tandem regions of 160 amino acids that arc 28% identical in amino acid sequence with a region of ThiF from E. coli. 

The more complex sulphur requirements of the marine animals are met largely by cysteine, cystine, methionine, biotin, and thiamine (Young and Maw, 1958) (Fig. 4). Cysteine is a component of the tripeptide glutathione and a precursor of taurine. Methionine is as an essential amino acid involved in biosynthesis of proteins, creatine and adrenaline. Adenosylmethionine is considered to be the active part in transmethylation, e.g. of choline. Methionine is part of the pathways to homocysteine, cystathionine and methylthioadenosine (Young and Maw, 1958). Various organisms convert cysteine and/or cystine into mercapturic acids, cysteine sulphinic acid, and thiazolidine derivatives (Zobell, 1963). 

S ATP + 4-methyl-5-(2-hydroxyethyl)thiazole <1, 2> (<1> enzyme involved in biosynthesis of thiamine [1] <1> the bifunctioinal enzyme hydroxyethylthiazole kinase/thiamine-phosphate pyrophosphorylase catalyzes two sequential steps in the synthesis of thiamin monophosphate from hydroxyethylthiazole [2] <2> the enzyme is a salvage enzyme in the thiamin biosynthetic pathway and enables the cell to use recycled 4-methyl-5-j8-hydroxyethylthiazole as an alternative to its synthesis from 1-deoxy-o-xylulose-5-phosphate, cysteine, and tyrosine [3]) (Reversibility <1, 2> [1,2,3]) [1,2, 3]... 

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