The Enzymatic Synthesis of Thiamine Monophosphate

OsTH1 is a key player in thiamin biosynthesis in rice

Thiamin is a vital nutrient that acts as a cofactor for several enzymes primarily localized in the mitochondria. These thiamin-dependent enzymes are involved in energy metabolism, nucleic acid biosynthesis, and antioxidant machinery. The enzyme HMP-P kinase/thiamin monophosphate synthase (TH1) holds a key position in thiamin biosynthesis, being responsible for the phosphorylation of HMP-P into HMP-PP and for the condensation of HMP-PP and HET-P to form TMP. Through mathematical kinetic model, we have identified TH1 as a critical player for thiamin biofortification in rice. We further focused on the functional characterization of OsTH1. Sequence and gene expression analysis, along with phylogenetic studies, provided insights into OsTH1 bifunctional features and evolution. The indispensable role of OsTH1 in thiamin biosynthesis was validated by heterologous expression of OsTH1 and successful complementation of yeast knock-out mutants impaired in thiamin production. We also proved that the sole OsTH1 overexpression in rice callus significantly improves B1 concentration, resulting in 50% increase in thiamin accumulation. Our study underscores the critical role of OsTH1 in thiamin biosynthesis, shedding light on its bifunctional nature and evolutionary significance. The significant enhancement of thiamin accumulation in rice callus upon OsTH1 overexpression constitutes evidence of its potential application in biofortification strategies.

The Biosynthesis of the Pyrimidine Moiety of Thiamin in Halobacterium salinarum

The biosynthetic pathway of the pyrimidine moiety of thiamin was studied in the archaean Halobacterium salinarum. Thiamin is biosynthesized from 4-amino-5-hydroxymethyl-2-methylpyrimidine (pyrimidine) and 5-(2-hydroxyethyl)-4-methylthiazole (thiazole). The pyrimidine and the thiazole are biosynthesized de novo in microorganisms. The biosynthetic routes of pyrimidine in microorganisms differ between eukaryote and eubacteria. In the eukaryote Saccharomyces cerevisiae, histidine and pyridoxine are the precursors of pyrimidine, while in the eubacterium Escherichia coli, pyrimidine is biosynthesized from 5-aminoimidazole ribonucleotide (AIR), an intermediate of purine biosynthesis. Tracer investigations revealed that [(15)N]-, [1-(13)C]- and [2-(13)C] glycine, precursors of AIR, were incorporated into the pyrimidine in H. salinarum. These results suggested that the biosynthetic route of the pyrimidine in H. salinarum is similar to that of E. coli.

The Biosynthesis of the Thiazole Moiety of Thiamin in the Archaeon Halobacterium salinarum

The biosynthetic pathways of the thiazole moiety of thiamin were studied in the archaeon Halobacterium salinarum. Thiamin is generated by the union of 4-amino-5-hydroxymethyl-2-methylpyrimidine (pyrimidine) and 5-(2-hydroxyethyl)-4-methylthiazole (thiazole). The biosynthesis of thiazole is different in facultative anaerobes, aerobes and eukaryotes. In eukaryotes, the C-4, -4', -5, -5' and -5" of the thiazole is biosynthesized from nicotinamide adenine dinucleotide (NAD), with cysteine as S donor and the C-2 and N atoms of glycine. In facultative anaerobic bacteria, such as Escherichia coli, the precursors of the thiazole are the N and C-2 atoms from tyrosine and C-4, -4', -5, -5' and -5" from 1-deoxy-D-xylurose-5-phosphate, again with cysteine as S donor. In aerobic bacteria, such as Bacillus subtilis, L-tyrosine is replaced by glycine. In Archaea, known as the third domain of life, the biosynthetic pathway of thiamin has not yet been elucidated. In the present study in the archaeon H. salinarum, it was shown that both the N and C-2 from glycine are incorporated into the thiazole, rather than the N atom coming from L-tyrosine. These results show that thiazole biosynthesis in H. salinarum more closely resembles the biosynthetic pathway found in eukaryotes.

Pathway of thiamine pyrophosphate synthesis in Micrococcus denitrificans

The pathway of thiamine pyrophosphate (TPP) biosynthesis, which is formed either from exogeneously added thiamine or from the pyrimidine and thiazole moieties of thiamine, in Micrococcus denitrificans was investigated. The following indirect evidence shows that thiamine pyrophosphokinase (EC 2.7.6.2) catalyzes the synthesis of TPP from thiamine: (i) [35S]thiamine incubated with cells of this microorganism was detected in the form of [35S]thiamine; (ii) thiamine gave a much faster rate of TPP synthesis than thiamine monophosphate (TMP) when determined with the extracts; and (iii) a partially purified preparation of the extracts can use thiamine, but not TMP, as the substrate. The activities of the four enzymes involved in TMP synthesis from pyrimidine and thiazole moieties of thiamine were detected in the extracts of M. denitrificans. The extracts contained a high activity of the phosphatase, probably specific for TMP. After M. denitrificans cells were grown on a minimal medium containing 3 mM adenosine, which causes derepression of de novo thiamine biosynthesis in Escherichia coli, the activities of the four enzymes involved with TMP synthesis, the TMP phosphatase, and the thiamine pyrophosphokinase were enhanced two- to threefold. These results indicate that TPP is synthesized directly from thiamine without forming TMP as an intermediate and that de novo synthesis of TPP from the pyrimidine and thiazole moieties involves the formation of TMP, followed by hydrolysis to thiamine, which is then converted to TPP directly. Thus, the pathway of TPP synthesis from TMP synthesized de novo in M. denitrificans is different from that found in E. coli, in which TMP synthesized de novo is converted directly to TPP without producing thiamine.