Pig tissues express a catalytically inefficient 25-kDa thiamine triphosphatase: insight in the catalytic mechanisms of this enzyme.
ABSTRACT Thiamine triphosphate (ThTP) is found in most organisms and may be an intracellular signal molecule produced in response to stress. We have recently cloned the cDNA coding for a highly specific mammalian 25-kDa thiamine triphosphatase. The enzyme was active in all mammalian species studied except pig, although the corresponding mRNA was present. In order to determine whether the very low ThTPase activity in pig tissues is due to the absence of the protein or to a lack of catalytic efficiency, we expressed human and pig ThTPase in E. coli as GST fusion proteins. The purified recombinant pig GST-ThTPase was found to be 2-3 orders of magnitude less active than human GST-ThTPase. Using site-directed mutagenesis, we show that, in particular, the change of Glu85 to lysine is responsible for decreased solubility and catalytic activity of the pig enzyme. Immunohistochemical studies revealed a distribution of the protein in pig brain very similar to the one reported in rodent brain. Thus, our results suggest that a 25-kDa protein homologous to hThTPase but practically devoid of enzyme activity is expressed in pig tissues. This raises the possibility that this protein may play a physiological role other than ThTP hydrolysis.
- SourceAvailable from: onlinelibrary.wiley.com[Show abstract] [Hide abstract]
ABSTRACT: The CYTH superfamily of proteins was named after its two founding members, the CyaB adenylyl cyclase from Aeromonas hydrophila, and the human 25-kDa thiamine triphosphatase (ThTPase). Members of this superfamily of proteins exist in all organisms, including bacteria, archaeons, fungi, plants, and animals (except birds), and can be traced back to the last universal common ancestor. Their sequences include several charged residues involved in divalent cation and triphosphate binding. Indeed, all members of the CYTH superfamily that have been characterized act on triphosphorylated substrates and require at least one divalent metal cation for catalysis. In most cases, the enzyme-substrate complex adopts a tunnel-like (β-barrel) conformation. The Nitrosomonas europaea, Escherichia coli and Arabidopsis thaliana CYTH proteins are specific inorganic tripolyphosphatases. We propose that inorganic tripolyphosphate, the simplest triphosphate compound, is the primitive substrate of CYTH proteins, other enzyme activities, such as adenylate cyclase (in A. hydrophila and Yersinia pestis), mRNA triphosphatase (in fungi and protozoans), and ThTPase (in metazoans), being secondary acquisitions. ThTPase activity is not limited to mammals, as sea anemone and zebrafish CYTH proteins are specific ThTPases. The acquisition of this enzyme activity is linked to the presence of a tryptophan involved in the binding of the thiazolium heterocycle of the thiamine molecule. Furthermore, we propose a conserved catalytic mechanism between a bacterial inorganic tripolyphosphatase and metazoan ThTPases, based on a catalytic dyad comprising a lysine and a tyrosine, explaining the alkaline pH optimum of these enzymes.FEBS Journal 08/2013; · 4.25 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Thiamine (vitamin B1) is an essential molecule for all life forms because thiamine diphosphate (ThDP) is an indispensable cofactor for oxidative energy metabolism. The less abundant thiamine monophosphate (ThMP), thiamine triphosphate (ThTP) and adenosine thiamine triphosphate (AThTP), present in many organisms, may have still unidentified physiological functions. Diseases linked to thiamine deficiency (polyneuritis, Wernicke-Korsakoff syndrome) remain frequent among alcohol abusers and other risk populations. This is the first comprehensive study on the distribution of thiamine derivatives in human biopsies, body fluids and cell lines. Thiamine derivatives were determined by HPLC. In human tissues, the total thiamine content is lower than in other animal species. ThDP is the major thiamine compound and tissue levels decrease at high age. In semen, ThDP content correlates with the concentration of spermatozoa but not with their motility. The proportion of ThTP is higher in humans than in rodents, probably because of a lower 25-kDa ThTPase activity. The expression and activity of this enzyme seems to correlate with the degree of cell differentiation. ThTP was present in nearly all brain and muscle samples and in ∼60% of other tissue samples, in particular fetal tissue and cultured cells. A low ([ThTP]+[ThMP])/([Thiamine]+[ThMP]) ratio was found in cardiovascular tissues of patients with cardiac insufficiency. AThTP was detected only sporadically in adult tissues but was found more consistently in fetal tissues and cell lines. The high sensitivity of humans to thiamine deficiency is probably linked to low circulating thiamine concentrations and low ThDP tissue contents. ThTP levels are relatively high in many human tissues, as a result of low expression of the 25-kDa ThTPase. Another novel finding is the presence of ThTP and AThTP in poorly differentiated fast-growing cells, suggesting a hitherto unsuspected link between these compounds and cell division or differentiation.PLoS ONE 01/2010; 5(10):e13616. · 3.53 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Thiamine triphosphate (ThTP) was discovered over 60 years ago and it was long thought to be a specifically neuroactive compound. Its presence in most cell types, from bacteria to mammals, would suggest a more general role but this remains undefined. In contrast to thiamine diphosphate (ThDP), ThTP is not a coenzyme. In E. coli cells, ThTP is transiently produced in response to amino acid starvation, while in mammalian cells, it is constitutively produced at a low rate. Though it was long thought that ThTP was synthesized by a ThDP:ATP phosphotransferase, more recent studies indicate that it can be synthesized by two different enzymes: (1) adenylate kinase 1 in the cytosol and (2) FoF1-ATP synthase in brain mitochondria. Both mechanisms are conserved from bacteria to mammals. Thus ThTP synthesis does not seem to require a specific enzyme. In contrast, its hydrolysis is catalyzed, at least in mammalian tissues, by a very specific cytosolic thiamine triphosphatase (ThTPase), controlling the steady-state cellular concentration of ThTP. In some tissues where adenylate kinase activity is high and ThTPase is absent, ThTP accumulates, reaching ≥ 70 % of total thiamine, with no obvious physiological consequences. In some animal tissues, ThTP was able to phosphorylate proteins, and activate a high-conductance anion channel in vitro. These observations raise the possibility that ThTP is part of a still uncharacterized cellular signaling pathway. On the other hand, its synthesis by a chemiosmotic mechanism in mitochondria and respiring bacteria might suggest a role in cellular energetics.Metabolic Brain Disease 03/2014; · 2.33 Impact Factor