Thiamine triphosphatase and the CYTH family of proteins

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, archaea, 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 family 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 (ß-barrel) conformation. The Nitrosomonas europaea and E.coli CYTH proteins are specific inorganic triphosphatases. We propose that inorganic triphosphate, the simplest triphosphate compound, is the primitive substrate of CYTH proteins. Other enzyme activities such as adenylate cyclase (in A. hydrophila and Y. pestis), mRNA triphosphatase (in fungi and protozoans) and ThTPase (in metazoans) are secondary acquisitions. ThTPase activity is not limited to mammals, but sea anemone and zebrafish CYTH proteins are already specific ThTPases and the acquisition of this enzyme activity is linked to the presence of a Trp (W53 in mammalian ThTPases) residue involved in the binding of the thiazole heterocycle of the thiamine molecule. Furthermore, we propose a conserved catalytic mechanism between a bacterial inorganic triphosphatase and metazoan ThTPases, based on a catalytic dyad comprising a Lys and a Tyr residue, explaining the alkaline pH optimum of these enzymes.