"ADP sulfurylase" from Thiobacillus denitrificans is an adenylylsulfate : phosphate adenylyltransferase and belongs to a new family of nucleotidyltransferases

During AMP-dependent sulfite oxidation by some sulfur bacteria, the liberat ion of sulfate from adenosine-5'-phosphosulfate (APS) is catalyzed by APS:p hosphate adenylyltransferase (APAT), Here we report the first biochemical a nd genetic characterization of APAT, We isolated this enzyme from the chemo lithoautotroph Thiobacillus denitrificans and cloned the corresponding gene , The enzyme is homodimeric with 41,387-Da subunits and exhibits a specific activity of 2100 mu mol min(-1) mg(-1). The K-m values are K-m(APS) = 300 mu M and K-m(Pi), = 12 mM. Catalysis occurs by a ping-pong mechanism with a covalently bound AMP as reaction intermediate. The arsenolysis of APS, but not of ADP, CDP, GDP, UDP, or IDP, is also catalyzed, indicating a specifi c and unidirectional function. The former enzyme name ADP-sulfurylase impli es that the reverse reaction is catalyzed; therefore, this name should not be used any longer. Histidine modification of APAT results in complete inac tivation that can be suppressed by substrate addition. APAT is highly simil ar to galactose-1-phosphate uridylyltransferase and also related to Ap(4)A phosphorylase, Active site residues of galactose-1-phosphate uridylyltransf erase are conserved in APAT and Ap(4)A phosphorylase, suggesting a histidin e as the nucleotide-binding residue in all three enzymes, which together fo rm a new family of nucleotidyltransferases.