'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 liberation of sulfate from adenosine-5'-phosphosulfate (APS) is catalyzed by APS:phosphate adenylyltransferase (APAT). Here we report the first biochemical and genetic characterization of APAT. We isolated this enzyme from the chemolithoautotroph Thiobacillus denitrificans and cloned the corresponding gene. The enzyme is homodimeric with 41,387-Da subunits and exhibits a specific activity of 2100 μmol min^-1 mg^-1. The K(m) values are K(m(aps)) = 300 μM and K(m(P(i))) = 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 specific and unidirectional function. The former enzyme name ADP-sulfurylase implies that the reverse reaction is catalyzed; therefore, this name should not be used any longer. Histidine modification of APAT results in complete inactivation that can be suppressed by substrate addition. APAT is highly similar to galactose-1-phosphate uridylyltransferase and also related to Ap₄A phosphorylase. Active site residues of galactose-1- phosphate uridylyltransferase are conserved in APAT and Ap₄A phosphorylase, suggesting a histidine as the nucleotide-binding residue in all three enzymes, which together form a new family of nucleotidyltransferases.