Adenosine 5'-triphosphate phosphoribosyltransferase (ATPPRT) catalyzes the first step in histidine biosynthesis, the condensation of ATP and 5-phospho-α-d-ribosyl-1-pyrophosphate to generate N¹-(5-phospho-β-d-ribosyl)-ATP and inorganic pyrophosphate. The enzyme is allosterically inhibited by histidine. Two forms of ATPPRT, encoded by the hisG gene, exist in nature, depending on the species. The long form, HisG_L, is a single polypeptide chain with catalytic and regulatory domains. The short form, HisG_S, lacks a regulatory domain and cannot bind histidine. HisG_S instead is found in complex with a regulatory protein, HisZ, constituting the ATPPRT holoenzyme. HisZ triggers HisG_S catalytic activity while rendering it sensitive to allosteric inhibition by histidine. Until recently, HisG_S was thought to be catalytically inactive without HisZ. Here, recombinant HisG_S and HisZ from the psychrophilic bacterium Psychrobacter arcticus were independently overexpressed and purified. The crystal structure of P. arcticus ATPPRT was determined at 2.34 Å resolution, revealing an equimolar HisG_S-HisZ hetero-octamer. Steady-state kinetics indicate that both the ATPPRT holoenzyme and HisG_S are catalytically active. Surprisingly, HisZ confers only a modest 2-4-fold increase in k_cat. Reaction profiles for both enzymes cannot be distinguished by ³¹P nuclear magnetic resonance, indicating that the same reaction is catalyzed. The temperature dependence of k_cat shows deviation from Arrhenius behavior at 308 K with the holoenzyme. Interestingly, such deviation is detected only at 313 K with HisG_S. Thermal denaturation by CD spectroscopy resulted in T_m's of 312 and 316 K for HisZ and HisG_S, respectively, suggesting that HisZ renders the ATPPRT complex more thermolabile. This is the first characterization of a psychrophilic ATPPRT.

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