Purification and Characterization of Two Extremely Thermostable Enzymes, Phosphate Acetyltransferase and Acetate Kinase, from the Hyperthermophilic Eubacterium Thermotoga maritima

Abstract

ABSTRACT Phosphate acetyltransferase (PTA) and acetate kinase (AK) of the hyperthermophilic eubacterium Thermotoga maritima have been purified 1,500- and 250-fold, respectively, to apparent homogeneity. PTA had an apparent molecular mass of 170 kDa and was composed of one subunit with a molecular mass of 34 kDa, suggesting a homotetramer (α 4 ) structure. The N-terminal amino acid sequence showed significant identity to that of phosphate butyryltransferases from Clostridium acetobutylicum rather than to those of known phosphate acetyltransferases. The kinetic constants of the reversible enzyme reaction (acetyl-CoA + P i ⇌ acetyl phosphate + CoA) were determined at the pH optimum of pH 6.5. The apparent K m values for acetyl-CoA, P i , acetyl phosphate, and coenzyme A (CoA) were 23, 110, 24, and 30 μM, respectively; the apparent V max values (at 55°C) were 260 U/mg (acetyl phosphate formation) and 570 U/mg (acetyl-CoA formation). In addition to acetyl-CoA (100%), the enzyme accepted propionyl-CoA (60%) and butyryl-CoA (30%). The enzyme had a temperature optimum at 90°C and was not inactivated by heat upon incubation at 80°C for more than 2 h. AK had an apparent molecular mass of 90 kDa and consisted of one 44-kDa subunit, indicating a homodimer (α 2 ) structure. The N-terminal amino acid sequence showed significant similarity to those of all known acetate kinases from eubacteria as well that of the archaeon Methanosarcina thermophila . The kinetic constants of the reversible enzyme reaction (acetyl phosphate + ADP ⇌ acetate + ATP) were determined at the pH optimum of pH 7.0. The apparent K m values for acetyl phosphate, ADP, acetate, and ATP were 0.44, 3, 40, and 0.7 mM, respectively; the apparent V max values (at 50°C) were 2,600 U/mg (acetate formation) and 1,800 U/mg (acetyl phosphate formation). AK phosphorylated propionate (54%) in addition to acetate (100%) and used GTP (100%), ITP (163%), UTP (56%), and CTP (21%) as phosphoryl donors in addition to ATP (100%). Divalent cations were required for activity, with Mn 2+ and Mg 2+ being most effective. The enzyme had a temperature optimum at 90°C and was stabilized against heat inactivation by salts. In the presence of (NH 4 ) 2 SO 4 (1 M), which was most effective, the enzyme did not lose activity upon incubation at 100°C for 3 h. The temperature optimum at 90°C and the high thermostability of both PTA and AK are in accordance with their physiological function under hyperthermophilic conditions.