Biochemical and molecular characterization of phenylacetate-coenzyme A ligase, an enzyme catalyzing the first step in aerobic metabolism of phenylacetic acid in Azoarcus evansii

Phenylacetate-coenzyme A ligase (PA-CoA ligase; AR IP forming, EC 6.2.1.30), the enzyme catalyzing the first step in the aerobic degradation of phenylacetate (PA) in Azoarcus evansii, has been purified and characterized. The gene (paaK) coding for this enzyme was cloned and sequenced. The enzyme catalyzes the reaction of PA. with CoA and MgATP to yield phenylacetyl-CoA (PACoA) plus AR IP plus PPI, The enzyme was specifically induced after aerobic grow-th in a chemically defined medium containing PA or phenylalanine (Phe) as the sole carbon source. Growth with 4-hydroxyphenylacetate, benzoate, adipate, or acetate did not induce the synthesis of this enzyme, This enzymatic activity was detected very early in the exponential phase of growth, and a maximal specific activity of 76 nmol min(-1) mg of cell protein-l was measured. After 117-fold purification to homogeneity, a specific activity of 48 mu mol min(-1) mg of protein(-1) was achieved with a turnover number (catalytic constant) of 40 s(-1). The protein is a monomer of 52 kDa and shows high specificity towards PA; other aromatic or aliphatic acids were not used as substrates. The apparent K-m values for PA, ATP, and CoA were 14, 60, and 45 mu M, respectively. The PA-CoA ligase has an optimum pH of 8 to 8.5 and a pi of 6.3. The enzyme is labile and requires the presence of glycerol for stabilization. The N-terminal amino acid sequence of the purified protein showed no homology with other reported PA-CoA ligases, The gene encoding this enzyme is 1,320 bp long and codes for a protein of 48.75 kDa (410 amino acids) which shows high similarity with other reported PA-CoA ligases, An amino acid consensus for an AMP binding motif (VX2SSGTTGXP) was identified. The biochemical and molecular characteristics of this enzyme are quite different from those of the isoenzyme catalyzing the same reaction under anaerobic conditions in the same bacterium.