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Nucleotide binding by multienzyme peptide synthetases.

Peptide synthetases consist of linearly arranged catalytic units, which by sequence alignment show equally spaced amino-acid-activating segments/modules of 600-700 amino acid residues. The consensus sequence comprises a new class of sequence motifs which are shared by some carboxyl-activating enzymes, but which do not occur in aminoacyl-tRNA synthetases. The catalytic properties of peptide synthetases with respect to the nucleotide substrate were investigated by enzyme kinetic studies. In the activation reaction ATP may be substituted by 2'-deoxy-ATP (dATP) and 7-deazaadenosine 5'-triphosphate, substrate analogues which are not recognised by many aminoacyl-tRNA synthetases, and may thus prove useful alternative substrates in the detection of peptide synthetases within complex protein mixtures. ATP derivatives substituted at C2 are substrates, while those substituted at C8 are not, indicating a preference for the anti-conformation in substrate binding. Kinetic studies revealed that coenzyme A is a non-competitive inhibitor of the activation reaction, suggesting the presence of a second nucleotide binding site which accommodates nucleotides with phosphate in the C2' or C3' position. This substrate and inhibition profile is markedly different from that of aminoacyl-tRNA synthetases and indicative of a separate homogeneous family of carboxyl-activating enzymes.