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Enhancement of catalytic activity by gene truncation: activation of L-aspartase from Escherichia coli.

Aspartase from Escherichia coli is activated by proteolysis at the carboxy-terminal. A systematic study has been undertaken with the goal of identifying the amino acids in this region that influence the catalytic activity of aspartase. Stop codons have been introduced at various positions to prematurely truncate the aspA gene that encodes for aspartase by sequentially eliminating each of the polar and charged amino acids in this region. The affinity of the enzyme for its substrate aspartic acid decreases systematically as each functionally significant amino acid is eliminated. However, enhanced catalytic activity (up to 2.5 times the kcat for native aspartase) is observed for those truncation mutants that end in a positively charged carboxy-terminal amino acid. The precise position of the proteolytic activation of aspartase has been defined, and this covalent activation has been shown to be independent of the allosteric activation of aspartase that is also observed.