ß-Aminopeptidases: Insight into Enzymes without a Known Natural Substrate.

ABSTRACT

ß-Aminopeptidases have the unique capability to hydrolyze N-terminal ß-amino acids, with varied preferences for the nature of ß-amino acid side chains. This unique capability makes them useful as biocatalysts for synthesis of ß-peptides and to kinetically resolve ß-peptides and amides for the production of enantiopure ß-amino acids. To date, six ß-aminopeptidases have been discovered and functionally characterized, five from Gram-negative bacteria and one from a fungus, Aspergillus Here we report on the purification and characterization of an additional four ß-aminopeptidases, one from a Gram-positive bacterium, Mycolicibacterium smegmatis (BapAMs), one from a yeast, Yarrowia lipolytica (BapAYlip), and two from Gram-negative bacteria isolated from activated sludge identified as Burkholderia spp. (BapABcA5 and BapABcC1). The genes encoding ß-aminopeptidases were cloned, expressed in Escherichia coli, and purified. The ß-aminopeptidases were produced as inactive preproteins that underwent self-cleavage to form active enzymes comprised of two different subunits. The subunits, designated α and ß, appeared to be tightly associated, as the active enzyme was recovered after immobilized-metal affinity chromatography (IMAC) purification, even though only the α-subunit was 6-histidine tagged. The enzymes were shown to hydrolyze chromogenic substrates with the N-terminal l-configurations ß-homo-Gly (ßhGly) and ß3-homo-Leu (ß3hLeu) with high activities. These enzymes displayed higher activity with H-ßhGly-p-nitroanilide (H-ßhGly-pNA) than previously characterized enzymes from other microorganisms. These data indicate that the new ß-aminopeptidases are fully functional, adding to the toolbox of enzymes that could be used to produce ß-peptides. Overexpression studies in Pseudomonas aeruginosa also showed that the ß-aminopeptidases may play a role in some cellular functions.IMPORTANCE ß-Aminopeptidases are unique enzymes found in a diverse range of microorganisms that can utilize synthetic ß-peptides as a sole carbon source. Six ß-aminopeptidases have been previously characterized with preferences for different ß-amino acid substrates and have demonstrated the capability to catalyze not only the degradation of synthetic ß-peptides but also the synthesis of short ß-peptides. Identification of other ß-aminopeptidases adds to this toolbox of enzymes with differing ß-amino acid substrate preferences and kinetics. These enzymes have the potential to be utilized in the sustainable manufacture of ß-amino acid derivatives and ß-peptides for use in biomedical and biomaterial applications. This is important, because ß-amino acids and ß-peptides confer increased proteolytic resistance to bioactive compounds and form novel structures as well as structures similar to α-peptides. The discovery of new enzymes will also provide insight into the biological importance of these enzymes in nature.