Hydrolyzes the N-terminal amino acid residue from a polypeptide chain, with a preference for substrates containing multiple alanine residues. Binds 2 cobalt ions per subunit. kcat is 1145 min(-1) and 122 min(-1) with N-succinyl-Ala-Ala-Ala-Pro-Phe-p-nitroanilide and N-succinyl-Ala-Ala-Ala-p-nitroanilide as substrate, respectively. Optimum pH is 4.0-5.0. Optimum temperature is about 40 degrees Celsius. Belongs to the peptidase M24B family.