an L-alpha-amino acid + L-proline => Xaa-L-Pro dipeptide + H2O
an l-alpha-amino acid + l-proline => xaa-l-pro dipeptide + h2o
an L-alpha-amino acid + L-proline => Xaa-L-Pro dipeptide + H2O
an l-alpha-amino acid + l-proline => xaa-l-pro dipeptide + h2o