Labelling of penicillin-binding proteins with a photoreactive peptidoglycan-peptide analogue

Transpeptidases, DD-carboxypeptidases and endopeptidases from bacteria are usually detected by labelling with radioactive beta-lactam antibiotics, due to a selective stabilization of the enzyme-antibiotic complex, and are therefore generally known as penicillin-binding proteins (PBPs). However, as a general rule, PBPs cannot be detected by labelling with real peptidoglycan substrate analogues other than beta-lactams, partly due to the fact that the acyl intermediates formed do not usually accumulate. We here report the chemical synthesis of a radioactive photoreactive derivative of the peptidoglycan substrate L-lysyl-D-alanyl-D-alanine which is able, due to the shortness of its activated state, to label a number of PBPs of Escherichia coli by quenching the reaction at the intermediate step. Furthermore, by using this derivative we have been able to label other PBPs of higher molecular mass (190, 170, 146, 125 and 87 kDa) that were previously detected only by using either photoreactive derivatives of beta-lactam or bis-beta-lactam antibiotics.