The pH dependence of the active‐site serine DD‐peptidase of Streptomyces R61

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Abstract
Titration of the active‐site serine DD‐peptidase of Streptomyces R61 shows that formation of acyl enzyme during hydrolysis of the substrate Ac2‐L‐Lys‐D‐Ala‐D‐Ala and enzyme inactivation by the β‐lactam compounds benzylpenicillin, N‐acetylampicillin and ampicillin relies on the acidic form of an enzyme's group of pK∼ 9.5. It is proposed that protonation of a lysine ɛ‐amino group facilitates initial binding by charge pairing with the free carboxylate of the substrate and the β‐lactam molecules. Lowering the pH from 7 to 5 has no effect on the second‐order rate constant of enzyme acylation by benzylpenicillin and N‐acetylampicillin but results in a decreased rate constant of acylation by ampicillin and Ac2‐L‐Lys‐D‐Ala‐D‐Ala. Protonation of the side‐chain amino group of ampicillin and a decreased efficacy of the initial binding of the peptide to the enzyme seem to be responsible for the observed effects. Whatever the molecule bound to the enzyme, there is no sign for the active involvement of an enzyme's histidine residue of pK 6.5 – 7.0 in the hydrolysis pathway.

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