BIOCHEMICAL-CHARACTERIZATION OF PENICILLIN-RESISTANT AND PENICILLIN-SENSITIVE PENICILLIN-BINDING PROTEIN 2X TRANSPEPTIDASE ACTIVITIES OF STREPTOCOCCUS-PNEUMONIAE AND MECHANISTIC IMPLICATIONS IN BACTERIAL-RESISTANCE TO BETA-LACTAM ANTIBIOTICS

To understand the biochemical basis of resistance of bacteria to beta- lactam antibiotics, we purified a penicillin-resistant penicillin-bind ing protein 2x (R-PBP2x) and a penicillin-sensitive PBP2x (S-PBP2x) en zyme of Streptococcus pneumoniae and characterized their transpeptidas e activities, using a thioester analog of stem peptides as a substrate . A comparison of the k(cat)/K-m values for the two purified enzymes ( 3,400 M-1 s(-1) for S-PBP2x and 11.2 M-1 s(-1) for R-PBP2x) suggests t hat they are significantly different kinetically. Implications of this finding are discussed. We also found that the two purified enzymes di d not possess a detectable level of beta-lactam hydrolytic activity; F inally, we show that the expression levels of both PBP2x enzymes were similar during different growth phases.