Penicillin-Binding Proteins in Pathogens: Characterizations of Catalytic Properties in Antibiotic Resistance

Treponema pallidum is the causative agent of syphilis and is exquisitely sensitiveto penicillins and other §-lactam antibiotics. The Tp47, the novel 47 kDa membrane-boundlipoprotein is abundant penicillin-binding protein (PBP) in T. pallidum.Interestingly its X-ray structure reveals that it is distinct from any known PBPs or §-lactamases. Our studies further revealed that Tp47 hydrolyzes the §-lactam bonds ofpenicillins. The reaction is unique and represents a bona fide novel mechanistic strategyin turnover of these antibiotics. The activity of Tp47 as a §-lactamase is held back by asignificant inhibition of the activity by the products of turnover, accounting for thesensitivity of T. pallidum to penicillins. The penicillin-binding protein activity and the §-lactamase reaction take place at different active sites in this protein, with the latterreaction proceeding at a rate of over 2000-fold faster than that of the former.Staphylococci have evolved to produce both §-lactamases and PBP 2a asresistance mechanism. The expression of these two proteins to have resistant phenotypeis regulated by BlaR1 and MecR1. BlaR1 is the sensor-transducer protein that binds with§-lactam antibiotics on the cell surface (C-terminus) and transduces the signal into thecytoplasm. The C-terminal domain of BlaR1 (BlaRS) is related to class D §-lactamases. The carboxylation on active site lysine is important for the activation of serine as anucleophile. BlaRS has slow deacylation step, which exhibits the characteristic of PBPs.BlaRS differs from class D §-lactamases by exhibiting barrierless N§-decarboxylation inthe event of acylation, which traps acyl-enzyme complex. The recarboxylation isprevented by interaction between the side chain of decarboxylated, free lysine andasparagine. The significant conformational changes of BlaRS are observed by circulardichroism and FT-IR upon interaction with §-lactams, which may indicate criticalinvolvement in signal transduction. The C-terminal surface domain of MecR1 (MecRS)has also been extensively studied. This protein also possesses N§-carboxylated lysine inthe active site. Unlike its homolog BlaRS, the kinetics of acylation of MecRS issubstantially defective, which may explain the reason why mec regulating system isdiminishing from clinical strains that exhibit high resistance level against drugs.