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The equally essential class A and class B HMM PBPs have been linked to processes involved in cell elongation, shape determination and septation, and suppression of these PBPs variously causes cell lysis, cessation of cell division, filamentation and the formation of spherical cells. HMM PBPs are involved in the late stages of PG synthesis with class A HMM PBPs catalyzing both polymerization of the PG precursor-lipid II via transglycosylation and 4–3 crosslinking-transpeptidation, while class B HMM PBPs can only carry out transpeptidation. Generally, gram-positive and -negative bacteria have multiple HMM and LMM PBPs. Furthermore, PBPs have confusing nomenclatures based upon their SDS-PAGE gel migration patterns and are divided into high-molecular-mass (HMM) and low-molecular-mass (LMM) PBPs.
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These enzymes are named penicillin-binding proteins (PBPs), based on their ability to bind penicillin rather than describing their catalytic activity. The periplasmic/extra-cytoplasmic targets of penicillin are a family of enzymes with a highly conserved catalytic activity involved in the final stage of bacterial cell wall biosynthesis: cross-linking of the structural polymer peptidoglycan (PG). Penicillin and the wider family of β-lactams have remained the single most important class of antibiotics since their introduction in the early 1940s.