PBP1b+(mrcB)

=__**PBP1b (mrcB)**__=

PBP1b is a High Molecular Mass (HMM), class A penicillin binding protein, which acts as one of the major transpeptidase-transglycosylases along with PBP1a. As a HMM PBP, PBP1b is responsible for peptidoglycan polymerisation and insertion into the pre-existing cell wall (Sauvage et al, 2008).

The crystal structure of PBB1b from E coli has recently been determined (Sung et al, 2009).
 * __Structure:__**

PBP1b consists of a cytoplasmic tail, a transmembrane anchor and two domains (a transpeptidase and transglycosylase domain) located on the outer surface of the cytoplasmic membrane joined by a beta-rich linker regions (Sauvage et al, 2008). The C-terminal penicillin binding domain of the protein possesses the transpeptidase activity, catalysing the cross-linking of peptides between two glycan strands. As a class A PBP, the N-terminal domain is responsible for the glycosytransferase activity, catalysing the elongation of uncross-linked glycan chains (Sauvage et al, 2008).

PBP1b forms a dimer, and to a lesser extent trimers, in vivo (Born et al, 2006; Vollmer and Bertsche, 2008).

The PBP1b protein exists in three isoforms (alpha, beta and gamma). The only difference between these, is the length of their short cytoplasmic tail and hence their different migration distances during SDS-gel electrophoresis. It is suggested that prehaps PBP1b-alpha could be involved in murein synthesis during cell division, while PBP1b-gamma could synthesise new material for the elongation of the sacculus (Vollmer and Bertsche, 2008).

PBP1b has been shown to form dimers and trimers in vivo (Vollmer and Bertsche, 2008). In addition, it has been shown to bind to PBP3 (Bertsche et al, 2006). It has been demonstrated that PBP1B dimers catalyse the formation of peptidoglycan with a greater efficiency than PBP1B monomers, especially with respect to the transpeptidation reaction (Bertsche et al, 2005).
 * __Interactions in Vivo:__**

E. coli mutants lacking both PBP1a ad PBP1b proteins (the two major transpeptidase-transglycosylase enzymes) have a lethal autolysis phenotype. However, deletion of PBP1a or PBP1b alone is not lethal (Sauvage et al, 2008).
 * __Deletion Mutants:__**

E. coli PBP1b use monomeric, tripeptide, tetrapeptide and pentapeptide as a acceptor in an in vitro transpeptidation reaction assay, with lipid II as the substrate. The activities of PBP1b, with respect to transpeptidation in particular, are highest at conditions favouring dimerisation of the enzyme, implying that is operates as a dimer in vivo. In addition, in the presence of lipid II, both the transglycosylation and transpeptidation reactions ocur simultaneously, which suggests that a PBP1b dimer may form two strands of peptidoglyccan, while similtaneously cross-linking between them (Vollmer and Bertsche, 2008).
 * __Substrates:__**

Unlike PBP1a, PBP1b can form trimeric muropeptide structures.

The average length of glycan strands obtained via polymerisation with PBP1b is >25 disaccharide units, and almost 50% of peptides engage in cross-linking.
 * __Products:__**

The catalytic efficiency of the PBP1b transglycosylase is about 10 times higher than that of PBP1a.
 * __Efficiency:__**