PBP1a+(mrcA)

__**PBP1a (mrcA)**__
PBP1a is a High Molecular Mass (HMM), class A penicillin binding protein, which comprises one of the main enzymes responsible for peptidoglycan biosynthesis. Along with PBP1b, it acts as one of the major peptidoglycan transpeptidase-transglycosylases and is responsible for the majority of peptidoglycan polymerisation and insertion into the pre-existing cell wall (Sauvage et al, 2008; Born et al, 2006).

PBP1a consists of a cytoplasmic tail, a transmembrane anchor and two enzyme domains (a transpeptidase and a transglycosylase) located within the periplasm. As a result most of the enzyme is situated within the periplasm, the site of the reactions which is catalyses (Born et al, 2006). The transmembrane portion of the protein is comprised of an alpha helix, and functions to anchor the protein to the cytoplasmic membrane.
 * __Structure:__**

The transpeptidase activity of the enzyme is located at the C-terminus, and this is responsible for catalysing the cross-linking of peptides between two glycan strands. Meanwhile, the N-terminal domain is responsible for the glycosytransferase activity, catalysing the elongation of uncross-linked glycan chains. The transpeptidase and transglycosylase domains are joined by a beta-rich linker region (Sauvage et al, 2008).

PBP1a proteins form homodimers in vivo, but do not form heterodimers with PBP1b proteins, indicating both serve distinct, unique functions (Born et al, 2006; Vollmer and Bertsche, 2008).

__**Deletion Mutants:**__ Deletion of the mraA gene coding for PBP1a has no efffect on cell growth, morphology or the sensitivity of the cell towards beta-lactam antibiotics (Vollmer and Bertsche, 2008). However, E. coli mutants lacking both PBP1a ad PBP1b proteins (the two major transpeptidase-transglycosylase enzymes) have a lethal autolysis phenotype.

Furthermore, PBP1b deficient mutants, but not PBP1a deficient mutants, lose cell integrity upon inactivation of PBP2, PBP3 or the cell division protein FtsQ (Born et al, 2006; Volllmer and Bertsche, 2008).

__**Location:**__ Most of the enzyme, including the transglycosylase and transpeptidatse domains, lies in the bacterial periplasm (Born et al, 2006).

__**Substrates:**__ E. coli PBP1a acts as a transglycosylase enzyme, catalysing the polymerisation of lipid II monoers to form glycan strands.

PBP1a catalyses the transpeptidation reaction between glycan strands, previously formed from transglycosylation of lipid II. It can not cross link individual lipid II molecules, and this is supported by the fact that the transpeptidase activity is dependent on transglycosylase activity. Indeed, PBP1a mutants which have a non-functional peptidase domain can still function as transglycosylases, but PBP1a mutants with non-functional glycosylase domains can't function as transpeptidases (Born et al 2006).

PBP1a will catalyse the cross-linking between pentapeptides in two newly synthesised glycan strands, or between pentapeptides in a newly synthesised glycan strand, and monomeric tri-, tetra- and pentapeptides in the mature peptidoglycan strand (Born et al, 2006).

use monomeric, tripeptide, tetrapeptide and pentapeptide as a acceptor in an in vitro transpeptidation reaction assay, with lipid II as the substrate.

__**Products:**__ The average length of glycan strands obtained via polymerisation with PBP1b is 20 disaccharide units, and 18-26% of peptides engage in cross-linking. PBP1a also catalyses the attachment of nascent peptidoglycan to the mature cell wall via transpeptidation.

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