Biochemistry and Comparative Genomics of SxxK Superfamily Acyltransferases Offer a Clue to the Mycobacterial Paradox: Presence of Penicillin-Susceptible Target Proteins versus Lack of Efficiency of Penicillin as Therapeutic Agent

Abstract

The bacterial acyltransferases of the SxxK superfamily vary enormously in sequence and function, with conservation of particular amino acid groups and all-α and α/β folds. They occur as independent entities (free-standing polypeptides) and as modules linked to other polypeptides (protein fusions). They can be classified into three groups. The group I SxxK d,d-acyltransferases are ubiquitous in the bacterial world. They invariably bear the motifs SxxK, SxN(D), and KT(S)G. Anchored in the plasma membrane with the bulk of the polypeptide chain exposed on the outer face of it, they are implicated in the synthesis of wall peptidoglycans of the most frequently encountered (4→3) type. They are inactivated by penicillin and other β-lactam antibiotics acting as suicide carbonyl donors in the form of penicillin-binding proteins (PBPs). They are components of a morphogenetic apparatus which, as a whole, controls multiple parameters such as shape and size and allows the bacterial cells to enlarge and duplicate their particular pattern. Class A PBP fusions comprise a glycosyltransferase module fused to an SxxK acyltransferase of class A. Class B PBP fusions comprise a linker, i.e., protein recognition, module fused to an SxxK acyltransferase of class B. They ensure the remodeling of the (4→3) peptidoglycans in a cell cycle-dependent manner. The free-standing PBPs hydrolyze d,d peptide bonds. The group II SxxK acyltransferases frequently have a partially modified bar code, but the SxxK motif is invariant. They react with penicillin in various ways and illustrate the great plasticity of the catalytic centers. The secreted free-standing PBPs, the serine β-lactamases, and the penicillin sensors of several penicillin sensory transducers help the d,d-acyltransferases of group I escape penicillin action. The group III SxxK acyltransferases are indistinguishable from the PBP fusion proteins of group I in motifs and membrane topology, but they resist penicillin. They are referred to as Penr protein fusions. Plausible hypotheses are put forward on the roles that the Penr protein fusions, acting as l,d-acyltransferases, may play in the (3→3) peptidoglycan-synthesizing molecular machines. Shifting the wall peptidoglycan from the (4→3) type to the (3→3) type could help Mycobacterium tuberculosis and Mycobacterium leprae survive by making them penicillin resistant

Crystal Structures of Complexes of Bacterial dd-Peptidases with Peptidoglycan-Mimetic Ligands: The Substrate Specificity Puzzle

“…It seems now quite well established that the essential crosslinking reaction is catalyzed by a high-mole-cular-mass (60-100 kDa) group of DD-peptidases, while a lower-molecular-mass (40-60 kDa) group appears to have in vivo only DD-carboxypeptidase and DD-endopeptidase activities. 2,3 In vitro, however, several members of the latter group are also able to catalyze transpeptidation reactions with small peptides and esters. 3,4 The enzymatic activity of these enzymes has recently been reviewed.…”

“…2,3 In vitro, however, several members of the latter group are also able to catalyze transpeptidation reactions with small peptides and esters. 3,4 The enzymatic activity of these enzymes has recently been reviewed. 5 In view of their in vivo role as DD-peptidases, one would anticipate that these enzymes would exhibit substrate specificity towards peptides that are analogous in structure to oligopeptide elements of peptidoglycan.…”

The Basis for Resistance to β-Lactam Antibiotics by Penicillin-binding Protein 2a of Methicillin-resistant Staphylococcus aureus

“…The gene product of mecA is a penicillin-binding protein (PBP) designated PBP2a. S. aureus normally produces four PBPs (3), enzymes that are anchored on the cytoplasmic membrane, the functions of which are the assembly and regulation of the latter stages of the cell wall biosynthesis (4,5). Whereas these four PBPs are susceptible to modification by ␤-lactam antibiotics, an event that leads to bacterial death, PBP2a is refractory to the action of all available ␤-lactam antibiotics.…”

Diversity of Penicillin-binding Proteins

“…We show that FmtA ⌬27 is capable of binding to peptidoglycan in vitro with high binding affinity. It also appears that FmtA is a membrane-associated protein, with its C terminus serving as a membrane anchor, a feature that is common for low molecular weight and membrane-bound PBPs with transpeptidase activity (48,59). In light of our findings that FmtA forms acyl-enzyme species with ␤-lactams, it is conceivable that FmtA could be a carboxypeptidase or a transpeptidase.…”

“…It is of note that the second conserved residue in the SXN loop in FmtA is occupied by Asp-213. Typically, this position is occupied by Asn in PBPs and ␤-lactamases, but the presence of other amino acids such as Val, Ser, or Gly has also been reported (59). The Asn side chain forms hydrogen bonds with the active-site residues and the substrate (60,61), which can also be facilitated by a protonated aspartate, as in the case of FmtA.…”