The architecture of peptidoglycan
Most bacteria contain in their envelope a peptidoglycan (or “murein”) layer which protects the cell from rupture as a result of osmotic pressure (turgor) of the cytoplasm and which maintains the specific shape (sphere, rod, spiral or other) of the bacterial cell. Although the chemical composition varies in different species, peptidoglycan is always made of glycan strands that are crosslinked by short peptides. Characteristic structural features include N-acetylmuramic acid (MurNAc), d-amino acids and unusual amide bonds. Many important questions on the structure and growth of the sacculus remain unanswered. Of particular interest is the architecture of peptidoglycan, the knowledge of which is needed to understand the mechanism(s) of enlargement of the sacculus in growing and dividing cells.
A crucial feature of the architecture of peptidoglycan is the orientation of the glycan strands and peptides with regard to surface and axis of the cell. However, the large size, structural heterogeneity and flexibility of the peptidoglycan sacculi preclude crystal formation and hence determination of the crystal structure. Furthermore, the glycan strands and peptides cannot be seen by conventional electron microscopy because their dimensions are of the same scale as the inevitable staining and fixation artifacts. Thus, current models of the architecture are based on chemical and biophysical data which, however, are limited to just a few species. Two mutually exclusive models are currently being discussed: the classical “layered” model in which both the glycan strands and peptide crosslinks run parallel (horizontal) to the cytoplasmic membrane, and the “scaffold” model in which the glycan strands run perpendicular to the membrane and the peptide crosslinks run parallel to the membrane. The existence of these fundamentally different models and the arguments about them have had one very positive effect: structural biologists are beginning to regain their interest in this problem utilizing current technologies and new methods.
In 2008, two important articles on the architecture of peptidoglycan were published. The first describes the architecture of sacculi isolated from Gram-negative species analyzed by electron cryotomography. The second reports on the architecture of peptidoglycan isolated from the Gram-positive, rod-shaped Bacillus subtilis as observed by atomic force microscopy (AFM). Although the new data support a layered architecture in Gram-negative bacteria, the situation appears to be more complex in B. subtilis. The data suggest that neither of the two models apply. This article summarizes the state of knowledge of peptidoglycan, discusses current models for Gram-negative and Gram-positive bacteria and notes the many unanswered questions about the architecture of peptidoglycan which remain.
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Tags: Bacteria, Biology, Microbiology, Science
