section 11.2
Peptidoglycans
191
NAc
ALTERNATE PATHWAY
H .C O f ë )
H C O H
H C O ( § )
ß-Hexosam inidase A
5
Y
NAc
FIGURE 11-13
Stepwise degradation of keratan sulfate. The deficiency diseases
corresponding to the numbered reactions are: 1 = MPS IV A, Morquio
syndrome type A; 2 = MPS IVB, Morquio syndrome type B; 3 = MPS
III D, Sanfilippo’s syndrome type D; 4 = Sandhoff’s disease; and 5 =
Tay-Sachs and Sandhoff’s disease. The alternate pathway releases intact
N-acetylglucosamine-6-sulfate, a departure from the usual stepwise
cleavage of sulfate and sugar residues. [Reproduced with permission
from E.F. Neufeld and J. Muenzer: In
M eta b o lic B a sis o f In h erited
D isease,
7th ed., C. R. Scriver, A. L. Beaudet, W. S. Sly, and D. Valle
(Eds). McGraw-Hill, New York, 1995, p. 2468.]
structure similar to that of eukaryotes and a thick, bag-
shaped cell wall. The cell wall, about 25 nm wide, consists
of peptidoglycans (Figure 11-14) and polyol phosphate
polymers known as teichoic acids. Gram-negative bacte-
ria (e.g.,
Escherichia coli
and
Salmonella typhimurium)
do not contain teichoic acids but have an outer mem-
brane system external to the plasma membrane and the
peptidoglycan. The constituents of this second outer
membrane system are phospholipids, lipopolysaccharides,
and proteins. The peptidoglycan layer is connected to the
outer layer via lipoprotein. The region between the in-
ner membrane and the peptidoglycan layer is known as
the periplasmic space. This space contains enzymes and
other proteins that digest impermeable nutrients and trans-
port low-molecular-weight nutrients, such as amino acids,
sugars, and specific ions.
Lipopolysaccharides are complex amphipathic mole-
cules located on the outer leaflet of the outer membrane.
Their structure in Enterobacteriaceae comprises three re-
gions: a phospholipid (A) (the hydrophobic portion of the
molecule), a phosphorylated core oligosaccharide, and a
polysaccharide chain known as the
0
side chain or
0
anti-
gen. The core oligosaccharide and the 0 side chain re-
gions contain several unusual sugars and form the hy-
drophilic portion of the molecule that projects outward.
Lipopolysaccharides serve as a barrier against invading
organisms, are highly toxic and are also known as endo-
toxins. The fever that occurs in gram-negative bacterial
infections is caused by endotoxins.
The outer membrane also contains a protein known as
porin.
The trimeric forms of porin form channels for the
passage of small ionic molecules through the outer mem-
brane. Other transport systems exist for higher molecular
weight ionic substances.
Both gram-negative and gram-positive bacteria contain
peptidoglycan as the main structural component of their
cell walls. However, gram-negative bacteria have only a
single layer of peptidoglycan, whereas gram-positive cells
have several layers with cross-linkages between them. The
polymeric structure of peptidoglycan surrounds the entire
inner membrane and the cytoplasm of the cell. Consisting
of a glycan composed of amino sugars in one dimension, a
cross-linked peptide moiety in the second dimension, and
an interpeptide bridge in the third dimension, peptidogly-
can projects in different planes (Figure 11-14). This three
dimensional, covalently bound network offers consider-
able resistance to outward pressure and confers mechani-
cal stability on the cell wall.
The glycan is called
murein
(from the Latin word
for “wall”) and is a polymer of alternating units of N-
acetylglucosamine (GlcNAc) and N-acetylmuramic acid
(MurNAc). The sugar residues are linked by /3(1
4)
glycosidic bonds (Figure 11-15). N-Acetylmuramic acid
is the 3-O-D-lactic acid ether of N-acetylglucosamine,
the carboxyl group of its lactic acid side chain be-
ing condensed with the cross-linking peptide. This pep-
tide usually consists of four amino acid residues that
are alternately L- or D-amino acids. The bacterial cell
wall is one of the few structures containing a signif-
icant amount of D-amino acid residues. In
S. aureus,
