192
chapter 11
Heteropolysaccharides II: Proteoglycans and Peptidoglycans
FIGURE 11-14
Structure of peptidoglycan in the cell wall of the gram-positive bacterium
S ta p h ylo co ccu s a u reu s.
The structure consists
of an alternating polymer of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) linked by /3( 1
-> 4)
glycosidic bonds in one dimension, a cross-linking peptide that contains alternating L- and D-amino acid residues in the
second dimension, and a pentaglycine moiety that cross-links the peptide chains in the final dimension. This three-
dimensional network is continuous and surrounds the entire cell. [Reproduced with permission from M. J. Osborn, Struc-
ture and biosynthesis of the bacterial cell wall.
A n n u . Rev. B io ch em .
38, 301 (1969). ©1969 by Annual Reviews Inc.]
the tetrapeptide sequence is L-alanyl-D-y-isoglutaminyl-
L-lysyl-D-alanine. The interpeptide chain is a pentaglycine
chain that connects the terminal D-alanyl residue of one
tetrapeptide to an L-lysyl residue in the third amino
о
н
II
I
- C — D-Ala— L -L y s — D-isoGIn— L-AIa— N
Gly
I
Gly
G jy
Gly
I
Gly
N — H
\
/
K
HO
r-CH OH
/
/
\r
^OH
IGIcNAcI
,CH,
Lady!
NH
C H
V
/
^CH,
P(1—-4) linkage
C H
OH
/ I
A
'p H C H ^
u
[MurNAcl
i*
° = <
t
C H ,
b
FIGURE 11-15
Repeating unit of the peptidoglycan of
S ta p h ylo co ccu s aureus.
acid position in another tetrapeptide (Figure 11-14). The
chemical linkages are as follows: the amino-terminal
group of pentaglycine is joined with the carboxyl group of
D-alanine, and the carboxyl terminal of pentaglycine is
linked with the £-NH
2
of L-lysine. Both of the sequence
variations of the tetrapeptide and the cross-linking pattern
are characteristic of the shape and species of the bacterium.
In some bacteria, in addition to the cell wall, sub-
stances form capsules or slime layers external to the
cell wall. These layers are not essential for growth and
multiplication but may be important for survival of the
organism in harsh environments, e.g., in preventing des-
iccation and serving as a barrier against phage (bacterial
virus) attack. The capsular layer also provides a charged
surface.
Streptococcus mutans,
which plays a significant
role in dental caries and plaque formation, produces an
extracellular 1,3-glucan that enables the bacteria to ad-
here to teeth. In the formation of dental caries, possi-
bly the most widespread human pathological process,
dietary carbohydrates (sugar in general and sucrose in
particular) promote bacterial growth and produce noxious
metabolites.
Penicillins prevent cell wall synthesis in susceptible or-
ganisms by inhibiting a late step in the enzymatic synthesis
of peptidoglycan (Chapter 16).
