Peptidoglycan .Types of Bacterial Cell Walls and their Taxonomic ...

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peptide bond can be substituted by an amide
group (188, 406).
L-Lysine is also a fairly common diamino
acid at position 3. Less frequent are L-OM (109,
167, 292, 301, 302, 427), L,L-Dpm (68, 69, 76,
345, 427), meso-2,6-diamino-3-hydroxy-,Bpimelic
acid (m-HyDpm) (290), and hydroxylysine
(HyLys) (265, 356). Since all these amino
acids possess an additional amino group, they
are an excellent anchoring point for the crosslinking
of the peptide subunits. Indeed, almost
all peptide subunits containing one of
these diamino acids in position 3 are crosslinked
by means of these diamino acids.
In a few bacteria, however, the diamino acid
in position 3 is not involved in the cross-linkage
and remains unsubstituted (129, 343). In the
case of Corynebacterium insidiosum the distant
amino group of the diamino acid, Ldiaminobutyric
acid (L-Dab), is acetylated
(289). In some coryneform bacteria the diamino
acid in position 3 is replaced by a monoamino
acid like L-homoserine (L-Hsr) (286, 291, 335),
L-Ala or L-Glu (81). These types of peptide
subunits are cross-linked in a different way.
Since the amino acid in position 3 contains no
reactive group for forming a peptide bond
(N-acetyl-L-Dab, L-Hsr, L-Ala) or the group is
unreactive (L-Lys, L-Orn, L-Dab, L-Glu),
another trifunctional amino acid must be found
as starting point of the cross-linking. The only
other trifunctional amino acid besides the diamino
acid occurring in the peptide subunit is
D-Glu at position 2. Therefore, the cross-linking
starts in these types at position 2.
Position 4 is almost always occupied by
D-Ala, with very little variation. The carboxyl
group of D-Ala is usually blocked by the interpeptide
bridge, but a portion of the peptide
subunits is not cross-linked (386). In such cases
the C-terminal D-Ala is either split off if D-Ala
carboxypeptidases are present (158), or it remains
substituted by another D-Ala. Therefore,
tri- and pentapeptides can also occur besides
tetrapeptides, whereby the pentapeptide represents
a remainder of the peptidoglycan precursor
(369).
Variation of the mode of cross-linkage.
Most variations of the peptide moiety of the
peptidoglycan do not occur in the peptide
subunit but in the interpeptide bridge and in
the mode of cross-linkage. Ghuysen (109) divided
the peptidoglycans into four different
main types. Since then many new amino acid
sequences of the peptidoglycan have been established
and the knowledge of their biosynthesis
is more complete. We shall use a new
classification system based on the mode of
SCHLEIFER AND KANDLER
cross-linkage and the proposed paths of biosynthesis.
There are two main groups of cross-linkage
called A and B, depending upon the anchoring
point of the cross-linkage to the peptide
subunit. They are divided in subgroups which
carry Arabic figures and are characterized by
the presence or absence of an interpeptide
bridge, the kind of interpeptide bridges, and
their mode of biosynthesis.
The variations within the subgroups reflect
the diversity of the amino acids in position 3 of
the peptide subunit. The variations are marked
by small Greek letters. The variations can be
subdivided into distinct peptidoglycan types
based on the different amino acid sequence of
the interpeptide bridges and on the differences
in the substitution of the a-carboxyl group of
D-glutamic acid. In the following chapter we
shall discuss these different variations.
Besides a short descriptive text, figures and
tables will be given for an easier understanding.
The amino acid sequence of the various subgroups
will be depicted in figures. The tables
contain the kind of variation and all known
types of each variation, together with the name
of the organism in which the structure was first
elucidated and the reference of the fiist description.
Group A: cross-linkage between position 3
and 4. The cross-linkage of group A extends
from the w-amino group of the diamino acid in
position 3 of one peptide subunit to the carboxyl
group of D-Ala in position 4 of another
adjacent peptide subunit. This is the most
common kind of cross-linkage. The first known
example of this group is the directly crosslinked,
m-Dpm containing peptidoglycan of E.
coli (403) which we call variation Aly.
A fragment of a subgroup Al structure (direct
cross-linkage) is depicted in Fig. 6. The amino
group of the D-asymmetric carbon of m-Dpm
forms a peptide bond with the carboxyl group
of D-alanine of an adjacent peptide subunit.
Since there is no interpeptide bridge involved,
this kind of cross-linkage is called "direct
- G-M -G-
BACTERIOL. REV.
L- Ala
I
D - Glu ----.(NH2)
1Y
(NH2 *- m - Dpm *--5- D-Ala
I I
(D-Ala) m-Dpm - (NH2)
FIG. 6. Fragment of the primary structure of a
directly cross-linked, m-Dpm containing peptidoglycan
(Aly).
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