October 21, 2002
Researchers Determine How "Hospital Staph" Resists Antibiotics
Structural studies of a key enzyme have revealed
how dangerous strains of the bacterium,
Staphylococcus aureus
, become
resistant to antibiotics.
Resistant strains of
Staphylococcus aureus
, which are also called
"hospital staph" because of their prevalence in hospitals, constitute
34 percent of the clinical isolates in the United States, more than 60 percent
in Japan, Singapore and Taiwan, and more than 50 percent in Italy and Portugal.
And the emergence of strains of
Staphylococcus
that are resistant to
vancomycin -- the antibiotic of last resort -- makes public health concerns
about drug- resistant strains of the bacterium even more urgent.
“A computer-generated model of the penicillin-binding protein 2a from the bacterium,
Staphylococcus aureus
.”
In an article published online on October 21, 2002, in the journal
Nature
Structural Biology
, Daniel Lim and Natalie Strynadka, who is a Howard
Hughes Medical Institute international research scholar, reported structural
studies of the enzyme known as penicillin-binding protein 2A (PBP2a). Lim and
Strynadka are at the University of British Columbia.
Before the advent of drug-resistant strains of
Staphylococcus aureus
,
staph infections were treated using beta-lactam antibiotics such as
methicillin, which block the bacterial enzyme PBP. This enzyme -- called a
transpeptidase -- normally catalyzes the cross-linking of structural molecules
in the bacterial cell wall. Blocking PBP with methicillin weakens the cell
wall, which ultimately bursts, killing the bacterium.
However, a methicillin-resistant strain of the bacteria has evolved that has
acquired the gene for a new version of PBP -- PBP2a --from another bacterium.
The challenge, as well as the opportunity, said Strynadka, is to understand why
PBP2a is resistant to beta-lactam antibiotics.
"What is very attractive from a therapeutic point of view is that PBP2a
constitutes a single target, in terms of developing new antibiotics that can
overcome this resistance," she said.
To understand the detailed structure of PBP2a, Lim produced a version of the
enzyme that lacked a segment that anchored it to the cell membrane, but which
retained the enzymes catalytic activity. Eliminating the anchoring segment
rendered the protein soluble, so that the researchers could crystallize the
protein for use in x-ray crystallography studies. In x-ray crystallography,
researchers direct an x-ray beam through crystals of a protein to deduce its
structure by analyzing the pattern of diffraction that is produced. Analysis by
Lim and Strynadka revealed critical differences between the structures of PBP2a
and other beta-lactam antibiotic sensitive PBPs.
"By comparing the native enzyme with previously known structures of
transpeptidases, we came to understand that PBP2a had evolved distortions of
the active site that prevent an effective reaction with the antibiotic,"
said Strynadka. An enzymes active site is the pocket within which the enzyme
carries out its catalytic reaction. In the case of PBP2a, this catalytic
reaction drives the essential cross-linking of cell-wall proteins in the
bacterium.
"Although beta-lactam-sensitive bacteria still have a number of these
normal transpeptidases, they also have PBP2a, which because of its distorted
active site doesnt react easily with the antibiotic," said Strynadka.
"Thus, PBP2a can produce sufficient cross-linking in the cell wall so that
the bacterium survives."
The researchers studies showed that PBP2a is different from normal PBPs
throughout its structure, and not just at the active site. This suggests that
the distorted active site is an integral part of the enzyme, said Strynadka.
The good news is that the PBP2a active site structure has unique features which
can be used to design new types of antibiotics that block its resistance
activity.
"The active site of PBP2a is quite extended and relatively
hydrophobic," said Strynadka. "The structures we observe now allow
for the rational design of specific PBP2a inhibitors that are tailored to better
fit these features of the PBP2a active site allowing better affinity and inactivation
of the enzyme."
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