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Introduction
Lysins are
peptidoglycan hyrdolases encoded by bacteriophage. They are
utilized late in the phage lifecycle to release progeny phage
from the host cell. Lysins have potential as alternatives to
antibiotics due to their bacteriolytic activity. The catalytic
domains and cell-wall binding domains of the lysins allow for
specificity. The cell wall binding domain is especially
important in this regard as it is able to distinguish between
discrete epitopes on the cell wall such as carbohydrates or
teichoic acids, accounting for the species or strain specific
activities of a lysin. It is even possible to combine the
catalytic domain of one lysin with the cell wall binding domain
of another in order to make a new enzyme with altered
specificity or activity. Lysins have been shown to be capable of
total bacterial lysis in vitro of Gram-positive bacteria within
seconds when added in their purified form.
PlyC is a
lysin with amidase activity encoded by the streptococcal
bacteriophage C1. PlyC is the most potent bacteriophage-derived
enzyme studied to date requiring only nanogram quantities to
affect their target bacteria. This unique lysin is a combination
of two gene products. A single PlyCA catalytic domain is coupled
with a multimeric PlyCB binding domain. The structure of PlyCB
is displayed in this JMol tutorial.
PlyCA, the
catalytic domain (not shown in this JMol tutorial), functions as
a cysteine and histidine-dependent amidohydolase/peptidase.
Though PlyC shows no similarities with other lysin proteins, it
contains a conserved CHAP domain, which is common among many
amidase proteins. PlyC functions by cleaving the amide bonds
between N-acetyl muramic acid and L-alanine on the streptococcal
peptidoglycan. Specifically Cys-333 and His-420 are responsible
for lytic activity in PlyC. Point mutations on these residues
(Cys-333 to serine and His-420 to alanine) halted all lytic
activities within PlyC.
The interaction between
adjacent subunits
PlyCB is a multimeric structure
composed of eight identical subunits (homo-octomer). The
interaction of adjacent subunits is represented by the
highlighted residues. Unlike most lysins, PlyC has eight
peptides that make up the binding domain (PlyCB) and one peptide
that makes up the catalytic domain (PlyCA, not seen in
structure). This 8:1 ratio, found in the PlyC holoenzyme
(catalytically active form), is completely unique to PlyC. PlyC
is the only known multimeric lysin composed of two distinct gene
products. In comparison, most other identified lysins are single
gene products functioning as a single polypeptide. This unique
characteristic of PlyC may attribute to its lytic activity, as
it is more than 200 times as active compared to other lysin
enzymes.
Charge/polarity distribution on
surface of PlyCB
Highlight hydrophobic (khaki) and hydrophilic
(blue) residues
Highlight charged residues
(positive,
negative)
One surface of the PlyCB has a
distinctive charged surface created largely by charged residues
at the carboxyl end of each of the peptides. In contrast,
the opposite surface has a hydophilic surface without an
abundance of charged residues.
View protein topology of single subunit
Each subunit is composed of two
α-helices
and four chains forming a
β-sheet.
The β-sheet
is composed of three parallel and one anti-parallel chains. A single
peptide of the PlyCB structure is shown here in cartoon view to highlight the
topology and directionality of the chains.
Model Designs
Wireframe model
protein backbone 300, amino acid sidechains wireframe 250.
lightblue subunit will be removable and held with magnets.
Spacefill model
Spacefill "on", close to VDW surface. Colored based on chemical property
of residues (hydrophobic,
hydrophilic,
positive,
negative)
Tutorial and scripts prepared by
Mac Cordrey, Bozhena Lisko, and
Will Pinke
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