Projects
Expanding Diversity Space Using Carbohydrates
We use chemistry to study the biological mechanisms
of natural products. Currently projects are focused on antibiotics
because resistance to existing antibiotics has become an enormous
problem, and strategies to overcome resistance present interesting
challenges from both a chemical and a biological perspective. Moreover,
antibiotics are useful tools for dissecting biological pathways. The
following projects illustrate some of our current interests:
Glycopeptide Antibiotics
Glycopeptide antibiotics
inhibit bacterial cell wall synthesis by binding to peptidoglycan
precursors. The prototypical glycopeptide antibiotic, vancomycin,
is an important drug that is used to treat a wide variety of Gram-positive
infections. Resistance to vancomycin is becoming a major
problem. We have been studying a class of glycopeptide derivatives
that are able to kill vancomycin resistant strains in order to
elucidate the mechanism of action of these compounds and to understand
the structural basis for biological activity. We are exploring
both chemical and chemoenzymatic approaches to make glycopeptide
derivatives. We use the compounds we make to dissect the
structural features that are important for activity and also to
probe biological pathways involved in bacterial cell death.
Ansamycin Antibiotics
The
ansamycin family of antibiotics includes rifamycin, derivatives
of which are used to treat tuberculosis. Resistant TB infections
have become a major health problem and we would like to understand
the mechanism of action of rifamycin derivatives in order to design
compounds that overcome resistance. Rifamycin and its derivatives
inhibit RNA polymerase and the mechanism of resistance involves
mutations in the polymerase. We have discovered that some
rifamycin derivatives are able to inhibit mutant RNA polymerases
and we are using a combination of chemical synthesis, enzymology,
and structural studies to elucidate the molecular basis for inhibition
by these compounds.
Transglycosylase
Inhibitors
Bacterial transglycosylases
catalyze the polymerization of Lipid II to form the glycan chains
of peptidoglycan. Because they are essential enzymes and
found on the outside of the cell membrane they are important targets
for the development of new antibiotics to treat bacterial infections. Moenomycin
is the only known transglycosylase inhibitor, but its mechanism
of action is not understood in detail. We are developing
synthetic approaches to moenomycin and derivatives for two reasons: One
is that we can use these analogues to evaluate the mechanism of
action of moenomycin; the second is that the construction of moenomycin
presents numerous challenges and we have a longstanding interest
in developing more efficient methods to make glycoconjugates. Moenomycin,
therefore, serves as a model system for testing chemical and enzymatic
strategies for the construction of complicated oligosaccharides.
Outer Membrane Biogenesis
We are interested
in how asymmetry is established and maintained in biological membranes,
and we use the outer membrane of Gram-negative bacteria as a model
system to study this problem. The outer membrane of Gram-negative
bacteria, which serves as a permeability barrier, consists of an
asymmetric bilayer in which the inner leaflet is primarily comprised
of phospholipids and the outer leaflet is primarily comprised of
lipopolysaccharides. Transmembrane proteins span the bilayer
and lipoproteins project from the inner leaflet into the periplasmic
space. We have recently discovered that small molecules that
block periplasmic targets are useful for probing outer membrane
permeability in Gram-negative organisms. Therefore, they
can be used to identify components of the protein machinery involved
in the assembly of the outer membrane. We design genetic
selections against various small molecule antibiotics to report
on outer membrane biogenesis. By mapping mutants that confer
resistance to these molecules, we have been able to identify genes
involved in outer membrane assembly. Biochemical analysis
of the protein products of these genes provides insight into their
functions and their interactions with other components of the machinery. We
hope eventually to elucidate the pathway by which the outer membrane
of E. coli is assembled, and we think that the approach we have
taken to understanding this process of outer membrane assembly
can also be applied to the biosynthesis of other organelles.
Personnel
 |
Daniel Kahne, Ph.D.
Project Leader
kahne@chemistry.harvard.edu |
Daniel Kahne
recently moved to Harvard University from Princeton University
where he was on the faculty for 16 years. He holds
appointments in the departments of Chemistry and Chemical
Biology (CCB) and Biological Chemistry and Molecular Pharmacology
(BCMP). He trained as a synthetic organic chemist with
Gilbert Stork and continued postdoctoral training at Columbia
with Clark Still.
He has longstanding interests in the chemistry
and biology of natural products, and in recent years has
become interested in how natural products can be used to
probe cellular pathways. Professor Kahne’s research
group is divided into students who develop synthetic methods
to make and modify complex natural products, and students
who combine some chemistry with molecular and cellular biology
to address questions relating to how various natural products
function. In the last five years, the Kahne group has
become interested in antibiotic resistance, and has made
significant contributions to understanding the mechanisms
of action of glycopeptide antibiotics and derivatives that
overcome glycopeptide resistance. The Kahne group has
also been using glycopeptide derivatives and other antibiotics
in conjunction with genetics to probe pathways involving
cell wall biosynthesis and outer membrane biogenesis.
Kahne
Lab Home Page
|
|
