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Zachary T. Ball research summary and research interests
July, 2012
1. Introduction
Polypeptides are the biological
solution to ligand design for transitionmetal catalysis. Metal cofactors within
protein folds achieve reactivity that is
impossible with current traditional
transition-metal catalysts. Efforts to
understand enzyme attributes and apply
them to new, designed reactivity have
been extensive: work by numerous
groups has resulted in tremendous
capabilities to build new metalloproteins
with novel structure and spectroscopy.
However, designing new and useful Fig. 2. Functional metallopeptides: (Path A) Proximity-driven catalytic
reactivity and catalytic function remains protein modification; (Path B) Cooperative inhibition of protein–
a daunting challenge. We have focused protein interactions with a hybrid organic-inorganic approach.
on designing metallopeptides with new
catalytic capabilities that combine features of enzymes and traditional transition-metal catalysts. One
crucial feature of biological catalysis that we have sought to emulate is the ability to override inherent
chemical reactivity, performing site-specific chemistry in a functional-group-rich environment. These
broad goals have brought us to study rhodium(II) complexes with peptide ligands. From a practical
perspective, peptides as ligands for transition-metal catalysis facilitates (A) efficient screening of ligand
diversity and (B) design of large-length-scale
(nanometer) structure necessary for molecular
recognition.
A fundamental concept of our work in this area is
a peptide as a molecular recognition element to
deliver a rhodium center to a specific site in a complex
environment (Fig. 2). Localization allows both catalysis
(path A) and protein inhibition stabilized by metalligand interactions with peripheral side chains (path B).
2. Metallopeptide synthesis and structure
Our lab has developed methods for direct
metalation of carboxylate side chains of fully
deprotected peptides with reactive rhodium(II)
complexes (Fig. 1a, pub 20†). The resulting rhodium
metallopeptides are stable in serum and are readily
purified by RP–HPLC. We have developed protectinggroup methods for synthesizing complex target
structures with multiple carboxylates (pub 31) and
demonstrated the ability of rhodium metalation to
control helical structure and peptide assembly (Fig.
1b,c, pubs 20,24).
3. Metallopeptides for protein modification
Fig. 1. (a) Synthesis of a rhodium metallopeptide.
(b,c) Rhodium metalation and de-metalation
controls peptide structure and assembly.
†
Chemical protein modification is an important and
unsolved problem that would create valuable tools for
chemical biology and facilitate the development of
next-generation protein drugs. The natural world
Publication numbers (e.g. “pub 24”) refer to numbering on my CV and on the group website (http://ztb.rice.edu/publications.html).
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Z. T. Ball
Fig. 3. Site-specific modificaiton of polyfunctional peptides based on a proximity-driven mechanism. Reaction is
possible at many side chains X that are unreactive with simple small-molecule catalysts (pubs 25, 28, 33).
solves the problem of protein modification with enzymes: biological catalysts capable of site-specific
modification because they achieve selectivity on the basis of molecular shape, allowing reactions at
“stable” functional groups in the presence of inherently reactive ones. Chemical catalysts, on the other
hand, are largely limited to
selectivity on the basis of inherent
reactivity. Our approach to “nontraditional” selectivity is based on
proximity-accelerated
catalytic
reactivity.
Dirhodium
metallopeptides
use
peptide–
protein supramolecular assembly to
localize the dirhodium catalyst at a
specific protein. The dirhodium
center then catalyzes covalent
functionalization of a protein of
interest with a diazo reagent. In a
series of papers laying out the
concept, we discovered that the
proximity-driven concept delivers
remarkable
shape-selectivity,
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affording >10 rate enhancement
over background reactivity (pub 25)
in initial studies of coiled-coil
assemblies (Fig. 3). The true
potential of this approach was
revealed upon discovering that
many other amino acids—wholly
unreactive with Rh2(OAc)4—are
efficient substrates for proximitydriven modification (Fig. 3, pub 28).
Fig. 4. Site-specific biotinylation of (a) recombinant MBP and (b)
different proteins in lysate based on catalyst choice (pubs 30,33). (c)
The natural Fyn SH3 domain (orange) is biotinylated selectively with a
metallopeptide designed from natural peptide ligands (red) (pub 33).
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Proximity-driven
side-chain
modification is a designed catalytic
reaction that exhibits many
Z. T. Ball
Fig. 5. Stabilization of a coiled-coil dimerization with
reversible coordination (pub 35).
features of enzymatic catalysis. For example, we
demonstrated that it is possible to perform
selective chemistry on glutamine or glutamate
residues in the presence of tryptophan residues.
Even though the inherent reactivity of
tryptophan is more than an order of magnitude
higher, we obtained 45:1 selectivity in favor of
glutamine modification, based on sequence
recognition (pub 28).
A new tool for site-specific protein
modification. Initial peptide work has been
extended to modifying whole proteins. In recombinant systems, we developed recognition elements
that allow orthogonal modification of multiple proteins in lysate by judicious choice of catalyst (Fig. 4a,b,
pubs 30, 33). The ability to modify specific proteins in lysate is a remarkable and enabling attribute that
speaks to the utility and robustness of our methods.
These results have been extended to entirely natural protein sequences. Modification of the c-Fos
bZip (pub 28) was achieved with a metallopeptide based on its natural binding partner, Jun.
Modification of different residues along the c-Fos sequence was possible by changing the
metallopeptide design. More recently, we demonstrated selective modification of the Fyn SH3 domain
(Fig. 4c, pub 33). SH3 domains are an entirely different peptide recognition fold that is present in a
number of medically relevant protein interactions.
4. Medicinal organometallic chemistry: hybrid inhibitors for “undruggable” targets
The other major goal of localization-induced metal function has been to engineer reversible metalligand with peripheral histidine, methionine, or cysteine near the binding site (Fig. 2, Path B). Many
protein-protein interactions are potential therapeutic targets, but are considered “undrugable” because
they feature shallow binding clefts and generally weak, transient associations with binding partners. By
combining traditional shape-selective organic ligands with reveresible coordination to side-chain Lewis
bases, such as histidine or methionine, we develop inhibitors for
“undruggable” interactions that are both potent and specific. We
initially reported rhodium-based stabilization of coiled coils (Fig.
5, pub 35), and have recently expanded the concept in reporting
the first sub-micormolar inhibitor of the CFTR-associated ligand
(CAL), an important therapeutic target for cystic fibrosis (Fig. 6,
pub 34). This approach has allowed us to demonstrate nanomolar
inhibition of NHERF1, another PDZ-containing protein of
therapeutic interest (Fig. 6), and we have received collaborative
funding to extend these successes to new classes of
therapeutically relevant protein–protein interactions, including
SH3 and SH2 domains.
5. Small-molecule catalysis with metallopeptides.
Because chiral ligand design remains a largely empirical
exercise, methods to synthesize catalyst libraries efficiently and in
parallel facilitate the search for optimal structure. In this regard,
polypeptides an ideal platform for ligand discovery. Following an
initial report of catalysts for asymmetric Si–H insertion (Fig. 7, pub
27), our more recent efforts have focused on the role of axial
ligands, peptides with non-traditional axial–equatorial
coordination modes, and high-throughput catalyst discovery
methods (pubs 29, 36). With on-bead catalyst screening, an entire
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Fig. 6. Inhibitors of CFTR regulatory
proteins based on a natural ligand
(in green). CALP stabilization can be
pinpointed to His301 (pub 34). N1P1
data is unpublished.
Z. T. Ball
Fig. 7. (a) High-throughput, on-bead screens for functional rhodium metallopeptides. (b) Enantioselective
cyclopropanation with different peptide ligands delivering both product enantiomers. (c) Catalyst structures that
allow access to either product enantiomer from natural L-amino acids.
96-well plate of catalyst candidates can be synthesized and screened within one week. Present studies
are extending high-throughput analysis to other transition-metal catalyzed reactions and bringing
molecular recognition ideas to bear on small-molecule targets.
6. Copper catalysis and other research
The group has developed novel copper catalysts for C–Si and C–H activation (Fig. 8). For C–Si
activation, stable copper(I) fluoride complexes efficiently transmetalate with silanes to afford
organocopper compounds, in the first direct evidence of copper–silicon transmetallation from simple
sp2 silanes (pub 18). This discovery has been extended to the development of catalytic methods
employing silane activation (pubs 22,
23).
For
C–H
activation,
we
demonstrated a radical method based
on atom-transfer ideas to achieve
remote functionalization of unactivated
sp3 C–H bonds (pub 26). We have also
received funding for two separate
projects involving the synthesis of small
molecules (pub 19) and responsive
polymers for upstream oil research.
Fig. 8. Cu-catalyzed methods for C–Si bond activation (eqn 1, pubs
7. Ongoing and Future Work
18,22,23) and remote alkyl C–H functionalization (eqn 2, pub 26).
My group works at the union of inorganic, organic, medicinal, and biological chemistry. In the area
of medicinal inorganic chemistry, the PI was recently funded (NSF AGEP–GRS and Simmons Family
Foundation) to pursue SH2 domains as drug targets in collaboration with clinical oncologists (Tweardy,
Redell). In addition, we have a funded collaboration (Ladbury) to understand and inhibit SH3
interactions of the Grb2 pathway. In these studies, we are moving beyond metallopeptides to examine
other metal-organic conjugates with more drug-like structures. In NSF-funded work on protein
modification, we are interested in protein chemistry for the creation of modified protein therapeutics
and are pursuing proximity-driven modification as a tool for chemical biology to understand and identify
weak, transient protein-protein interactions. In the area of small-molecule catalysis, we are focused on
developing peptides as a general ligand platform for selective and diverse transition-metal catalysts,
including chromium, palladium, and nickel. We are interested in next-generation peptide ligands that
bring molecular recognition ideas from our protein work into small-molecule catalysis.
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