Biomedical subjects

Charles M Forsyth

Publications and source records attributed to Charles M Forsyth.

4 recordsLinked to original sources

PubMed2006

Molecular evolution of antibody cross-reactivity for two subtypes of type A botulinum neurotoxin.

Broadening antibody specificity without compromising affinity should facilitate detection and neutralization of toxin and viral subtypes. We used yeast display and a co-selection strategy to increase cross-reactivity of a single chain (sc) Fv antibody to botulinum neurotoxin type A (BoNT/A). Starting with a scFv that binds the BoNT/A1 subtype with high affinity (136 pM) and the BoNT/A2 subtype with low affinity (109 nM), we increased its affinity for BoNT/A2 1,250-fold, to 87 pM, while maintaining high-affinity binding to BoNT/A1 (115 pM). To find the molecular basis for improved cross-reactivity, we determined the X-ray co-crystal structures of wild-type and cross-reactive antibodies complexed to BoNT/A1 at resolutions up to 2.6 A, and measured the thermodynamic contribution of BoNT/A1 and A2 amino acids to wild-type and cross-reactive antibody binding. The results show how an antibody can be engineered to bind two different antigens despite structural differences in the antigen-antibody interface and may provide a general strategy for tuning antibody specificity and cross-reactivity.

Antibodies, Monoclonal↗

PubMed2006

A structural perspective of the sequence variability within botulinum neurotoxin subtypes A1-A4.

Botulinum neurotoxin (BoNT) is a category A toxin that has been classified within seven serotypes, designated A-G. Recently, it has been discovered that sequence variability occurs in BoNTs produced by serotype A (BoNT/A) variant strains, designated as subtypes A1 and A2, which have significantly different antibody-binding properties. We have therefore made efforts to understand at the molecular level the diversity and its effects on the biological actions of the toxin, including receptor binding, substrate recognition, and catalysis. We provide the results of these studies, including the analysis of two newly sequenced BoNT/A variants, Loch Maree (A3) and 657Ba (A4), and their comparison to A1 and A2. Using sequence analysis, available functional data, molecular modeling, and comparison of models with the crystal structures of BoNT/A1 and the light chain of BoNT/A2, we conclude that these sequence differences within subtypes will impact development of broad-spectrum antibody and small ligand therapeutics, and suggest dissimilarities in binding affinity and cleavage efficiency of the SNAP-25 substrate. In particular, sequence variation in subtypes BoNT/A3 and BoNT/A4 will likely effect alpha-exosite and S1' subsite recognition, respectively.

Amino Acid Sequence↗

PubMed2005

De novo design of an IL-4 antagonist and its structure at 1.9 A.

An IL-4 antagonist was designed based on structural and biochemical analysis of unbound IL-4 and IL-4 in complex with its high-affinity receptor (IL-4Ralpha). Our design strategy sought to capture a protein-protein interaction targeting the high affinity that IL-4 has for IL-4Ralpha. This strategy has impact due to the potential relevance of IL-4Ralpha as a drug target in the treatment of asthma. To mimic the IL-4 binding surface, critical side chains for receptor binding were identified, and these side chains were transplanted onto a previously characterized, de novo-designed four-helix protein called designed helical protein 1 (DHP-1). This first-generation design resolved the ambiguity previously described for the connectivity between helices in DHP-1 and resulted in a protein capable of binding to IL-4Ralpha. The second-generation antagonist was based upon further molecular modeling, and it succeeded in binding IL-4Ralpha better than the first-generation. This protein, termed DHP-14-AB, yielded a protein with a cooperative unfolding transition (DeltaGu0=8.1 kcal/mol) and an IC50 of 27 microM when in competition with IL-4 whereas DHP-1 had no affinity for IL-4Ralpha. The crystal structure of DHP-14-AB was determined to 1.9-A resolution and was compared with IL-4. This comparison revealed how design strategies targeting protein-protein interactions require high-resolution 3D data and the incorporation of orientation-specific information at the level of side-chains and secondary structure element interactions.

Binding Sites↗

PubMed2003

Crystal structure of an invertebrate caspase.

Caspases play an essential role in the execution of apoptosis. These cysteine proteases are highly conserved among metazoans and are translated as inactive zymogens, which are activated by proteolytic cleavages to generate the large and small subunits and remove the N-terminal prodomain. The 2.3 A resolution crystal structure of active Sf-caspase-1, the principal effector caspase of the insect Spodoptera frugiperda, is presented here. The structure represents the first nonhuman caspase to be resolved. The structure of the cleaved and active protease was determined with the tetrapeptide inhibitor N-acetyl-Asp-Glu-Val-Asp-chloromethylketone covalently bonded to the active site cysteine. As expected, the overall fold of Sf-caspase-1 is exceedingly similar to that of the five active caspases from humans solved to date. The overall structure and active site arrangement of Sf-caspase-1 is most comparable with that of the human effector caspases, with which it shares highest sequence homology. The most prominent structural difference with Sf-caspase-1 is the position of the N-terminal region of the large subunit. Unlike the N terminus of human caspases, the N terminus of Sf-caspase-1 originates from the active site side where it interacts with active site loop L2 and then extends to the backside of the heterodimer. This unusual structural arrangement raises the possibility that the N-terminal prodomain plays a regulatory role during effector caspase activation or enzyme activity in insects.

Amino Acid Chloromethyl Ketones↗