Azidohomoalanine: A Minimally Invasive, Versatile, and Sensitive Infrared Label in Proteins To Study Ligand Binding

The noncanonical amino acid azidohomoalanine (Aha) is known to be an environment-sensitive infrared probe for the site-specific investigation of protein structure and dynamics. Here, the capability of that label is explored to detect protein ligand interactions by incorporating it in the vicinity of the binding groove of a PDZ2 domain. Circular dichroism and isothermal titration calorimetry measurements reveal that the perturbation of the protein system by mutation is negligible, with minimal influence on protein stability and binding affinity. Two-dimensional infrared spectra exhibit small (1-3 cm(-1)) but clearly measurable red shifts of the Aha vibrational frequency upon binding of two different peptide ligands, while accompanying molecular dynamics simulations suggest that these red shifts are induced by polar contacts with side chains of the peptide ligands. Hence, Aha is a versatile and minimally invasive vibrational label that is not only able to report on large structural changes during, e.g., protein folding, but also on very subtle changes of the electrostatic environment upon ligand binding

2D-IR Spectroscopy of an AHA Labeled Photoswitchable PDZ2 Domain

We explore the capability of the non-natural amino acid azidohomoalanine (AHA) as an IR label to sense relatively small structural changes in proteins with the help of 2D IR difference spectroscopy. To that end, we AHA-labeled an allosteric protein (the PDZ2 domain from human tyrosine-phosphatase 1E) and furthermore covalently linked it to an azobenzene-derived photoswitch as to mimic its conformational transition upon ligand binding. To determine the strengths and limitations of the AHA label, in total six mutants have been investigated with the label at sites with varying properties. Only one mutant revealed a measurable 2D IR difference signal. In contrast to the commonly observed frequency shifts that report on the degree of solvation, in this case we observe an <i>intensity</i> change. To understand this spectral response, we performed classical MD simulations, evaluating local contacts of the AHA labels to water molecules and protein side chains and calculating the vibrational frequency on the basis of an electrostatic model. Although these simulations revealed in part significant and complex changes of the number of intraprotein and water contacts upon <i>trans–cis</i> photoisomerization, they could not provide a clear explanation of why this one label would stick out. Subsequent quantum-chemistry calculations suggest that the response is the result of an electronic interaction involving charge transfer of the azido group with sulfonate groups from the photoswitch. To the best of our knowledge, such an effect has not been described before

Clinical Trials for Immunosuppression in Transplantation: The Case for Reform and Change in Direction

Currently trials of immunosuppression in transplantation are in decline because their objectives remain focused on improving acute rejection rates and graft survival in the first 12 months. With 1 year renal graft survival rates of greater than 90% the best that can be hoped for is noninferiority trial outcomes compared with current standard of care. Current trial design is not leading to novel therapies improving long-term outcomes and safety, and hence important unmet clinical needs in transplantation remain unanswered. Issues that need to be addressed include but are not limited to: prevention of subclinical rejection in the first year, better 5- and 10-year graft outcomes, more effective treatment for high immunological risk and sensitized (including donor-specific antibody) patients, immunosuppressive combinations that are better tolerated by patients with fewer side effects and less morbidity and mortality. In September 2015, the Transplantation Society convened a group of transplant clinical trial experts to address these problems. The aims were to substantially realign the priorities of clinical trials for renal transplant immunosuppression with the current unmet needs and to propose new designs for clinical trials for transplant immunosuppression. Moving forward, the transplant community needs to provide trial data that will identify superior treatment options for patient subgroups and allow new agents to be evaluated for efficacy and safety and achieve timely regulatory approval. Trial designs for new transplant immunosuppression must be intelligently restructured to ensure that short- and long-term clinical outcomes continue to improve.status: publishe