The melanocortin-3 receptor (MC3R) is a G-protein coupled receptor that regulates appetite and is a potential therapeutic target for anorexia and weight loss treatments. The report of a highly selective synthetic antagonist of MC3R, Cpd11, represents a major advance towards therapeutic targeting of MC3R. However, Cpd11 is challenging to access synthetically, severely limiting its use and additional structural optimization. Here, we outline an improved synthesis of Cpd11 that addresses three major synthetic challenges, including the formation of Cpd11's structurally unique bicyclic core. With these changes, Cpd11 was readily produced (2.3 mg from a .05 mmol scale versus << 0.1 mg using the original synthetic methodology) and utilized in MC3R studies in C57BL/6J male mice. Thus, this new approach will increase the accessibility of Cpd11 and is translatable to related bicyclic agonists and antagonists for other melanocortin receptors and may have general applicability toward the synthesis of other multicyclic poly-cysteine peptides.
The COVID-19 pandemic drove a uniquely fervent pursuit to explore the potential of peptide, antibody, protein, and small-molecule-based antiviral agents against severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2). The interaction between the SARS-CoV2 spike protein with the angiotensin-converting enzyme 2 (ACE2) receptor that mediates viral cell entry was a particularly interesting target given its well-described protein-protein interaction (PPI). This PPI is mediated by an alpha-helical portion of ACE2 binding to the receptor binding domain (RBD) of the spike protein and thought to be susceptible to blockade through molecular mimicry. Small numbers of hydrocarbon-stapled synthetic peptides designed to disrupt or block this interaction were tested individually and were found to have variable efficacy despite having related or overlapping sequences and similarly increased alpha-helicity. Reasons for these differences are unclear and reported preclinical successes have been limited. This study sought to better understand reasons for these differences through evaluation of a comprehensive collection of hydrocarbon-stapled peptides, designed based on four distinct principles: stapling position, number of staples, amino acid sequence, and primary sequence length. Surprisingly, we observed that the helicity and amino acid sequence iterations of hydrocarbon-stapled peptides did not correlate with their bioactivity. Our results highlight the importance of iterative and combinatorial testing of these compounds to determine a configuration that best mimics natural binding and allows for chain flexibility while sacrificing structural helicity.
Simple and efficient total synthesis of homogeneous and chemically modified protein samples remains a significant challenge. Here, we report development of a convergent hybrid phase native chemical ligation (CHP‐NCL) strategy for facile preparation of proteins. In this strategy, proteins are split into ~100‐residue blocks, and each block is assembled on solid support from synthetically accessible peptide fragments before ligated together into full‐length protein in solution. With the new method, we increase the yield of CENP‐A synthesis by 2.5‐fold compared to the previous hybrid phase ligation approach. We further extend the new strategy to the total chemical synthesis of 212‐residue linker histone H1.2 in unmodified, phosphorylated, and citrullinated forms, each from eight peptide segments with only one single purification. We demonstrate that fully synthetic H1.2 replicates the binding interactions of linker histones to intact mononucleosomes, as a proxy for the essential function of linker histones in the formation and regulation of higher order chromatin structure.
Coiled coils are one of most common protein quaternary structures and represent the best understood relationship between amino acid sequence and protein conformation. Whereas the roles of residues at the canonical heptad positions the a, d, e, and g are understood in precise detail, conventional approaches often assume that the solvent-exposed b-, c-, and f-positions can be varied broadly for application-specific purposes with minimal consequences. However, a growing body of evidence suggests that interactions among these b, c, and f residues can contribute substantially to coiled-coil conformational stability. In the trimeric coiled coil described here, we find that b-position Glu10 engages in a stabilizing long-range synergistic interaction with c-position Lys18 (Delta Delta Delta G(f) = -0.65 +/- 0.02 kcal/mol). This favorable interaction depends strongly on the presence of two nearby f-position residues: Lys 7 and Tyr14. Extensive mutational analysis of these residues in the presence of added salt versus denaturant suggests that this long-range synergistic interaction is primarily electrostatic in origin, but also depends on the precise location and acidity of a side-chain hydrogen-bond donor within f-position Tyr14.
Monoclonal antibodies (mAbs) that target the β‐amyloid peptide (Aβ) are important Alzheimer's disease research tools and are now being used as Alzheimer's disease therapies. Conformation‐specific mAbs that target oligomeric and fibrillar Aβ assemblies are of particular interest, as these assemblies are associated with Alzheimer's disease pathogenesis and progression. This article reports the generation of rabbit mAbs against two different triangular trimers derived from Aβ. These antibodies are the first mAbs generated against Aβ oligomer mimics in which the high‐resolution structures of the oligomers are known. We describe the isolation of the mAbs using single B‐cell sorting of peripheral blood mononuclear cells (PBMCs) from immunized rabbits, the selectivity of the mAbs for the triangular trimers, the immunoreactivity of the mAbs with aggregated Aβ 42 , and the immunoreactivity of the mAbs in brain tissue from the 5xFAD Alzheimer's disease mouse model. The characterization of these mAbs against structurally defined trimers derived from Aβ enhances understanding of antibody‐amyloid recognition and may benefit the development of diagnostics and immunotherapies in Alzheimer's disease.
