Annularins

Annularins are a proposed class of ring-shaped macromolecules, commonly envisioned as cyclic peptides or small cyclic proteins with a closed backbone. The name derives from annulus, reflecting their circular topology. In theoretical discussions, annularins may form through post-translational cyclization of linear precursors or via ribosomal synthesis followed by enzymatic cyclization, and they are usually considered in the context of synthetic biology or natural product analogs.

Structural features described in current models include a closed loop backbone containing a limited number of residues, typically between 6 and 20, with tight conformational constraints. Some proposals envision single-ring architectures while others allow multi-subunit assemblies that create hollow or partially hollow structures. Side-chain interactions and, in some designs, central channels or cavities are noted as possible elements that influence ligand binding, selectivity, and stability. Stability is often attributed to covalent linkages that resist common proteolytic or thermal stress.

Function and significance attributed to annularins are primarily hypothetical. Proposed roles include serving as rigid scaffolds for catalysis or molecular recognition, components of nanoscale compartments, or protective elements against proteolysis. They are also discussed as potential building blocks for nanomaterials and programmable nanoreactors. However, there is no broad consensus on natural occurrence, and most discussions remain theoretical or rely on designed analogs rather than confirmed biological examples.

Research approaches emphasize computational modeling, synthesis of designed cyclic peptides, and structural characterization of annularin-like rings using mass spectrometry, NMR, X-ray crystallography, or cryo-EM. See also: cyclic peptides, protein rings, macrocycles.