Dodecins are so far the smallest known flavoproteins (68 -71 amino acids) and are most likely involved in prokaryotic flavin storage. The dodecin monomers adopt a simple ␤␣␤␤-fold and assemble to hollow sphere-like dodecameric complexes. Flavin binding by the dodecin from Thermus thermophilus showed a 1:1 stoichiometry and apparent dissociation constants in the submicromolar to nanomolar range as characterized by isothermal titration calorimetry and fluorescence titrations. The x-ray structures of the flavin-prebound and FMN-reconstituted state of the T. thermophilus dodecin revealed binding of FMN dimers in a novel si-si-rather than the re-re-orientation of their isoalloxazine moieties as found before in an archaeal dodecin. Electron paramagnetic resonance studies demonstrated that upon reduction the excess electron is localized only on one flavin, thus making dodecin-bound flavins highly refractory to redox chemistry. Besides FMN dimers, trimers of coenzyme A are additionally bound to this eubacterial dodecin along the 3-fold symmetry face II of the dodecin complex. Therefore, dodecins can act as bifunctional cofactor storage proteins that sequester catalytic cofactors in prokaryotes very efficiently in an aggregated and unreactive state.Flavoproteins are ubiquitous proteins using flavins as prosthetic groups. Because riboflavin (vitamin B 2 ) serves mostly as a biosynthetic intermediate, FMN and FAD are the principal flavin cofactors (1). They are involved in the catalysis of a wide range of biological redox reactions, such as the dehydrogenation of NAD(P)H, lipid esters, and D-amino acids, the oxidation of amines to imines, the formation and cleavage of disulfide bonds, the hydroxylation of aromatic substrates, or the activation of molecular oxygen. Furthermore, flavoproteins serve as electron transmitters in electron transfer processes like oxidative phosphorylation. Here, they act as mediators between typical 2-electron donors like NADH and 1-electron acceptors like the heme group. This versatility to engage in one-and twoelectron transfer reactions is due to their isoalloxazine moiety that adopts either an oxidized, a semiquinoid, or fully reduced redox state (2). Flavins are not only involved in redox reactions but also in the sensing of blue or ultraviolet light (3). In cryptochromes, flavin chromophores mediate the flowering and daily light/dark cycles in plants (4), in phototropins they regulate phototropism (5), and in photolyases they are involved in DNA repair (6). Beside catalytic or sensoric actions, flavoproteins are also involved in the binding or transport of flavins. During pregnancy, a specific carrier system evolves in vertebrates including the riboflavin-binding proteins (RCPs or RfBPs), since adequate amounts of vitamin B 2 are essential for the normal fetal development (7).Dodecins are a novel family of flavin-binding proteins of unknown function that were discovered in the Archaea Halobacterium salinarum during an inverse structural genomics project on halophilic proteins (8). So far it...

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The suitability for chemical engineering of the highly symmetrical Mycobacterium tuberculosis dodecin was investigated, its inner cavity providing a large compartment shields introduced compounds from bulk solvent. Hybrids were obtained by S-alkylation of cysteine mutants and characterized by spectroscopic methods, including the crystal structures of wild type and biohybrid dodecins.

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