Nature has evolved several unique biomineralization strategies to direct the synthesis and growth of inorganic materials. These natural systems are complex, involving the interaction of multiple biomolecules to catalyze biomineralization and template growth. Herein we describe the first report to our knowledge of a single enzyme capable of both catalyzing mineralization in otherwise unreactive solution and of templating nanocrystal growth. A recombinant putative cystathionine γ-lyase (smCSE) mineralizes CdS from an aqueous cadmium acetate solution via reactive H 2 S generation from L-cysteine and controls nanocrystal growth within the quantum confined size range. The role of enzymatic nanocrystal templating is demonstrated by substituting reactive Na 2 S as the sulfur source. Whereas bulk CdS is formed in the absence of the enzyme or other capping agents, nanocrystal formation is observed when smCSE is present to control the growth. This dualfunction, single-enzyme, aerobic, and aqueous route to functional material synthesis demonstrates the powerful potential of engineered functional material biomineralization.cadmium sulfide | quantum dot | biomineralization | enzyme | nanoparticle B iological systems have evolved a diverse array of mechanisms to synthesize inorganic materials from aqueous solutions under ambient conditions. This inherent control over material properties has created interest in using these biological routes to synthesize materials (1-3) such as biosilica from sponges and diatoms (4-8), biogenic CaCO 3 from mollusks (9-12), and magnetic particles from magnetotactic bacteria (13-15). Designing a biomineralization strategy requires control of both the material composition and structure; in nature, this control is typically achieved through the assembly of a multiprotein complex, including both structuredirecting proteins and proteins responsible for mineralization of a specific composition. In the current work, we demonstrate the reduction of this complexity to its simplest form: a single enzyme capable of both catalyzing CdS mineralization and controlling particle size within the quantum confined size range to form functional biomineralized CdS quantum dots.Two of the most studied biomineralization proteins are perlucin and silicatein. Perlucin (16) has been shown to mineralize crystalline forms of CaCO 3 , a common structural material that constitutes the shell of many marine organisms, in the form of organic-inorganic composites. The role of the nacre protein perlucin in crystallite templating has been elucidated through experiments demonstrating crystallite formation in the presence of purified perlucin, and perlucin selectively being removed from solution during crystallite formation in the presence of a mixture of water-soluble, nacre-associated proteins (17). Native silicatein harvested from sea sponge or engineered forms produced recombinantly are active for biomineralization of silica and titania into structures that are amorphous or crystalline (7,18,19). In particular, biomineralizatio...
Light-induction of an anionic semiquinone (SQ) flavin radical in Drosophila cryptochrome (dCRY) alters the dCRY conformation to promote binding and degradation of the circadian clock protein Timeless (TIM). Specific peptide ligation with sortase A attaches a nitroxide spin-probe to the dCRY C-terminal tail (CTT) while avoiding deleterious side reactions. Pulse dipolar electron-spin resonance spectroscopy from the CTT nitroxide to the SQ shows that flavin photoreduction shifts the CTT ~1 nm and increases its motion, without causing full displacement from the protein. dCRY engineered to form the neutral SQ serves as a dark-state proxy to reveal that the CTT remains docked when the flavin ring is reduced but uncharged. Substitutions of flavin-proximal His378 promote CTT undocking in the dark or diminish undocking in the light, consistent with molecular dynamics simulations and TIM degradation activity. The His378 variants inform on recognition motifs for dCRY cellular turnover and strategies for developing optogenetic tools.
Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for investigating the structure and dynamics of proteins. The introduction of paramagnetic moieties at specific positions in a protein enables precise measurement of local structure and dynamics. This technique, termed site-directed spin-labeling, has traditionally been performed using cysteine-reactive radical-containing probes. However, large proteins are more likely to contain multiple cysteine residues and cysteine labeling at specific sites may be infeasible or impede function. To address this concern, we applied three peptide-ligating enzymes (sortase, asparaginyl endopeptidase, and inteins) for nitroxide labeling of N- and C-termini of select monomeric and dimeric proteins. Continuous wave and pulsed EPR (double electron electron resonance) experiments reveal specific attachment of nitroxide probes to ether N-termini (OaAEP1) or C-termini (sortase and intein) across three test proteins (CheY, CheA, and iLOV), thereby enabling a straightforward, highly specific, and general method for protein labeling. Importantly, the linker length (3, 5, and 9 residues for OaAEP1, intein, and sortase reactions, respectively) between the probe and the target protein has a large impact on the utility of distance measurements by pulsed EPR, with longer linkers leading to broader distributions. As these methods are only dependent on accessible N- and C-termini, we anticipate application to a wide range of protein targets for biomolecular EPR spectroscopy.
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