Nature (May 2016): Abstract
Yuta Suzuki, Giovanni Cardone, David Restrepo, Pablo D. Zavattieri, Timothy S. Baker & F. Akif Tezcan
Two-dimensional (2D) crystalline materials possess unique structural, mechanical and electronic properties that make them highly attractive in many applications. Although there have been advances in preparing 2D materials that consist of one or a few atomic or molecular layers, bottom-up assembly of 2D crystalline materials remains a challenge and an active area of development. More challenging is the design of dynamic 2D lattices that can undergo large-scale motions without loss of crystallinity. Dynamic behaviour in porous three-dimensional (3D) crystalline solids has been exploited for stimuli-responsive functions and adaptive behaviour. As in such 3D materials, integrating flexibility and adaptiveness into crystalline 2D lattices would greatly broaden the functional scope of 2D materials. Here we report the self-assembly of unsupported, 2D protein lattices with precise spatial arrangements and patterns using a readily accessible design strategy. Three single- or double-point mutants of the C4-symmetric protein RhuA were designed to assemble via different modes of intermolecular interactions (single-disulfide, double-disulfide and metal-coordination) into crystalline 2D arrays. Owing to the flexibility of the single-disulfide interactions, the lattices of one of the variants (C98RhuA) are essentially defect-free and undergo substantial, but fully correlated, changes in molecular arrangement, yielding coherently dynamic 2D molecular lattices. C98RhuA lattices display a Poisson’s ratio of −1 — the lowest thermodynamically possible value for an isotropic material—making them auxetic.
JACS September 2015: Abstract
Cedric P. Owens, Faith E. H. Katz, Cole H. Carter, Maria A. Luca, and F. Akif Tezcan
Nitrogenase is the only enzyme that can convert atmospheric dinitrogen (N2) into biologically usable ammonia (NH3). To achieve this multielectron redox process, the nitrogenase component proteins, MoFe-protein (MoFeP) and Fe-protein (FeP), repeatedly associate and dissociate in an ATP-dependent manner, where one electron is transferred from FeP to MoFeP per association. Here, we provide experimental evidence that encounter complexes between FeP and MoFeP play a functional role in nitrogenase catalysis. The encounter complexes are stabilized by electrostatic interactions involving a positively charged patch on the β-subunit of MoFeP. Three single mutations (βAsn399Glu, βLys400Glu, and βArg401Glu) in this patch were generated in Azotobacter vinelandii MoFeP. All of the resulting variants displayed decreases in specific catalytic activity, with the βK400E mutation showing the largest effect. As simulated by the Thorneley–Lowe kinetic scheme, this single mutation lowered the rate constant for FeP-MoFeP association 5-fold. We also found that the βK400E mutation did not affect the coupling of ATP hydrolysis with electron transfer (ET) between FeP and MoFeP. These data suggest a mechanism where FeP initially forms encounter complexes on the MoFeP β-subunit surface en route to the ATP-activated, ET-competent complex over the αβ-interface.
JACS Communication August 2015: Abstract
Pamela A. Sontz, Jake B. Bailey, Sunhyung Ahn, and F. Akif Tezcan
We describe here the construction of a three-dimensional, porous, crystalline framework formed by spherical protein nodes that assemble into a prescribed lattice arrangement through metal–organic linker-directed interactions. The octahedral iron storage enzyme, ferritin, was engineered in its C3 symmetric pores with tripodal Zn coordination sites. Dynamic light scattering and crystallographic studies established that this Zn-ferritin construct could robustly self-assemble into the desired bcc-type crystals upon coordination of a ditopic linker bearing hydroxamic acid functional groups. This system represents the first example of a ternary protein–metal–organic crystalline framework whose formation is fully dependent on each of its three components.