Protein engineering in the development of functional hydrogels.

Proteins, which are natural heteropolymers, have evolved to exhibit a staggering array of functions and capabilities. As scientists and engineers strive to tackle important challenges in medicine, novel biomaterials continue to be devised, designed, and implemented to help to address critical needs. This review aims to cover the present advances in the use of protein engineering to create new protein and peptide domains that enable the formation of advanced functional hydrogels. Three types of domains are covered in this review: (a) the leucine zipper coiled-coil domains, (b) the EF-hand domains, and (c) the elastin-like polypeptides. In each case, the functionality of these domains is discussed as well as recent advancements in the use of these domains to create novel hydrogel-based biomaterials. As protein engineering is used to both create and improve protein domains, these advances will lead to exciting new biomaterials for use in a variety of applications.

Bioelectrocatalytic hydrogels from electron-conducting metallopolypeptides coassembled with bifunctional enzymatic building blocks.

Here, we present two bifunctional protein building blocks that coassemble to form a bioelectrocatalytic hydrogel that catalyzes the reduction of dioxygen to water. One building block, a metallopolypeptide based on a previously designed triblock polypeptide, is electron-conducting. A second building block is a chimera of artificial alpha-helical leucine zipper and random coil domains fused to a polyphenol oxidase, small laccase (SLAC). The metallopolypeptide has a helix-random-helix secondary structure and forms a hydrogel via tetrameric coiled coils. The helical and random domains are identical to those fused to the polyphenol oxidase. Electron-conducting functionality is derived from the divalent attachment of an osmium bis-bipyrdine complex to histidine residues within the peptide. Attachment of the osmium moiety is demonstrated by mass spectroscopy (MS-MALDI-TOF) and cyclic voltammetry. The structure and function of the alpha-helical domains are confirmed by circular dichroism spectroscopy and by rheological measurements. The metallopolypeptide shows the ability to make electrical contact to a solid-state electrode and to the redox centers of modified SLAC. Neat samples of the modified SLAC form hydrogels, indicating that the fused alpha-helical domain functions as a physical cross-linker. The fusion does not disrupt dimer formation, a necessity for catalytic activity. Mixtures of the two building blocks coassemble to form a continuous supramolecular hydrogel that, when polarized, generates a catalytic current in the presence of oxygen. The specific application of the system is a biofuel cell cathode, but this protein-engineering approach to advanced functional hydrogel design is general and broadly applicable to biocatalytic, biosensing, and tissue-engineering applications.

Broadening the cofactor specificity of a thermostable alcohol dehydrogenase using rational protein design introduces novel kinetic transient behavior.

Cofactor specificity in the aldo-keto reductase (AKR) superfamily has been well studied, and several groups have reported the rational alteration of cofactor specificity in these enzymes. Although most efforts have focused on mesostable AKRs, several putative AKRs have recently been identified from hyperthermophiles. The few that have been characterized exhibit a strong preference for NAD(H) as a cofactor, in contrast to the NADP(H) preference of the mesophilic AKRs. Using the design rules elucidated from mesostable AKRs, we introduced two site-directed mutations in the cofactor binding pocket to investigate cofactor specificity in a thermostable AKR, AdhD, which is an alcohol dehydrogenase from Pyrococcus furiosus. The resulting double mutant exhibited significantly improved activity and broadened cofactor specificity as compared to the wild-type. Results of previous pre-steady-state kinetic experiments suggest that the high affinity of the mesostable AKRs for NADP(H) stems from a conformational change upon cofactor binding which is mediated by interactions between a canonical arginine and the 2'-phosphate of the cofactor. Pre-steady-state kinetics with AdhD and the new mutants show a rich conformational behavior that is independent of the canonical arginine or the 2'-phosphate. Additionally, experiments with the highly active double mutant using NADPH as a cofactor demonstrate an unprecedented transient behavior where the binding mechanism appears to be dependent on cofactor concentration. These results suggest that the structural features involved in cofactor specificity in the AKRs are conserved within the superfamily, but the dynamic interactions of the enzyme with cofactors are unexpectedly complex.

Catalytic biomaterials: engineering organophosphate hydrolase to form self-assembling enzymatic hydrogels.

Organophosphate (OP) neurotoxins have contaminated the environment, contributed to millions of poisoning annually, and have been used as chemical weapons. Biomaterials incorporating the native activity of the OP hydrolase (OPH) enzyme are of interest for applications including OP sensing, environmental bioremediation and prophylactic decontamination. We have engineered and characterized four novel hydrogel-forming OPH variants by genetically fusing the OPH enzyme with alpha-helical leucine zipper domains (H), unstructured soluble linker domains (S) and polyhistidine purification tags. The appended H domains form physical cross-links between the enzymes and enable self-assembly of the enzymes into hydrogels. The addition of the H and S fusions significantly increased the expression levels of soluble protein. OPH constructs with biterminal H domains form hydrogels at lower protein weight percents and exhibit higher enzymatic activity than those variants modified with a single H domain fusion. Polyhistidine tags were not useful for purification but they were not benign, as the addition of the 6His tags increased the hydrogel-forming abilities of the proteins with a concomitant reduction in both the k(cat) and K(M) values. Active enzymatic hydrogels could be made from concentrated unpurified crude protein lysates, significantly simplifying the processing and utilization of the biomaterials. And, a simple proteinaceous bioactive surface coating exhibiting OPH activity is demonstrated. The hydrogels were stable over long-term storage, as activity was retained after cold storage in buffer after 5 months. These new protein constructs further show the use of rational protein design to create novel, bifunctional, self-assembling units for the formation of catalytic biomaterials.

A chimeric fusion protein engineered with disparate functionalities-enzymatic activity and self-assembly.

The fusion of protein domains is an important mechanism in molecular evolution and a valuable strategy for protein engineering. We are interested in creating fusion proteins containing both globular and structural domains so that the final chimeric protein can be utilized to create novel bioactive biomaterials. Interactions between fused domains can be desirable in some fusion protein applications, but in this case the optimal configuration will enable the bioactivity to be unaffected by the structural cross-linking. To explore this concept, we have created a fusion consisting of a thermostable aldo-keto reductase, two alpha-helical leucine zipper domains, and a randomly coiled domain. The resulting protein is bifunctional in that (1) it can self-assemble into a hydrogel material as the terminal leucine zipper domains form interprotein coiled-coil cross-links, and (2) it expresses alcohol dehydrogenase and aldo-keto reductase activity native to AdhD from Pyrococcus furiosus. The kinetic parameters of the enzyme are minimally affected by the addition of the helical appendages, and rheological studies demonstrate that a supramolecular assembly of the bifunctional protein building blocks forms a hydrogel. An active hydrogel is produced at temperatures up to 60 degrees C, and we demonstrate the functionality of the biomaterial by monitoring the oxidation and reduction of the native substrates by the gel. The design of chimeric fusion proteins with both globular and structural domains is an important advancement for the creation of bioactive biomaterials for biotechnology applications such as tissue engineering, bioelectrocatalysis, and biosensing and for the study of native assembled enzyme structures and clustered enzyme systems such as metabolons.

Bioactive proteinaceous hydrogels from designed bifunctional building blocks.

Stimulus-responsive, or "smart" protein-based hydrogels are of interest for many bioengineering applications, but have yet to include biological activity independent of structural functionality. We have genetically engineered bifunctional building blocks incorporating fluorescent proteins that self-assemble into robust and active hydrogels. Gelation occurs when protein building blocks are cross-linked through native protein-protein interactions and the aggregation of alpha-helical hydrogel-forming appendages. Building blocks constructed from different fluorescent proteins can be mixed to enable tuning of fluorescence loading and hydrogel strength with a high degree of independence. FRET experiments suggest a macro-homogeneous structure and that intragel and interprotein reactions can be engineered. This design approach will enable the facile construction of complex hydrogels with broad applicability.

Biomimetics with a self-assembled monolayer of catalytically active tethered isoalloxazine on Au.

A new biomimetic nanostructured electrocatalyst comprised of a self-assembled monolayer (SAM) of flavin covalently attached to Au by reaction of methylformylisoalloxazine with chemisorbed cysteamine is introduced. Examinations by Fourier transform infrared spectroscopy and scanning tunneling microscopy (STM) show that the flavin molecules are oriented perpendicular to the surface with a 2 nm separation between flavin molecules. As a result of the contrast observed in the STM profiles between areas only covered by unreacted cysteamine and those covered by flavin-cysteamine moieties, it can be seen that the flavin molecules rise 0.7 nm above the chemisorbed cysteamines. The SAM flavin electrocatalyst undergoes fast electron transfer with the underlying Au and shows activity toward the oxidation of enzymatically active beta-NADH at pH 7 and very low potential (-0.2 V vs Ag/AgCl), a requirement for use in an enzymatic biofuel cell, and a 100-fold increase in activity with respect to the collisional reaction in solution.