Tandem repeat proteins, which are formed by repetition of modular units of protein sequence and structure, play important biological roles as macromolecular binding and scaffolding domains, enzymes, and building blocks for the assembly of fibrous materials1,2. The modular nature of repeat proteins enables the rapid construction and diversification of extended binding surfaces by duplication and recombination of simple building blocks3,4. The overall architecture of tandem repeat protein structures – which is dictated by the internal geometry and local packing of the repeat building blocks – is highly diverse, ranging from extended, super-helical folds that bind peptide, DNA, and RNA partners5–9, to closed and compact conformations with internal cavities suitable for small molecule binding and catalysis10. Here we report the development and validation of computational methods for de novo design of tandem repeat protein architectures driven purely by geometric criteria defining the inter-repeat geometry, without reference to the sequences and structures of existing repeat protein families. We have applied these methods to design a series of closed alpha-solenoid11 repeat structures (alpha-toroids) in which the inter-repeat packing geometry is constrained so as to juxtapose the N- and C-termini; several of these designed structures have been validated by X-ray crystallography. Unlike previous approaches to tandem repeat protein engineering12–20, our design procedure does not rely on template sequence or structural information taken from natural repeat proteins and hence can produce structures unlike those seen in nature. As an example, we have successfully designed and validated closed alpha-solenoid repeats with a left-handed helical architecture that – to our knowledge – is not yet present in the protein structure database21.

Site-directed mutagenesis and Laue diffraction data to 2.5 Å resolution were used to solve the structures of two sequential intermediates formed during the catalytic actions of isocitrate dehydrogenase. Both intermediates are distinct from the enzyme-substrate and enzyme-product complexes. Mutation of key catalytic residues changed the rate determining steps so that protein and substrate intermediates within the overall reaction pathway could be visualized.

Altering the specificity of an enzyme requires precise positioning of side-chain functional groups that interact with the modified groups of the new substrate. This requires not only sequence changes that introduce the new functional groups but also sequence changes that remodel the structure of the protein backbone so that the functional groups are properly positioned. We describe a computational design method for introducing specific enzyme-substrate interactions by directed remodeling of loops near the active site. Benchmark tests on 8 native protein-ligand complexes show that the method can recover native loop lengths and, often, native loop conformations. We then use the method to redesign a critical loop in human guanine deaminase such that a key side-chain interaction is made with the substrate ammelide. The redesigned enzyme is 100-fold more active on ammelide and 2.5e4-fold less active on guanine than wild-type enzyme: The net change in specificity is 2.5e6-fold. The structure of the designed protein was confirmed by X-ray crystallographic analysis: The remodeled loop adopts a conformation that is within 1-Å C␣ RMSD of the computational model. computational protein design ͉ loop modeling C omputational protein design methodology has been used to optimize properties such as protein stability (1, 2) and to introduce functions such as binding of small molecules (3), proteins (4), and nucleic acids (5), as well as enzymatic activity (6, 7). In most of these studies, the implicit assumption that the structure of the polypeptide backbone would remain largely fixed despite mutations of amino acid side chains was made for the sake of computational tractability.Explicit remodeling of the polypeptide backbone makes possible further optimization of these and other structural or functional properties. The set of combinations of protein sequences and structures is considerably larger when backbone flexibility is allowed and is likely to contain conformations that optimize a desired property to a greater degree than the original scaffold. This is illustrated by the backbone shifts that accompany functional divergence in natural protein evolution. Previous studies have achieved functional changes by backbone alteration, but relied on grafting methods that are restricted to sequences of known structure and function (8-10), which may be suboptimal with respect to the desired property.De novo protein structure prediction methods are well suited for sampling novel backbone conformations (11). These methods have recently been extended to focus sampling on conformations that satisfy specific positional constraints (12, 13). Computational design algorithms that iterate between sequence design and backbone optimization using structure-prediction methods have been used to design previously unobserved protein fold and loop conformations (2, 14), but have not yet been applied to achieving functional changes such as alteration of an enzyme's substrate specificity.We have developed a computational design algorithm that uses constr...

We determined the crystal structure of a bifunctional group I intron splicing factor and homing endonuclease, termed the I-AniI maturase, in complex with its DNA target at 2.6 Å resolution. The structure demonstrates the remarkable structural conservation of the ␤-sheet DNA-binding motif between highly divergent enzyme subfamilies. DNA recognition by I-AniI was further studied using nucleoside deletion and DMS modification interference analyses. Correlation of these results with the crystal structure provides information on the relative importance of individual nucleotide contacts for DNA recognition. Alignment and modeling of two homologous maturases reveals conserved basic surface residues, distant from the DNA-binding surface, that might be involved in RNA binding. A point mutation that introduces a single negative charge in this region uncouples the maturase and endonuclease functions of the protein, inhibiting RNA binding and splicing while maintaining DNA binding and cleavage.

Genetically encoded unnatural amino acids could facilitate the design of proteins and enzymes of novel function, but correctly specifying sites of incorporation, and the identities and orientations of surrounding residues represents a formidable challenge. Computational design methods have been used to identify optimal locations for functional sites in proteins and design the surrounding residues, but have not incorporated unnatural amino acids in this process. We extended the Rosetta design methodology to design metalloproteins in which the amino acid (2,2’-bipyridin-5yl)alanine (Bpy-Ala) is a primary ligand of a bound metal ion. Following initial results that indicated the importance of buttressing the Bpy-Ala amino acid, we designed a buried metal binding site with octahedral coordination geometry consisting of Bpy-Ala, two protein based metal ligands, and two metal bound water molecules. Experimental characterization revealed a Bpy-Ala mediated metalloprotein with the ability to bind divalent cations including Co2+, Zn2+, Fe2+, and Ni2+, with a Kd for Zn2+ of ~40 pM. X-ray crystallographic analysis of the designed protein shows only slight deviation from the computationally designed model.