The Protein Geometry Database: exploring protein conformational features

Have you ever seen a feature in your structure and asked, "I wonder how novel this is?" For instance a residue with a certain phi/psi angle, or an Asp residue followed by three further residues that make a tight turn centered on the side chain, or a peptide unit deviating 25° from planarity? If so, the Protein Geometry Database (PGD) is something you'll want to know about. The PGD web service (pgd.science.oregonstate.edu, Berkholz 2009a) manages access to a database containing the geometric details of 1.9 million amino acids. Working with the PGD involves two easy steps - using a search form to find a set of peptides matching your interests, and analyzing the geometric details of the set. The search form allows you to find examples of peptide fragments that have any specified combination of backbone conformations and sequence. Filtering can also be performed based on side chain conformations, or bond angles. You can also specify the quality of models included in the search. Once a set of peptides has been identified, their geometric properties can be analyzed on the web site. You can look at the averages and standard deviations for any bond length, bond angle, or conformational angle, or you can explore relationships between properties by displaying highly customizable plots. An option to export the search results allows you to perform any further analyses you might devise. We will show how the PGD has been used to develop a Conformation Dependent Library of main chain bond angle targets for crystallographic refinement (Berkholz 2009b), as well as to advance our understanding of peptide non-planarity, and of conformational preferences for pairs of residues and of cis-peptides. We will also describe how simple searches for outliers in bond lengths and angles are a powerful validation tool that can both uncovering errors in PDB models and can lead to the discovery of very interesting and real deviations from what is normally considered ideal geometry.

Crystal structure and conformational flexibility of the unligated FK506-binding protein FKBP12.6

The primary known physiological function of FKBP12.6 involves its role in regulating the RyR2 isoform of ryanodine receptor Ca2+channels in cardiac muscle, pancreatic β islets and the central nervous system. With only a single previously reported X-ray structure of FKBP12.6, bound to the immunosuppressant rapamycin, structural inferences for this protein have been drawn from the more extensive studies of the homologous FKBP12. X-ray structures at 1.70 and 1.90 Å resolution fromP21andP3121 crystal forms are reported for an unligated cysteine-free variant of FKBP12.6 which exhibit a notable diversity of conformations. In one monomer from theP3121 crystal form, the aromatic ring of Phe59 at the base of the active site is rotated perpendicular to its typical orientation, generating a steric conflict for the immunosuppressant-binding mode. The peptide unit linking Gly89 and Val90 at the tip of the protein-recognition `80s loop' is flipped in theP21crystal form. Unlike the >30 reported FKBP12 structures, the backbone conformation of this loop closely follows that of the first FKBP domain of FKBP51. The NMR resonances for 21 backbone amides of FKBP12.6 are doubled, corresponding to a slow conformational transition centered near the tip of the 80s loop, as recently reported for 31 amides of FKBP12. The comparative absence of doubling for residues along the opposite face of the active-site pocket in FKBP12.6 may in part reflect attenuated structural coupling owing to increased conformational plasticity around the Phe59 ring.

Redrawing the Ramachandran plot after inclusion of hydrogen-bonding constraints

A protein backbone has two degrees of conformational freedom per residue, described by its φ,ψ-angles. Accordingly, the energy landscape of a blocked peptide unit can be mapped in two dimensions, as shown by Ramachandran, Sasisekharan, and Ramakrishnan almost half a century ago. With atoms approximated as hard spheres, the eponymous Ramachandran plot demonstrated that steric clashes alone eliminate ¾ of φ,ψ-space, a result that has guided all subsequent work. Here, we show that adding hydrogen-bonding constraints to these steric criteria eliminates another substantial region of φ,ψ-space for a blocked peptide; for conformers within this region, an amide hydrogen is solvent-inaccessible, depriving it of a hydrogen-bonding partner. Yet, this “forbidden” region is well populated in folded proteins, which can provide longer-range intramolecular hydrogen-bond partners for these otherwise unsatisfied polar groups. Consequently, conformational space expands under folding conditions, a paradigm-shifting realization that prompts an experimentally verifiable conjecture about likely folding pathways.