The Rosetta loop modeling benchmark [Mandell09, Stein13] tests the ability of a protocol to reconstruct the backbone conformation of loop segments in experimentally determined protein structures. The short loops benchmark set consists of 45 12-residue protein segments without regular secondary structure. In each case, the given segment is deleted from the protein structure and then reconstructed de novo, given a fixed backbone environment for the rest of the protein. All segment side chains and those within 10Å of the segment are modeled based on a rotamer library that does not include the native side chain conformations.
The long loops benchmark [Stein13] analogously tests whether protocols are able to reconstruct longer loop segments of 14-17 residues. This benchmark set consists of 27 non-redundant long loops. De novo loop reconstruction and side chain optimization are performed as described above for the standard loop modeling benchmark.
After superposition of each generated model onto the native structure (excluding the reconstructed loop), the loop backbone heavy-atom RMSD of the model to the native loop conformation is calculated. Overall benchmark performance of each protocol is then evaluated using two different metrics across the entire benchmark set: (i) median loop backbone RMSD of the lowest-energy model to the native structure (or median lowest loop backbone RMSD of the 5 lowest-energy models, which is less susceptible to stochastic fluctuations [Leaver-Fay12]), and (ii) the median percentage of models generated that have a sub-angstrom loop backbone RMSD.
The cyclic coordinate descent (CCD) protocol uses insertion of fragments from proteins of known structure to efficiently sample the loop backbone degrees of freedom [Wang07], followed by torsion angle adjustments via cyclic coordinate descent to close the resulting chain break [Canutescu03].
The kinematic closure (KIC) protocol is a robotics-based method [Mandell09], which samples all but six loop backbone degrees of freedom probabilistically from Ramachandran space. Those remaining three pairs of φ/ψ torsion angles are then solved analytically through kinematic closure to close the chain break [Coutsias05]. The figure below demonstrates a KIC move, which is a purely local conformational change in the chosen protein segment without affecting the rest of the structure.
Next-generation KIC (NGK) adds four additional sampling strategies to the standard KIC protocol: (i) the selection of pairs of φ/ψ torsions from neighbor-dependent Ramachandran distributions, (ii) sampling of ω degrees of freedom, as well as annealing methods that gradually ramp the weights of (iii) the repulsive and (iv) Ramachandran terms of the Rosetta energy function to overcome energy barriers [Stein13].
All three loop modeling protocols use Monte-Carlo simulated annealing for rotamer-based side-chain optimization ("repacking") of the loop residues and those within 10Å of the loop, followed by gradient-based minimization. Both KIC and NGK protocols have been shown to successfully sample sub-angstrom protein loop conformations in many cases [Mandell09, Stein13], with NGK generating a higher median percentage of sub-angstrom models than standard KIC [Stein13].
Note that the protocols described below do not make use native/homologous fragments as we wish to present a fair comparison between the methods. If you have homologous fragments for your protein of interest then you may wish to use one of the methods which supports the use of fragments (e.g. CCD).