Modelling the polypeptide backbone with 'spare parts' from known protein structures.

An automatic procedure for building a protein polyalanine backbone from C alpha positions and 'spare parts' retrieved from a data base of 66 high-resolution protein structures is described. Protein backbones are constructed from overlapping fragments of variable length, which allows the backbone of regular secondary structure elements to be built in one block. The procedure is shown to yield backbones which compare very favourably with those from highly refined X-ray structures (r.m.s. deviation between generated and crystal structures less than 1A). The method is furthermore quite insensitive to experimental errors in C alpha positions as well as to the size of the data base, and is seen to yield valuable insight into the relationships between sequence and 3-D structure: one example on triose phosphate isomerase, a beta-barrel protein, shows that beta alpha loops can be considered as structurally more uncommon than alpha beta loops. The 'spare parts' approach is also found to be useful for general-purpose modelling of local structural changes produced by insertion or deletion of residues. It should, however, be used with caution. Crude selection criteria based solely on fragment length and geometric fit to the loop base regions yield realistic backbones in about two-thirds of the test cases (r.m.s. deviations from refined crystal structure approximately 1A). In the remaining cases, sequence information, in particular the presence of glycine residues which tend to adopt more unusual backbone conformations, must be considered to obtain comparable results.