An efficient dynamic programming algorithm for phosphorylation site assignment of large-scale mass spectrometry data

Phosphorylation site assignment of large-scale data from high throughput tandem mass spectrometry (LC-MS/MS) data is an important aspect of phosphoproteomics. Correct assignment of phosphorylated residue(s) is important for functional interpretation of the data within a biological context. Common search algorithms (Sequest etc.) for mass spectrometry data are not designed for accurate site assignment; thus, additional algorithms are needed. In this paper, we propose a linear-time and linear-space dynamic programming strategy for phosphorylation site assignment. The algorithm, referred to as PhosSA, optimizes the objective function defined as the summation of peak intensities that are associated with theoretical phosphopeptide fragmentation ions. Quality control is achieved through the use of a post-processing criteria whose value is indicative of the signal-to-noise (S/N) properties and redundancy of the fragmentation spectra. The algorithm is tested using experimentally generated data sets of peptides with known phosphorylation sites while varying the fragmentation strategy (CID or HCD) and molar amounts of the peptides. The algorithm is also compatible with various peptide labeling strategies including SILAC and iTRAQ. PhosSA is shown to achieve > 99% accuracy with a high degree of sensitivity. The algorithm is extremely fast and scalable (able to process up to 0.5 million peptides in an hour). The implemented algorithm is freely available at http://helixweb.nih.gov/ESBL/PhosSA/ for academic purposes.