Uncovering immobilized trypsin digestion features from large-scale proteome data generated by high-resolution mass spectrometry.

Immobilized trypsin produces very fast protein digestion, which is attractive for application to high throughput bottom-up proteomics. While there is a rich literature on the preparation of immobilized trypsin, there are very few studies that investigate its application to complex proteomic samples. In this work, we compared solution-phase trypsin with trypsin immobilized on magnetic microspheres for digestion of two complex proteomes, Escherichia coli and the MCF7 cell line. The digests were separated by HPLC, and detected with a Q-Exactive mass spectrometer, which generated high resolution and high quality parent- and fragment-ion mass spectra. The data were analyzed using MaxQuant. We make several conclusions about the features of immobilized trypsin digestion of complex proteomes. First, both immobilized and solution-phase trypsin generate peptides that sample the same protein pool. Second, immobilized trypsin can digest complex proteomes two orders of magnitude faster than solution-phase trypsin while retaining similar numbers of protein identifications and proteome depth. Digestion using immobilized trypsin for 5-min produces a similar number of missed cleavages as solution-based trypsin digestion for 4-h; digestion using immobilized trypsin for 20-min produces a similar number of missed cleavages as solution-based trypsin digestion for 12-h. Third, immobilized trypsin produces quantitatively reproducible digestion of complex proteomes. Finally, there is small but measurable loss of peptide due to non-specific adsorption to the immobilization matrix. This adsorption generates a bias against detection of basic peptides.