Why wait ? Sample prep/protein digestion in as little as 30 minutes!

While many proteases are used in bottom-up mass spectrometric (MS) analysis, trypsin (4,5) is the de facto protease of choice for most applications. There are several reasons for this: Trypsin is highly efficient, active and specific. Tryptic peptides produced after proteolysis are ideally suited, in terms of both size (350–1,600 Daltons) and charge (+2 to +4), for MS analysis. One significant drawback to trypsin digestion is the long sample preparation times, which typically range from 4 hours to overnight for most protocols. Achieving efficient digestion usually requires that protein substrates first be unfolded either with surfactants or denaturants such as urea or guanidine. These chemical additives can have negative effects, including protein modification, inhibition of trypsin or incompatibility with downstream LC-MS/MS. Accordingly, additional steps are typically required to remove these compounds prior to analysis.

To shorten the time required to prepare samples for LC-MS/MS analysis, we have developed a specialized trypsin preparation that supports rapid and efficient digestion at temperatures as high as 70°C. There are several benefits to this approach. First, proteolytic reaction times are dramatically shortened. Second, because no chemical denaturants have been added, off -line sample cleanup is not necessary, leading to shorter preparation times and diminished sample losses.

The Rapid Digestion trypsin protocols are highly flexible. They can accommodate a variety of additives including reducing and alkylating agents. There are no restrictions on sample volume or substrate concentrations with these kits. Furthermore, the protocol is simple to follow and requires no laboratory equipment beyond a heat block. Digestion is achieved completely using an in-solution approach, and since the enzyme is not immobilized on beads, the protocol does not have strict requirements for rapid shaking and off -line filtering to remove beads.

In addition to the benefits of this flexibility, we also developed a Rapid Digestion–Trypsin/Lys-C mixture. Like the Trypsin/Lys-C Mix previously developed to prepare maximally efficiently proteolytic digests, particularly for complex mixtures, Rapid Digestion–Trypsin/Lys C is ideally suited for studies that require improved reproducibility across samples.

 

Widening the Proteolysis Bottleneck: A New Protein Sample Preparation Tool

The poster featured in this blog provides background information and data on development of Rapid Digestion-Trypsin.

The poster featured in this blog provides background information and data on development of Rapid Digestion-Trypsin.

Improvements in Protein Bioprocessing

As more and more protein-based therapeutics enter research pipelines, more efficient protocols are needed for characterization of protein structure and function, as well as means of quantitation. One main step in this pipeline, proteolysis of these proteins into peptides, presents a bottleneck and can require optimization of multiple steps including reduction, alkylation and digestion time.

We have developed a new trypsin reagent, Rapid Digestion–Trypsin, that streamlines the protein sample preparation process, reducing the time to achieve proteolysis to about 1 hour, a remarkable improvement over existing overnight sample preparation times.

How Does it Work?

With this new trypsin product, proteolysis is performed at 70°C, incorporating both denaturation and rapid digestion. The protocol can be used with multiple protein types, including pure proteins and complex mixtures, and is compatible with digestion under native, reduced or nonreduced conditions.

Continue reading

Optimization of Alternative Proteases for Bottom-Up Proteomics

Alternate Proteases CoverBottom-up proteomics focuses on the analysis of protein mixtures after enzymatic digestion of the proteins into peptides. The resulting complex mixture of peptides is analyzed by reverse-phase liquid chromatography (RP-LC) coupled to tandem mass spectrometry (MS/MS). Identification of peptides and subsequently proteins is completed by matching peptide fragment ion spectra to theoretical spectra generated from protein databases.

Trypsin has become the gold standard for protein digestion to peptides for shotgun proteomics. Trypsin is a serine protease. It cleaves proteins into peptides with an average size of 700-1500 daltons, which is in the ideal range for MS (1). It is highly specific, cutting at the carboxyl side of arginine and lysine residues. The C-terminal arginine and lysine peptides are charged, making them detectable by MS. Trypsin is highly active and tolerant of many additives.

Even with these technical features, the use of trypsin in bottom-up proteomics may impose certain limits in the ability to grasp the full proteome, Tightly-folded proteins can resist trypsin digestion. Post-translational modifications (PTMs) present a different challenge for trypsin because glycans often limit trypsin access to cleavage sites, and acetylation makes lysine and arginine residues resistant to trypsin digestion.

To overcome these problems, the proteomics community has begun to explore alternative proteases to complement trypsin. However, protocols, as well as expected results generated when using these alternative proteases have not been systematically documented.

In a recent reference (2), optimized protocols for six alternative proteases that have already shown promise in their applicability in proteomics, namely chymotrypsin, Lys-C, Lys-N, Asp-N, Glu-C and Arg-C have been created.

Data describe the appropriate MS data analysis methods and the anticipated results in the case of the analysis of a single protein (BSA) and a more complex cellular lysate (Escherichia coli). The digestion protocol presented here is convenient and robust and can be completed in approximately in 2 days.

References

  1. Laskay, U. et al. (2013) Proteome Digestion Specificity Analysis for the Rational Design of Extended Bottom-up and middle-down proteomics experiments. J of Proteome Res. 12, 5558–69.
  2. Giansanti, P. et. al. (2016) Six alternative protease for mass spectrometry based proteomics beyond trypsin. Nat. Protocols 11, 993–6

Optimizing Trypsin Digestion Parameters for Plasma Proteins

TrypsinLysC Page 2Biomarkers in biological fluids in particular have the potential to inform regarding risk of disease or to allow early detection for more effective treatment. Plasma/serum is considered the universal source of biomarkers. This fluid is, indeed, easily collected, and the important point is that plasma collects proteins from each and every tissue, compared to other fluids such as urine or cerebrospinal fluid. Optimizing experimental conditions (i.e., use of trypsin for the digestion of target proteins) used to discover or monitor biomarkers in plasma is critical to successful detection of biomarkers.

In a recent publication by Proc et al., plasma denaturation/digestion protocols were compared using quanititation methods. In this reference 14 combinations of heat, solvent (acetonitrile, methanol, trifluoroethanol), chaotropic agents (guanidine hydrochloride, urea) and surfactants (sodium dodecyl sulfate (SDS)and sodium deoxycholate (DOC) with effectiveness in improving tryptic digestion. Digestion efficiency was monitored by quantitating the peptides from 45 moderate- to high-abundance plasma proteins using tandem mass spectrometry in multiple reaction mode with a mixture of stable isotope labeled analogues of these peptides as internal standards. In the results, Proc et al. noted that use of either DOC and SDS produced an increase in the overall yield of tryptic peptides. Since SDS is not compatible with mass spectrometry and DOC can be easily remove by acid precipitation, the overall recommendation was the use of DOC with a nine hour digestion procedure.

Literature Cited:

Proc, J.L. et al. (2010) A Quantitative Study of the Effects of Chaotropic  Agents Surfactants and Solvents on the Digestion Efficiency of Human Plasma Proteins by Trypsin J. Proteome Res. 9, 5422–37.

Optimizing Tryptic Digestions for Phosphoproteomics Analysis

11296971-DC-CR-KinaseProtein phosphorylation is the most widespread type of post-translational modification. It affects every basic cellular process, including metabolism, growth, division, differentiation, motility, organelle trafficking, membrane transport, muscle contraction, immunity, learning and memory (1,2). Protein kinases catalyse the transfer of the phosphate from ATP to specific amino acids in proteins. In eukaryotes, these are usually Ser, Thr and Tyr residues. Due to the development of specific phosphopeptide enrichment techniques and highly sensitive MS instruments, phosphoproteomics has enabled researchers to gain a comprehensive view on the dynamics of protein phosphorylation and phosphorylation based signaling networks.

Due to its high cleavage specificity, trypsin is the commonly used proteolytic enzyme in MS-based proteomics, cleaving peptides carboxyterminal of the amino acids lysine and arginine. However, various factors such as the tertiary structure of a protein, adjacent basic amino acids or negatively charged residues close to cleavage sites as well as PTMs are known to impair proteolysis.

To gain closer insights into the impact of phosphorylation on tryptic digestion, a recent publication(3) systematically characterized the digestion efficiency of model peptide sequences that are known to be prone to incomplete digestion.

The results indicated that increasing trypsin concentrations up to a trypsin to peptide ratio of 1:10 led to a significant gain (1) in the overall number of phosphorylation sites (up to 9%) and in the intensities of individual phosphopeptides, thereby improving the sensitivity of phosphopeptide quantification.

The effect of organic solvents (ACN, acetonitrile and TFE trifuorethanol was also evaluated). Positive results were noted with TFE when determining the digestion of individual peptides. However TFE interfered with TiO2 phosphopeptide enrichment and therefore was not recommended for use with complex samples.

  1. Engholm-Keller, K and Larsen, M.R. (2013) Technologies and challenges in large scale phosphoroproteomics. Proteomics 13, 910–31.
  2. Beausoleil, S. A. et al. (2010) Tissue-specific atlas of mouse protein phosphorylation and expression. Cell 143, 1174–89.
  3. Dickhut, C. et al. (2014) Impact of Digestion Conditions on phosphoproteomics. J. Proteome Res. 13, 2761–70.

Enhanced Protein Mass Spectrometry Analysis with Trypsin/Lys-C Mix

We recently presented a webinar illustrating the technical benefits of the new Trypsin/Lys-C Mix, Mass Spec Grade. The following is a summary of key attributes highlighted during the presentation:

Side-by-side Comparison of Trypsin and Trypsin/Lys-C Digestion for Missed Cleavages (% of total cleavages). All the digests used overnight 37°C incubation.

Side-by-side Comparison of Trypsin and Trypsin/Lys-C Digestion for Missed Cleavages (% of total cleavages). All the digests used overnight 37°C incubation.

Efficient proteolysis is a major requirement for protein mass spectrometry analysis. Incomplete digestion has multiple ramifications including decreased number of identified proteins, compromised analytical reproducibility and protein quantitation, etc. Trypsin is one of the most robust proteases and is characterized by efficient proteolysis. Typical trypsin reactions do not digest proteins to completion, missing 15–30% of cleavage sites. Incomplete digestion affects protein identification, reproducibility of mass spectrometry analysis and accuracy of protein quantitation. Supplementing Trypsin with Lys-C compensates for the majority of missed cleavages. Continue reading

Improving Protein Digestion for Mass Spec Analysis

The latest Promega Webinar covered trypsin and protein analysis for mass spectrometry. Here we summarize some of the discussion from this webinar.

Guest Blog by Lynn Litterer, Promega Technical Services Scientist

You are studying your favorite biological system and fascinating questions about how it works. Chances are good that some of those questions involve protein interaction with other proteins and molecules in the cell. Where and when proteins are active is often regulated by post-translational modification. Mass spectrometry (mass spec, MS) is a powerful tool to identify protein sequence and modifications. Strategic use of mass spec tools can substantially improve the analysis of your samples.

Why is trypsin the first choice for protein sequencing? Continue reading

Use of Multiple Proteases for Improved Protein Digestion

ResearchBlogging.orgOne of the approaches to identify proteins by mass spectrometry includes the separation of proteins by gel electrophoresis or liquid chromatography. Subsequently the proteins are cleaved with sequence-specific endoproteases. Following digestion the generated peptides are investigated by determination of molecular masses or specific sequence. For protein identification the experimentally obtained masses/sequences are compared with theoretical masses/sequences compiled in various databases.

Trypsin is the favored enzyme for this application, for the following reasons: A) the peptides contain a basic residue (Arg or Lys) on the C terminus and thus are good candidates for collision induced activation (CAD) in tandem experiments (low charge states and high mass-to-charge ratios); B) it is relatively Inexpensive; and C) optimal digestion conditions have been well characterized.

An inherent limitation of trypsin is the size of the peptides that it generates. For most organisms > 50% of tryptic peptides are less than 6 amino acids, too small for mass spectrometry based sequencing.

One recent publication examined the use of multiple proteases (trypsin, LysC, ArgC , AspN and GluC) in combination with either CAD or electron-based fragmentation (ETD) to improve protein identification (1). Their results indicated a significant improvement from a single protease digestion (trypsin), which yielded 27,822 unique peptides corresponding to 3313 proteins. In contrast using a combination of proteases with either CAD or ETD fragmentation methods yielded 92,095 unique peptides mapping to 3908 proteins.

Swaney DL, Wenger CD, & Coon JJ (2010). Value of using multiple proteases for large-scale mass spectrometry-based proteomics. Journal of proteome research, 9 (3), 1323-9 PMID: 20113005

Trypsin: Innovative Applications

Tryptic digestion of samples and subsequent analysis by mass spectrometry is a popular technique for the identification of proteins typically those related to interaction partners or biomarkers characterization. This powerful tool can also be used for less traditional experimental designs. Three examples are: Continue reading

Enhanced In-Solution Tryptic Digestions: Immobilized Trypsin

In many proteome studies, sample analysis is performed directly from the liquid phase. This method avoids many of the time consuming steps associated with in-gel digestion. As with in-gel digestions, trypsin is the enzyme of choice. Trypsin cleaves at C-terminal of lysine (K) and arginine (R) amino acids resulting in a N-terminal amine group that can accept a proton and a basic side chain residue of the K/R that will also take up a proton. These charged particles can be easily ionized when analyzed by mass spectrometry.

Immobilized Trypsin provides a fast and convenient method for digesting a range of concentrations of purified protein, or complex protein mixtures. Continue reading