Protein Molecular Mass to 1 Da by13C,15N Double-Depletion and FT-ICR Mass Spectrometry

Isotopic Depletion Increases the Spatial Resolution of FPOP Top-Down Mass Spectrometry Analysis

Protein radical labeling, like fast photochemical oxidation of proteins (FPOP), coupled to a top-down mass spectrometry (MS) analysis offers an alternative analytical method for probing protein structure or protein interaction with other biomolecules, for instance, proteins and DNA. However, with the increasing mass of studied analytes, the MS/MS spectra become complex and exhibit a low signal-to-noise ratio. Nevertheless, these difficulties may be overcome by protein isotope depletion. Thus, we aimed to use protein isotope depletion to analyze FPOP-oxidized samples by top-down MS analysis. For this purpose, we prepared isotopically natural (IN) and depleted (ID) forms of the FOXO4 DNA binding domain (FOXO4-DBD) and studied the protein-DNA interaction interface with double-stranded DNA, the insulin response element (IRE), after exposing the complex to hydroxyl radicals. As shown by comparing tandem mass spectra of natural and depleted proteins, the ID form increased the signal-to-noise ratio of useful fragment ions, thereby enhancing the sequence coverage by more than 19%. This improvement in the detection of fragment ions enabled us to detect 22 more oxidized residues in the ID samples than in the IN sample. Moreover, less common modifications were detected in the ID sample, including the formation of ketones and lysine carbonylation. Given the higher quality of ID top-down MSMS data set, these results provide more detailed information on the complex formation between transcription factors and DNA-response elements. Therefore, our study highlights the benefits of isotopic depletion for quantitative top-down proteomics. Data are available via ProteomeXchange with the identifier PXD044447.

Ultralight Ultrafast Enzymes

Inorganic materials depleted of heavy stable isotopes are known to deviate strongly in some physicochemical properties from their isotopically natural counterparts. Here we explored for the first time the effect of simultaneous depletion of the heavy carbon, hydrogen, oxygen and nitrogen isotopes on the bacterium E. coli and the enzymes expressed in it. Bacteria showed faster growth, with most proteins exhibiting higher thermal stability, while for recombinant enzymes expressed in depleted media, faster kinetics was discovered. At room temperature, luciferase, thioredoxin and dihydrofolate reductase and Pfu DNA polymerase showed up to a 250 % increase in activity compared to the native counterparts, with an additional ∼50 % increase at 10 °C. Diminished conformational and vibrational entropy is hypothesized to be the cause of the accelerated kinetics. Ultralight enzymes may find an application where extreme reaction rates are required.

Hyphenation of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with separation methods: The art of compromises and the possible - A review

This review provides an overview of the online hyphenation of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) with separation methods to date. The online coupling between separation techniques (gas and liquid chromatography, capillary electrophoresis) and FT-ICR MS essentially raises questions of compromise and is not look as straightforward as hyphenation with other analyzers (QTOF-MS for instance). FT-ICR MS requires time to reach its highest resolving power and accuracy in mass measurement capabilities whereas chromatographic and electrophoretic peaks are transient. In many applications, the strengths and the weaknesses of each technique are balanced by their hyphenation. Untargeted "Omics" (e.g. proteomics, metabolomics, petroleomics, …) is one of the main areas of application for FT-ICR MS hyphenated to online separation techniques because of the complexity of the sample. FT-ICR MS achieves the required high mass measurement accuracy to determine accurate molecular formulae and resolution for isobar distinction. Meanwhile separation techniques highlight isomers and reduce the ion suppression effects extending the dynamic range. Even if the implementation of FT-ICR MS hyphenated with online separation methods is a little trickier (the art of compromise), this review shows that it provides unparalleled results to the scientific community (the art of the possible), along with raising the issue of its future in the field with the relentless technological progress.

Determining the Location of the α-Synuclein Dimer Interface Using Native Top-Down Fragmentation and Isotope Depletion-Mass Spectrometry

α-Synuclein (αSyn), a 140-residue intrinsically disordered protein, comprises the primary proteinaceous component of pathology-associated Lewy body inclusions in Parkinson's disease (PD). Due to its association with PD, αSyn is studied extensively; however, the endogenous structure and physiological roles of this protein are yet to be fully understood. Here, ion mobility-mass spectrometry and native top-down electron capture dissociation fragmentation have been used to elucidate the structural properties associated with a stable, naturally occurring dimeric species of αSyn. This stable dimer appears in both wild-type (WT) αSyn and the PD-associated variant A53E. Furthermore, we integrated a novel method for generating isotopically depleted protein into our native top-down workflow. Isotope depletion increases signal-to-noise ratio and reduces the spectral complexity of fragmentation data, enabling the monoisotopic peak of low abundant fragment ions to be observed. This enables the accurate and confident assignment of fragments unique to the αSyn dimer to be assigned and structural information about this species to be inferred. Using this approach, we were able to identify fragments unique to the dimer, which demonstrates a C-terminal to C-terminal interaction between the monomer subunits. The approach in this study holds promise for further investigation into the structural properties of endogenous multimeric species of αSyn.

Analysis of Isotopically Depleted Proteins Derived from Escherichia coli and Caenorhabditis elegans Cell Lines by Liquid Chromatography 21 T Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry

Protein mass measurement by mass spectrometry is complicated by wide isotopic distributions that result from incorporation of heavy isotopes of C, H, N, O, and S, thereby limiting signal-to-noise ratio (SNR) and accurate intact mass determination, particularly for larger proteins [Fenselau et al. Anal. Chem. 1983, 55 (2), 353-356]. Observation of the monoisotopic mass-to-charge ratio (m/z) is the simplest and most accurate way to determine intact protein mass, but as mass increases, the relative abundance of the monoisotopic peak becomes so low that it is often undetectable. Here, we used an isotopically depleted growth medium to culture bacterial cells (Escherichia coli), resulting in isotopically depleted proteins. Isotopically depleted proteins show increased sequence coverage, mass measurement accuracy, and increased S/N of the monoisotopic peak by Fourier transform ion cyclotron resonance mass spectrometry analysis. We then grew Caenorhabditis elegans cells in a medium containing living isotopically depleted E. coli cells, thereby producing the first isotopically depleted eukaryotic proteins. This is the first time isotopic depletion has been implemented for four isotopes (¹H, ¹²C, ¹⁴N, and ¹⁶O), resulting in the highest degree of depletion ever used for protein analysis and further improving MS analysis.

Novel Insights into Quantitative Proteomics from an Innovative Bottom-Up Simple Light Isotope Metabolic (bSLIM) Labeling Data Processing Strategy

Simple light isotope metabolic labeling (SLIM labeling) is an innovative method to quantify variations in the proteome based on an original in vivo labeling strategy. Heterotrophic cells grown in U-[12C] as the sole source of carbon synthesize U-[12C]-amino acids, which are incorporated into proteins, giving rise to U-[12C]-proteins. This results in a large increase in the intensity of the monoisotope ion of peptides and proteins, thus allowing higher identification scores and protein sequence coverage in mass spectrometry experiments. This method, initially developed for signal processing and quantification of the incorporation rate of 12C into peptides, was based on a multistep process that was difficult to implement for many laboratories. To overcome these limitations, we developed a new theoretical background to analyze bottom-up proteomics data using SLIM-labeling (bSLIM) and established simple procedures based on open-source software, using dedicated OpenMS modules, and embedded R scripts to process the bSLIM experimental data. These new tools allow computation of both the 12C abundance in peptides to follow the kinetics of protein labeling and the molar fraction of unlabeled and 12C-labeled peptides in multiplexing experiments to determine the relative abundance of proteins extracted under different biological conditions. They also make it possible to consider incomplete 12C labeling, such as that observed in cells with nutritional requirements for nonlabeled amino acids. These tools were validated on an experimental dataset produced using various yeast strains of Saccharomyces cerevisiae and growth conditions. The workflows are built on the implementation of appropriate calculation modules in a KNIME working environment. These new integrated tools provide a convenient framework for the wider use of the SLIM-labeling strategy.

Isotope Depletion Mass Spectrometry (ID-MS) for Accurate Mass Determination and Improved Top-Down Sequence Coverage of Intact Proteins

Top-down mass spectrometry (MS) is an increasingly important technique for protein characterization. However, in many biological MS experiments, the practicality of applying top-down methodologies is still limited at higher molecular mass. In large part, this is due to the detrimental effect resulting from the partitioning of the mass spectral signal into an increasing number of isotopic peaks as molecular mass increases. Reducing the isotopologue distribution of proteins via depletion of heavy stable isotopes was first reported over 20 years ago (Marshall, A. G.; Senko, M. W.; Li, W.; Li, M.; Dillon, S., Guan, S.; Logan, T. M.. Protein Molecular Mass to 1 Da by ¹³C, ¹⁵N Double-Depletion and FT-ICR Mass Spectrometry. J. Am. Chem. Soc. 1997, 119, 433-434.) and has been demonstrated for several small proteins. Here we extend this approach, introducing a new highly efficient method for the production of recombinant proteins depleted in ¹³C and ¹⁵N and demonstrating its advantages for top-down analysis of larger proteins (up to ∼50 kDa). FT-ICR MS of isotopically depleted proteins reveals dramatically reduced isotope distributions with monoisotopic signal observed up to 50 kDa. In top-down fragmentation experiments, the reduced spectral complexity alleviates fragment-ion signal overlap, the presence of monoisotopic signals allows assignment with higher mass accuracy, and the dramatic increase in signal-to-noise ratio (up to 7-fold) permits vastly reduced acquisition times. These compounding benefits allow the assignment of ∼3-fold more fragment ions than comparable analyses of proteins with natural isotopic abundances. Finally, we demonstrate greatly increased sequence coverage in time-limited top-down experiments-highlighting advantages for top-down LC-MS/MS workflows and top-down proteomics.

A Simple Light Isotope Metabolic Labeling (SLIM-labeling) Strategy: A Powerful Tool to Address the Dynamics of Proteome Variations In Vivo

Many quantitative proteomics strategies rely on in vivo metabolic incorporation of amino acids with modified stable isotope profiles into proteins. These methods give rise to multiple ions for each peptide, with possible distortion of the isotopolog distribution, making the overall analytical process complex. We validated an alternative strategy, simple light isotope metabolic labeling (SLIM-labeling), which alleviates many of these problems. SLIM-labeling is based on the in vivo reduction of the isotopic composition of proteins using metabolic precursors with a unique light isotope composition to label all amino acids. This brings a new dimension to in-depth, high resolution MS-based quantitative proteomics. Here, we describe a ¹²C-based SLIM-labeling strategy using U-[¹²C]-glucose as the metabolic precursor of all amino acids in the pathogenic yeast Candida albicans Monoisotopic ion intensity increased exponentially following ¹²C enrichment, substantially improving peptide identification scores and protein sequence coverage in bottom-up analyses. Multiplexing samples of ¹²C composition varying from natural abundance (98.93%) to 100% makes it possible to address relative quantification issues, keeping all the critical information for each peptide within a single isotopolog cluster. We applied this method to measure, for the first time, protein turnover at the proteome scale in Candida albicans and its modulation by inhibitors of the proteasome and vacuolar protein degradation systems.

A Comprehensive Guide for Performing Sample Preparation and Top-Down Protein Analysis

Methodologies for the global analysis of proteins in a sample, or proteome analysis, have been available since 1975 when Patrick O′Farrell published the first paper describing two-dimensional gel electrophoresis (2D-PAGE). This technique allowed the resolution of single protein isoforms, or proteoforms, into single ‘spots’ in a polyacrylamide gel, allowing the quantitation of changes in a proteoform′s abundance to ascertain changes in an organism′s phenotype when conditions change. In pursuit of the comprehensive profiling of the proteome, significant advances in technology have made the identification and quantitation of intact proteoforms from complex mixtures of proteins more routine, allowing analysis of the proteome from the ‘Top-Down’. However, the number of proteoforms detected by Top-Down methodologies such as 2D-PAGE or mass spectrometry has not significantly increased since O’Farrell’s paper when compared to Bottom-Up, peptide-centric techniques. This article explores and explains the numerous methodologies and technologies available to analyse the proteome from the Top-Down with a strong emphasis on the necessity to analyse intact proteoforms as a better indicator of changes in biology and phenotype. We arrive at the conclusion that the complete and comprehensive profiling of an organism′s proteome is still, at present, beyond our reach but the continuing evolution of protein fractionation techniques and mass spectrometry brings comprehensive Top-Down proteome profiling closer.

Heavy Sugar and Heavy Water Create Tunable Intact Protein Mass Increases for Quantitative Mass Spectrometry in Any Feed and Organism

Stable isotope labeling techniques for quantitative top-down proteomics face unique challenges. These include unpredictable mass shifts following isotope labeling, which impedes analysis of unknown proteins and complex mixtures and exponentially greater susceptibility to incomplete isotope incorporation, manifesting as broadening of labeled intact protein peaks. Like popular bottom-up isotope labeling techniques, most top-down labeling methods are restricted to defined media/feed as well as amino acid auxotrophic organisms. We present a labeling method optimized for top-down proteomics that overcomes these challenges. We demonstrated this method through the spiking of ¹³C-sugar or ²H-water into standard laboratory feedstocks, resulting in tunable intact protein mass increases (TIPMI). After mixing of labeled and unlabeled samples, direct comparison of light and heavy peaks allowed for the relative quantitation of intact proteins in three popular model organisms, including prokaryotic and eukaryotic microorganisms and an animal. This internal standard method proved to be more accurate than label-free quantitation in our hands. Advantages over top-down SILAC include working equally well in nutrient-rich media, conceivably expanding applicability to any organism and all classes of biomolecules, not requiring high-resolving power MS for quantitation and being relatively inexpensive.

Establishing drug resistance in microorganisms by mass spectrometry

A rapid method to determine drug resistance in bacteria based on mass spectrometry is presented. In it, a mass spectrum of an intact microorganism grown in drug-containing stable isotope-labeled media is compared with a mass spectrum of the intact microorganism grown in non-labeled media without the drug present. Drug resistance is determined by predicting characteristic mass shifts of one or more microorganism biomarkers using bioinformatics algorithms. Observing such characteristic mass shifts indicates that the microorganism is viable even in the presence of the drug, thus incorporating the isotopic label into characteristic biomarker molecules. The performance of the method is illustrated on the example of intact E. coli, grown in control (unlabeled) and (13)C-labeled media, and analyzed by MALDI TOF MS. Algorithms for data analysis are presented as well.

Multistage accurate mass spectrometry: a "basket in a basket" approach for structure elucidation and its application to a compound from combinatorial synthesis

A "basket in a basket" method based on a multistage accurate mass spectrometric (MAMS) technique was developed and demonstrated by obtaining a unique elemental composition of a compound (with a molecular weight of 517) from combinatorial synthesis. The accurate masses for the parent and the fragment ions were obtained with up to five stages of MAMS using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This approach requires only input of elements used in the synthetic processes and some constraints about unusual light elements, such as fluorine, while the compositions of the parent ions and their fragments are obtained for structure elucidation. Conversely, accuracy of better than 0.02 ppm (assuming elements C, H, N, O, S, and F are involved) would be required in order to define a unique composition for the same mass using a direct accurate mass measurement because the number of possible elemental compositions increases sharply as the mass increases. Similarly, due to the uncertainty in determining elemental compositions of fragments and complexity of possible internal fragmentation, tandem mass spectrometry may not provide enough information for structure elucidation of unknown compounds, especially of the organic molecules in the mass range of 300-1000 Da, typically encountered in combinatorial lead generation. The application of MAMS to combinatorial drug discovery is particularly advantageous since the built-in chemical information from the synthesis can be used as constraints. The implementation of a nanoelectrospray ionization technique makes this approach practical for characterization of small quantities of compounds typically available from lead generation processes.

Unit mass baseline resolution for an intact 148 kDa therapeutic monoclonal antibody by Fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) provides the highest mass resolving power and mass measurement accuracy for unambiguous identification of biomolecules. Previously, the highest-mass protein for which FTICR unit mass resolution had been obtained was 115 kDa at 7 T. Here, we present baseline resolution for an intact 147.7 kDa monoclonal antibody (mAb), by prior dissociation of noncovalent adducts, optimization of detected total ion number, and optimization of ICR cell parameters to minimize space charge shifts, peak coalescence, and destructive ion cloud Coulombic interactions. The resultant long ICR transient lifetime (as high as 20 s) results in magnitude-mode mass resolving power of ~420,000 at m/z 2,593 for the 57+ charge state (the highest mass for which baseline unit mass resolution has been achieved), auguring for future characterization of even larger intact proteins and protein complexes by FTICR MS. We also demonstrate up to 80% higher resolving power by phase correction to yield an absorption-mode mass spectrum.

On the scalability and requirements of whole protein mass spectrometry

Top-down proteomics has improved over the past decade despite the significant challenges presented by the analysis of large protein ions. Here, the detection of these high mass species by electrospray-based mass spectrometry (MS) is examined from a theoretical perspective to understand the mass-dependent increases in the number of charge states, isotopic peaks, and interfering species present in typical protein mass spectra. Integrating these effects into a quantitative model captures the reduced ability to detect species over 25 kDa with the speed and sensitivity characteristic of proteomics based on <3 kDa peptide ions. The model quantifies the challenge that top-down proteomics faces with respect to current MS instrumentation and projects that depletion of (13)C and (15)N isotopes can improve detection at high mass by only <2-fold at 100 kDa whereas the effect is up to 5-fold at 10 kDa. Further, we find that supercharging electrosprayed proteins to the point of producing <5 charge states at high mass would improve detection by more than 20-fold.

Advantages of isotopic depletion of proteins for hydrogen/deuterium exchange experiments monitored by mass spectrometry

Solution-phase hydrogen/deuterium exchange (HDX) monitored by mass spectrometry is an excellent tool to study protein-protein interactions and conformational changes in biological systems, especially when traditional methods such as X-ray crystallography or nuclear magnetic resonance are not feasible. Peak overlap among the dozens of proteolytic fragments (including those from autolysis of the protease) can be severe, due to high protein molecular weight(s) and the broad isotopic distributions due to multiple deuterations of many peptides. In addition, different subunits of a protein complex can yield isomeric proteolytic fragments. Here, we show that depletion of (13)C and/or (15)N for one or more protein subunits of a complex can greatly simplify the mass spectra, increase the signal-to-noise ratio of the depleted fragment ions, and remove ambiguity in assignment of the m/z values to the correct isomeric peptides. Specifically, it becomes possible to monitor the exchange progress for two isobaric fragments originating from two or more different subunits within the complex, without having to resort to tandem mass spectrometry techniques that can lead to deuterium scrambling in the gas phase. Finally, because the isotopic distribution for a small to medium-size peptide is essentially just the monoisotopic species ((12)C(c)(1)H(h)(14)N(n)(16)O(o)(32)S(s)), it is not necessary to deconvolve the natural abundance distribution for each partially deuterated peptide during HDX data reduction.

Distributive and directional behavior of lantibiotic synthetases revealed by high-resolution tandem mass spectrometry

The lantibiotic synthetases LctM and HalM2 are bifunctional enzymes that catalyze both the dehydration of serine and threonine residues and the Michael-type additions of cysteine residues to the resulting dehydroamino acids in their substrate peptides. Using Fourier transform mass spectrometry to analyze these activities in vitro, the dehydration is shown to take place by a distributive mechanism, with build-up of intermediates observed in electrospray mass spectra. The cyclization activity of HalM2 was monitored through alkylation of free cysteines in intermediates, providing access to the regioselectivity of lanthionine ring formation using high-resolution tandem mass spectrometry. HalM2 is shown to catalyze the cyclization process in a largely N- to C-terminal directional fashion, forming a total of four lanthionine rings in its HalA2 substrate. These studies advance a model for lantibiotic production where substrate binding via an N-terminal leader results in dehydration and cyclization on similar time scales and with a high, though not strict, propensity for N-to-C directionality.

Investigation of molecular interaction within biological macromolecular complexes by mass spectrometry

The advent of electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) has accelerated structural studies of biological macromolecular complexes. At present, mass spectrometry can provide accurate mass values not only of individual biological macromolecules but also of their assemblies. Furthermore, it can also give information on the interface sites of the biological macromolecular complexes. The present article focuses on the role of mass spectrometry in the investigation of biological molecular interactions, such as protein-protein, protein-DNA, and protein-ligand interactions, which play essential roles in various biological events.

Stepwise deamidation of ribonuclease A at five sites determined by top down mass spectrometry

Although deamidation at asparagine and glutamine has been found in numerous studies of a variety of proteins, in almost all cases the analytical methodology that was used could detect only a single site of deamidation. For the extensively studied case of reduced bovine ribonuclease A (13,689 Da), only Asn67 deamidation has been demonstrated previously, although one study found three monodeamidated fractions. Here top down tandem mass spectrometry shows that Asn67 deamidation is extensive before Asn71 and Asn94 react; these are more than half deamidated before Asn34 reacts, and its deamidation is extensive before that at Gln74 is initiated. Except for the initial Asn67 site, these large reactivity differences correlate poorly with neighboring amino acid identities and instead indicate residual conformational effects despite the strongly denaturing media that were used; deamidation at Asn67 could enhance that at Asn71, and these enhance that at Gln74. This success in the site-specific quantitation of deamidation in a 14 kDa protein mixture, despite the minimal 1 Da (-NH2 --> -OH) change in the molecular mass, is further evidence of the broad applicability of the top down MS/MS methodology for characterization of protein posttranslational modifications.

The dehydratase activity of lacticin 481 synthetase is highly processive

Lacticin 481 synthetase (LctM) is a bifunctional enzyme that undertakes dehydration and cyclization in the structural region of the pre-lacticin peptide (LctA) to introduce three thioether rings and one dehydrobutyrine residue. The order and timing of these events has been investigated employing high-resolution ESI-FTMS-based tandem MS/MS techniques and chemical derivatization. LctM demonstrates highly processive behavior as seen by MS analysis of the reaction course of dehydration. Furthermore, cyclization is not tightly coupled to dehydration and follows at a later stage of the enzymatic reaction.

Dissecting non-ribosomal and polyketide biosynthetic machineries using electrospray ionization Fourier-Transform mass spectrometry

Many virulence factors and bioactive compounds with antifungal, antimicrobial, and antitumor properties are produced via the non-ribosomal peptide synthetase (NRPS) or polyketide synthase(PKS) paradigm. During the biosynthesis of these natural products, substrates, intermediates and side products are covalently tethered to the NRPS or PKS catalyst, introducing mass changes, making these biosynthetic systems ideal candidates for interrogation by large molecule mass spectrometry. This review serves as an introduction into the application of electrospray ionization Fourier-Transform massspectrometry (ESI-FTMS) to investigate NRPS and PKS systems. ESI-FTMS can be used to understand substrate tolerance, timing of covalent linkages, timing of tailoring reactions and the transfer of substrates and biosynthetic intermediates from domain to domain. Therefore we not only highlight key mechanistic insights for thiotemplate systems as found on the enterobactin,yersiniabactin, epothilone, clorobiocin, coumermycin, pyoluteorin, gramicidin, mycosubtilin, C-1027,6-deoxyerythronolide B and FK520 biosynthetic pathways, but we also explain the approaches taken to identify active sites from complex digests and compare the FTMS based assay to traditional assays and other mass spectrometric techniques. Although mass spectrometry was introduced over two decades ago to investigate NRPS and PKS biosynthetic systems, this is the first review devoted to this methodology.

Contemporary mass spectrometry for the direct detection of enzyme intermediates

The field of enzymology has long used small-molecule mass spectrometry. However, the direct interrogation of covalent and non-covalent intermediates by large-molecule mass spectrometry of enzymes or large peptide substrates is illuminating an increasingly diverse array of chemistries used in nature. Recent advances now allow improved detection of several modifications formed at sub-stoichiometric levels on the same polypeptide, and elucidation of intermediate dynamics with low millisecond temporal resolution. Highlighting recent applications in both ribosomal and non-ribosomal biosynthesis of natural products, along with acetyl transferases, sulfonucleotide reducatases, and PEP-utilizing enzymes, the utility of small- and large-molecule mass spectrometry to reveal enzyme intermediates and illuminate mechanism is described briefly. From ever more complex mixtures, mass spectrometry continues to evolve into a key technology for a larger number of today's enzymologists.

Dichlorination of a pyrrolyl-S-carrier protein by FADH2-dependent halogenase PltA during pyoluteorin biosynthesis

The antifungal natural product pyoluteorin contains a 4,5-dichloropyrrole moiety. The timing of dichlorination in the heteroaromatic ring is now shown to occur after proline is tethered by thioester linkage to the carrier protein PltL and enzymatically desaturated to the pyrrolyl-S-PltL. Surprisingly, the FADH2-dependent halogenase PltA catalyzes chlorination at both positions of the ring, generating the 5-chloropyrrolyl-S-PltL intermediate and then the 4,5-dichloropyrrolyl-S-PltL product. PltA activity strictly depends on a heterologous flavin reductase that uses NAD(P)H to produce FADH2. Electrospray ionization-Fourier transform MS detected five covalent intermediates attached to the 11-kDa carrier protein PltL. Tandem MS localized the site of covalent modification on the carrier protein scaffold. HPLC analysis of the hydrolyzed products was consistent with the regiospecific chlorination at position 5 and then position 4 of the heteroaromatic ring. A mechanism for dichlorination is proposed involving formation of a FAD-4a-OCl intermediate for capture by the electron-rich C4 and C5 of the heteroaromatic pyrrole moiety.

Simultaneous kinetic characterization of multiple protein forms by top down mass spectrometry

Top down mass spectrometry, using a Fourier transform instrument, has unique capabilities for biomolecule kinetic studies, in that the concentration of large molecules in a reaction mixture can be monitored simultaneously from its mass spectrum produced by electrospray ionization. This is demonstrated with enzyme modifications occurring in the biosynthesis of the thiazole moiety of thiamin phosphate. The formation rate of ThiS-thiocarboxylate from ThiS was determined from the relative abundance of the corresponding m/z 10162 and 10146 isotopic peak clusters for all the observable charge states in the mass spectra measured at different reaction times. Even without measuring standard ionization efficiencies, the rate and precision of 0.018 +/- 0.004 min(-1) agree well with the 0.027 +/- 0.003 min(-1) obtained with a radiochemical assay, which requires a separate derivatization step. To illustrate the simultaneous characterization of the reaction kinetics of a native enzyme and its mutant, the imine formation rate of ThiG and its substrate DXP was compared between the native protein (M(r) = 26803.9) and its E98A (M(r) = 26745.9) or D182A (M(r) = 26759.9) mutant in the same reaction mixture. The kinetic data show clearly that neither the E98 nor the D182 residues participate in the imine formation. The high resolution and MS/MS capabilities of FTMS should make possible the extension of this kinetics approach to far more complicated systems, such as simultaneous monitoring of 24 native, intermediate, and reduced forms in the reductive unfolding of a mixture of ribonuclease A and the five isoforms of ribonuclease B. Stable intermediates with different SS bonding (same molecular weight) can be differentiated by MS/MS, while molecular ions differing by only 2 Da are distinguished clearly by synthesizing isotopically depleted proteins.

Two-dimensional mass spectra generated from the analysis of 15N-labeled and unlabeled peptides for efficient protein identification and de novo peptide sequencing

Protein identification has been greatly facilitated by database searches against protein sequences derived from product ion spectra of peptides. This approach is primarily based on the use of fragment ion mass information contained in a MS/MS spectrum. Unambiguous protein identification from a spectrum with low sequence coverage or poor spectral quality can be a major challenge. We present a two-dimensional (2D) mass spectrometric method in which the numbers of nitrogen atoms in the molecular ion and the fragment ions are used to provide additional discriminating power for much improved protein identification and de novo peptide sequencing. The nitrogen number is determined by analyzing the mass difference of corresponding peak pairs in overlaid spectra of (15)N-labeled and unlabeled peptides. These peptides are produced by enzymatic or chemical cleavage of proteins from cells grown in (15)N-enriched and normal media, respectively. It is demonstrated that, using 2D information, i.e., m/z and its associated nitrogen number, this method can, not only confirm protein identification results generated by MS/MS database searching, but also identify peptides that are not possible to identify by database searching alone. Examples are presented of analyzing Escherichia coli K12 extracts that yielded relatively poor MS/MS spectra, presumably from the digests of low abundance proteins, which can still give positive protein identification using this method. Additionally, this 2D MS method can facilitate spectral interpretation for de novo peptide sequencing and identification of posttranslational or other chemical modifications. We envision that this method should be particularly useful for proteome expression profiling of organelles or cells that can be grown in (15)N-enriched media.

Determination of thioxylo-oligosaccharide binding to family 11 xylanases using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and X-ray crystallography

Noncovalent binding of thioxylo-oligosaccharide inhibitors, methyl 4-thio-alpha-xylobioside (S-Xyl2-Me), methyl 4,4II-dithio-alpha-xylotrioside (S-Xyl3-Me), methyl 4,4II,4III-trithio-alpha-xylotetroside (S-Xyl4-Me), and methyl 4,4II,4III,4IV-tetrathio-alpha-xylopentoside (S-Xyl5-Me), to three family 11 endo-1,4-beta-xylanases from Trichoderma reesei (TRX I and TRX II) and Chaetomium thermophilum (CTX) was characterized using electrospray ionization Fourier transform ion cyclotron resonance (FT-ICR) MS and X-ray crystallography. Ultra-high mass-resolving power and mass accuracy inherent to FT-ICR allowed mass measurements for noncovalent complexes to within |DeltaM|average of 2 p.p.m. The binding constants determined by MS titration experiments were in the range 10(4)-10(3) M-1, decreasing in the series of S-Xyl5-Me>or=S-Xyl4-Me>S-Xyl3-Me. In contrast, S-Xyl2-Me did not bind to any xylanase at the initial concentration of 5-200 microM, indicating increasing affinity with increasing number of xylopyranosyl units, with a minimum requirement of three. The crystal structures of CTX-inhibitor complexes gave interesting insights into the binding. Surprisingly, none of the inhibitors occupied any of the aglycone subsites of the active site. The binding to only the glycone subsites is nonproductive for catalysis, and yet this has also been observed for other family 11 xylanases in complex with beta-d-xylotetraose [Wakarchuk WW, Campbell RL, Sung WL, Davoodi J & Makoto Y (1994) Protein Sci3, 465-475, and Sabini E, Wilson KS, Danielsen S, Schulein M & Davies GJ (2001) Acta CrystallogrD57, 1344-1347]. Therefore, the role of the aglycone subsites remains controversial despite their obvious contribution to catalysis.

Combining capillary electrophoresis with mass spectrometry for applications in proteomics

Mass spectrometry (MS)-based proteomics is currently dominated by the analysis of peptides originating either from digestion of proteins separated by two-dimensional gel electrophoresis (2-DE) or from global digestion; the simple peptide mixtures obtained from digestion of gel-separated proteins do not usually require further separation, while the complex peptide mixtures obtained by global digestion are most frequently separated by chromatographic techniques. Capillary electrophoresis (CE) provides alternatives to 2-DE for protein separation and alternatives to chromatography for peptide separation. This review attempts to elucidate how the most promising CE modes, capillary zone electrophoresis (CZE) and capillary isoelectric focusing (CIEF), might best be applied to MS-based proteomics. CE-MS interfacing, mass analyzer performance, column coating to minimize analyte adsorption, and sample stacking for CZE are considered prior to examining numerous applications. Finally, multidimensional systems that incorporate CE techniques are examined; CZE often finds use as a fast, final dimension before ionization for MS, while CIEF, being an equilibrium technique, is well-suited to being the first dimension in automated fractionation systems.

The biosynthesis of the thiazole phosphate moiety of thiamin: the sulfur transfer mediated by the sulfur carrier protein ThiS

Thiamin-pyrophosphate is an essential cofactor in all living systems. The biosynthesis of both the thiazole and the pyrimidine moieties of this cofactor involves new biosynthetic chemistry. Thiazole-phosphate synthase (ThiG) catalyses the formation of the thiazole moiety of thiamin-pyrophosphate from 1-deoxy-D-xylulose-5-phosphate (DXP), dehydroglycine and the sulfur carrier protein (ThiS), modified on its carboxy terminus as a thiocarboxylate (ThiS-thiocarboxylate). Thiazole biosynthesis is initiated by the formation of a ThiG/DXP imine, which then tautomerizes to an amino-ketone. In this paper we study the sulfur transfer from ThiS-thiocarboxylate to this amino-ketone and trap a new thioenolate intermediate. Surprisingly, thiazole formation results in the replacement of the ThiS-thiocarboxylate sulfur with an oxygen from DXP and not from the buffer, as shown by electrospray ionization Fourier transform mass spectrometry (ESI-FTMS) using (18)O labeling of the 13C-, 15N-depleted protein. These observations further clarify the mechanism of the complex thiazole biosynthesis in bacteria.

Detection and localization of protein modifications by high resolution tandem mass spectrometry

For interrogation of peptides with diverse modifications, no other instrument is as versatile as the Fourier-transform mass spectrometer (FTMS). Particularly using electrospray ionization (ESI), many intact proteins and their proteolytic products harboring post-translational and chemical modifications (PTMs) have been studied by high resolution tandem mass spectrometry (MS/MS). The widely touted analytical figures of merit for FTMS in fact have translated into clarity when analyzing PTMs from phosphorylations to disulfides, oxidations, methylations, acetylations, and even exotic PTMs found in the biosynthesis of antibiotics and other natural products. A top down approach to PTM detection and localization is proving extensible to an increasing variety of PTMs, some of which are stable to MS/MS at the protein level but unstable to amide bond cleavage by threshold dissociations at the level of small peptides <3 kDa. In contrast, MS/MS using electron capture dissociation (ECD) allows precise localization of even labile PTMs given enough sample and abundant molecular ions. Finally, this brief synopsis of recent literature highlights specific PTMs that perturb the protein backbone therefore altering MS/MS fragmentation patterns. Thus, FTMS will continue its expansion into more laboratories in part because of its ability to detect and deconvolute the regulatory mechanisms of biology written in the language of PTMs.

Protein conformations, interactions, and H/D exchange

Modern mass spectrometry (MS) is well known for its exquisite sensitivity in probing the covalent structure of macromolecules, and for that reason, it has become the major tool used to identify individual proteins in proteomics studies. This use of MS is now widespread and routine. In addition to this application of MS, a handful of laboratories are developing and using a methodology by which MS can be used to probe protein conformation and dynamics. This application involves using MS to analyze amide hydrogen/deuterium (H/D) content from exchange experiments. Introduced by Linderstøm-Lang in the 1950s, H/D exchange involves using (2)H labeling to probe the rate at which protein backbone amide protons undergo chemical exchange with the protons of water. With the advent of highly sensitive electrospray ionization (ESI)-MS, a powerful new technique for measuring H/D exchange in proteins at unprecedented sensitivity levels also became available. Although it is still not routine, over the past decade the methodology has been developed and successfully applied to study various proteins and it has contributed to an understanding of the functional dynamics of those proteins.

Examples of Fourier transform ion cyclotron resonance mass spectrometry developments: from ion physics to remote access biochemical mass spectrometry

The application of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) for high resolution biomolecular analysis has increased greatly after 30 years of innovation since its conception in 1974. FT- ICR-MS can now routinely be used for the analysis of complex organic mixtures such as biological or petrochemical samples. Many of these new possibilities have been the results of many different instrumental developments. This paper provides a mini review of selected instrumental developments that now allow these measurements. The development of soft ionization techniques such as electrospray ionization and matrix assisted laser desorption and ionisation was crucial for the analysis of biological macromolecules. Improved ion transport optics led to an increase in sensitivity. New ICR cell designs complement the capabilities of FT-ICR-MS by allowing a more thorough study of the mechanism and kinetics of ion reactions in the gas-phase. A selected example of electron capture dissociation (ECD) employs these developments to investigate the role of peptide conformation in ECD. Improved electronics and software allow faster and more flexible experiments. All these improvements led to an increase in speed and sensitivity that are necessary to couple FT-MS to fast separation techniques such as nano-high performance liquid chromatography. The modern FT-ICR-MS instruments can be incorporated in virtual organizations allowing remote access to unique infrastructure. This concept of remote experimentation opens new possibilities for scientific collaborations between expert scientists at different locations and allows the efficient use of this expensive instrumentation.

Improved mass analysis of oligoribonucleotides by 13C, 15N double depletion and electrospray ionization FT-ICR mass spectrometry

13C, 15N doubly depleted 32-ribonucleotide was synthesized enzymatically by in vitro transcription from nucleoside triphosphates isolated from E. coli grown in a minimal medium containing 12C, 14N-enriched glucose and ammonium sulfate. Following purification and desalting by reversed-phase HPLC, buffer exchange with Microcon YM-3, and ethanol precipitation, electrospray ionization Fourier transform ion cyclotron resonance mass spectra revealed greatly enhanced abundance of monoisotopic ions (by a factor of approximately 100) and a narrower isotopic distribution with higher signal-to-noise ratio. The abrupt onset and high magnitude of the monoisotopic species promise to facilitate accurate mass measurement of RNA's.

Subtle modification of isotope ratio proteomics; an integrated strategy for expression proteomics

Use of minor modification of isotope ratio to code samples for expression proteomics is being investigated. Alteration of (13)C abundance to approximately 2% yields a measurable effect on peptide isotopic distribution and inferred isotope ratio. Elevation of (13)C abundance to 4% leads to extension of isotopic distribution and background peaks across every unit of the mass range. Assessment of isotope ratio measurement variability suggests substantial contributions from natural measurement variability. A better understanding of this variable will allow assessment of the contribution of sequence dependence. Both variables must be understood before meaningful mixing experiments for relative expression proteomics are performed. Subtle modification of isotope ratio ( approximately 1-2% increase in (13)C) had no effect upon either the ability of data-dependent acquisition software or database searching software to trigger tandem mass spectrometry or match MSMS data to peptide sequences. More severe modification of isotope ratio caused a significant drop in performance of both functionalities. Development of software for deconvolution of isotope ratio concomitant with protein identification using LC-MSMS, or any other proteomics strategy, is underway (Isosolv). The identified peptide sequence is then be used to provide elemental composition for accurate isotope ratio decoding and the potential to control for specific amino acid biases should these prove significant. It is suggested that subtle modification of isotope ratio proteomics (SMIRP) offers a convenient approach to in vivo isotope coding of plants and might ultimately be extended to mammals including humans.

Proteomics techniques and their application to hematology

The recent sequencing of a number of genomes has raised the level of opportunities for studies on proteins. This area of research has been described with the all-embracing term, proteomics. In proteomics, the use of mass spectrometric techniques enables genomic databases to be used to establish the identity of proteins with relatively little data, compared to the era before genome sequencing. The use of related analytical techniques also offers the opportunity to gain information on regulation, via posttranslational modification, and potential new diagnostic and prognostic indicators. Relative quantification of proteins and peptides in cellular and extracellular material remains a challenge for proteomics and mass spectrometry. This review presents an analysis of the present and future impact of these proteomic technologies with emphasis on relative quantification for hematologic research giving an appraisal of their potential benefits.

Trace Labeling of Proteins with Stable Isotopes To Identify Fragments in Complex Mixtures

We describe a novel nonradioactive protein-labeling technique that permits mass spectrometric identification of fragments of labeled proteins. Proteins are labeled by modulating their content of carbon-13 and labeled fragments identified from the distinctive isotope pattern observed on MALDI-TOF mass spectrometry. We show that carbon-13 enrichment to just 2.3% of total carbon (about twice the natural abundance of 1.1%) is sufficient for all fragments to be distinguishable from fragments of natural carbon-13-content proteins. Distinguishing labeled fragments is easily accomplished by visual inspection of spectra, but importantly, we show that labeled fragments can also be identified by computer analysis of spectra using novel parameters we have derived. The technique is demonstrated for identification of fragments of carbon-13-enriched glutathione transferase within a complex mixture of unlabeled peptides by visual and computer analysis of MALDI-TOF mass spectra, but it could be developed to mass spectrometrically identify and characterize fragments of labeled proteins recovered from biological systems.

Proteome analysis by mass spectrometry

The coupling of high-performance mass spectrometry instrumentation with highly efficient chromatographic and electrophoretic separations has enabled rapid qualitative and quantitative analysis of thousands of proteins from minute samples of biological materials. Here, we review recent progress in the development and application of mass spectrometry-based techniques for the qualitative and quantitative analysis of global proteome samples derived from whole cells, tissues, or organisms. Techniques such as multidimensional peptide and protein separations coupled with mass spectrometry, accurate mass measurement of peptides from global proteome digests, and mass spectrometric characterization of intact proteins hold great promise for characterization of highly complex protein mixtures. Advances in chemical tagging and isotope labeling techniques have enabled quantitative analysis of proteomes, and highly specific isolation strategies have been developed aimed at selective analysis of posttranslationally modified proteins.

Use of double-depleted 13C and 15N culture media for analysis of whole cell bacteria by MALDI time-of-flight and Fourier transform mass spectrometry

In the present paper, results demonstrating the significant advantages of matrix-assisted laser desorption/ionization (MALDI) analysis of whole cell samples of bacteria grown on double isotopically-depleted (13C and 15N) media are presented. It is shown that several advantages accrue for MALDI with a 9.4 T Fourier transform mass spectrometer (FTMS). Of particular note, for analysis of whole cells, sample preparation is simple and chemical interference is reduced. Moreover, ion coalescence problems are minimized, and data-base identification of proteins facilitated. Furthermore, high resolution mass spectra obtained from such whole cells show significant improvement in apparent mass resolving power and mass measurement accuracy, whether time-of-flight or FTMS MALDI is used. As a consequence, it becomes possible to detect subtle details in the chemistry of the organism, such as the presence of both post-translationally modified and unmodified versions of the same proteins. This approach is also adaptable to direct assay of over-expressed proteins from Escherichia coli cultures and should facilitate studies aimed at the detection of medically important cellular biomarker proteins.

Electron capture dissociation and 13C,15N depletion for deuterium localization in intact proteins after solution-phase exchange

For localization of deuterium atoms after solution-phase exchange with D2O, intact proteins are often digested prior to analysis by mass spectrometry (MS) and tandem MS (MS/MS). Amelioration of limitations associated with this approach (e.g., <70% sequence coverage and some D atom scrambling during MS/MS) were sought using intact proteins and two newer methods applied to tracking H/D exchange dynamics for the first time. Using 2-4-fold signal enhancements through depletion of 13C and 15N isotopes and implementing the new MS/MS technique of electron capture dissociation (ECD) yielded an increased number of c and z* ions observed (43 vs 25) for recombinant yeast ubiquitin (9.3 kDa). Initial determination of D atom content in consecutive c ion series (c4-c7, c28, c31, c32, and c33) was demonstrated. The improved ion signal and experiment speed combined with narrower isotopic distributions markedly increases the degree of localization and feasibility of ECD-based MS/MS after solution-phase H/D exchange.

Advantages of Thermococcus kodakaraenis (KOD) DNA Polymerase for PCR-mass spectrometry based analyses

The advantages of the thermostable DNA polymerase from Thermococcus kodakaraensis (KOD) are demonstrated for PCR amplification with subsequent detection by mass spectrometry. Commonly used DNA polymerases for PCR amplification include those from Thermus aquaticus (Taq) and Pyrococcus furiosus (Pfu). A 116 base-pair PCR product derived from a vWA locus was amplified by Taq, Pfu, or KOD DNA polymerase and compared by agarose gel electrophoresis and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). KOD DNA polymerase demonstrated a 2- to 3-fold increase in PCR product formation compared to Pfu or Taq, respectively, and generated blunt-ended PCR product which allows facile interpretation of the mass spectrum. Additionally, we demonstrate the advantage of using high magnetic fields to obtain unit resolution of the same 116 base pair (approximately 72 kDa) PCR product at high m/z.

Crossing the phase boundary to study protein dynamics and function: combination of amide hydrogen exchange in solution and ion fragmentation in the gas phase

Protein dynamics are the key to understanding their behavior. The static protein structure alone in most cases is insufficient to describe the vast array of complex functions they perform in vivo. Until recently there were relatively few techniques available to investigate the dynamic nature of these proteins. Mass spectrometry has recently emerged as a powerful biophysical method, capable of providing both structural and dynamic information. By utilizing the labile nature of amide hydrogens as a marker of the backbone dynamics in solution, combined with gas-phase dissociation techniques, we now have a high-resolution tool to locate these exchanging hydrogens within the sequence of the protein and to probe the functional importance of its structural elements. In this paper we describe several applications of these methodologies to illustrate the importance of dynamics to the biological functions of proteins.

Baseline resolution of isobaric phosphorylated and sulfated peptides and nucleotides by electrospray ionization FTICR ms: another step toward mass spectrometry-based proteomics

Electrospray ionization broadband FTICR mass spectrometry at a mass resolving power, m/delta m50% > or = 400,000 has achieved the first direct mass spectral resolution of phosphorylated and sulfated peptides (or nucleotides) of the same nominal mass. The elemental composition difference in each case is PH versus S (9.5 mDa), requiring a minimum mass resolving power ((m2 - m1)/ml) of 118,000 (C terminal amidated cholecystekinin fragment 26-33 (CCK-8), DY(PO3H2)MGWMDF-NH2 versus DY(SO3H)MGWMDF-NH2) or 65,400 (adenosine triphosphate vs 3-phosphoadenosine 5'-phosphosulfate). The isobaric mass doublets were detected in broadband mode (400 < m/z <1400) in the presence of dozens of other species. It is therefore now possible to distinguish phosphorylated from sulfated peptides, even when both species are present at the same time in a protein digest.

Top down characterization of larger proteins (45 kDa) by electron capture dissociation mass spectrometry

The structural characterization of proteins expressed from the genome is a major problem in proteomics. The solution to this problem requires the separation of the protein of interest from a complex mixture, the identification of its DNA-predicted sequence, and the characterization of sequencing errors and posttranslational modifications. For this, the "top down" mass spectrometry (MS) approach, extended by the greatly increased protein fragmentation from electron capture dissociation (ECD), has been applied to characterize proteins involved in the biosynthesis of thiamin, Coenzyme A, and the hydroxylation of proline residues in proteins. With Fourier transform (FT) MS, electrospray ionization (ESI) of a complex mixture from an E. coli cell extract gave 102 accurate molecular weight values (2-30 kDa), but none corresponding to the predicted masses of the four desired enzymes for thiamin biosynthesis (GoxB, ThiS, ThiG, and ThiF). MS/MS of one ion species (representing approximately 1% of the mixture) identified it with the DNA-predicted sequence of ThiS, although the predicted and measured molecular weights were different. Further purification yielded a 2-component mixture whose ECD spectrum characterized both proteins simultaneously as ThiS and ThiG, showing an additional N-terminal Met on the 8 kDa ThiS and removal of an N-terminal Met and Ser from the 27 kDa ThiG. For a second system, the molecular weight of the 45 kDa phosphopantothenoylcysteine synthetase/decarboxylase (CoaBC), an enzyme involved in Coenzyme A biosynthesis, was 131 Da lower than that of the DNA prediction; the ECD spectrum showed that this is due to the removal of the N-terminal Met. For a third system, viral prolyl 4-hydroxylase (26 kDa), ECD showed that multiple molecular ions (+98, +178, etc.) are due to phosphate noncovalent adducts, and MS/MS pinpointed the overall mass discrepancy of 135 Da to removal of the initiation Met (131 Da) and to formation of disulfide bonds (2 x 2 Da) at C32-C49 and C143-C147, although 10 S-S positions were possible. In contrast, "bottom up" proteolysis characterization of the CoaBC and the P4H proteins was relatively unsuccessful. The addition of ECD substantially increases the capabilities of top down FTMS for the detailed structural characterization of large proteins.

CEPH family 1362 STR database: an online resource for characterization of PCR products using electrospray ionization mass spectrometry

An online database has been established in order to validate electrospray ionization mass spectrometry (ESI-MS) for genotyping and to publicize the procedures developed in our laboratory for the characterization of PCR products by ESI-MS. Genotypes derived from short tandem repeat (STR) loci that were obtained using ESI Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) have been posted for fifteen members of the CEPH family 1362 pedigree. The website provides specific information such as PCR parameters, PCR product cleanup approaches, and ESI solution compositions to enable other laboratories to reproduce our data. Links are provided to related websites in an effort to integrate information regarding the CEPH family, STR genotyping, and mass spectrometry. The database, currently available at http://www.people.vcu.edu/ -dcmuddim/genotype/ will be routinely updated with genotypes from additional STR loci including PCR parameters as well as PCR cleanup strategies as further developments are completed.

Improvement in the apparent mass resolution of oligonucleotides by using 12C/14N-enriched samples

The apparent mass resolution of oligonucleotides in time-of-flight (TOF) mass spectrometers has been examined. In a reflectron TOF instrument, where the isotopic profile can be completely resolved, the apparent resolution matches the instrument's resolving power. In a linear TOF instrument, unresolved isotopic profiles limit the apparent resolution to much lower values than the actual instrument resolution. By using 12C/14N-enriched oligonucleotides, the apparent resolution can be improved significantly. The isotope enrichment method also enhances the signal-to-noise ratio.

Reducing mass degeneracy in SAR by MS by stable isotopic labeling

Mass spectrometry (MS) promises to be an invaluable tool for functional genomics, by supporting low-cost, high-throughput experiments. However, large-scale MS faces the potential problem of mass degeneracy---indistinguishable masses for multiple biopolymer fragments (e.g., from a limited proteolytic digest). This paper studies the tasks of planning and interpreting MS experiments that use selective isotopic labeling, thereby substantially reducing potential mass degeneracy. Our algorithms support an experimental--computational protocol called structure-activity relation by mass spectrometry (SAR by MS) for elucidating the function of protein-DNA and protein-protein complexes. SAR by MS enzymatically cleaves a crosslinked complex and analyzes the resulting mass spectrum for mass peaks of hypothesized fragments. Depending on binding mode, some cleavage sites will be shielded; the absence of anticipated peaks implicates corresponding fragments as either part of the interaction region or inaccessible due to conformational change upon binding. Thus, different mass spectra provide evidence for different structure--activity relations. We address combinatorial and algorithmic questions in the areas of data analysis (constraining binding mode based on mass signature) and experiment planning (determining an isotopic labeling strategy to reduce mass degeneracy and aid data analysis). We explore the computational complexity of these problems, obtaining upper and lower bounds. We report experimental results from implementations of our algorithms.

Perspectives on the use of electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry for short tandem repeat genotyping in the post-genome era

The recent completion of the first rough draft of the human genome has provided fundamental information regarding our genetic make-up; however, the post-genome era will certainly require a host of new technologies to address complex biological questions. In particular, a rapid and accurate approach to characterize genetic markers, including short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs) is demanded. STRs are the most informative of the two polymorphisms owing to their remarkable variability and even dispersity throughout eukaryotic genomes. Mass spectrometry is rapidly becoming a significant method in DNA analysis and has high probability of revolutionizing the way in which scientists probe the human genome. It is our responsibility as biomolecular mass spectrometrists to understand the issues in genetic analysis and the capabilities of mass spectrometry so that we may fulfill our role in developing a rapid, reliable technology to answer specific biological questions. This perspective is intended to familiarize the mass spectrometry community with modern genomics and to report on the current state of mass spectrometry, specifically electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, for characterization of STRs.

Protein complexes and analysis of their assembly by mass spectrometry

The utility of mass spectrometry for the analysis of proteins has grown enormously in the past decade. Significant advances in detection and ionization techniques are allowing questions about noncovalent assembly to be addressed by the direct observation of gas phase complexes, their assembly in real time and their disassembly by perturbation of solution or instrument conditions. These technological innovations have plainly captured the imagination of biological researchers. Recent and novel developments include the combination of mass spectrometry with isotopic labeling, affinity labeling and genomic information. Collectively, these advances are opening new doors to the isolation of complexes, the identification of their substituents and the characterization of their conformations and assembly.

Stable isotope incorporation triples the upper mass limit for determination of elemental composition by accurate mass measurement

By comparing electrospray ionization Fourier-transform ion cyclotron resonance (FT-ICR) mass spectra and collision-induced dissociation (CID) FT-ICR mass spectra of a phospholipid (851 Da) extracted from natural abundance and 99% 13C bacterial growth media, we are able to reduce its number of possible elemental compositions (based on +/-10 ppm externally calibrated mass accuracy and biologically relevant compositional constraints) from 394 to 1. The basic idea is simply that the mass of a molecule containing N carbon atoms increases by N Da when 12C is replaced by 13C. Once the number of carbons is known, the number of possible combinations of other atoms in the molecule is greatly reduced. We demonstrate the method for a stored-waveform inverse Fourier transform-isolated phospholipid from an extract of membrane lipids from Rhodococcus rhodochrous hydrocarbon-degrading bacteria grown on either natural abundance or 99% 13C-enriched mixtures of n-hexadecane and n-octadecane. We project that this method raises the upper mass limit for unique determination of elemental composition from accurate mass measurement by a factor of at least 3, thereby extending "chemical formula" determination to identification and sequencing of larger synthetic and bio-polymers: phospholipids, oligopeptides of more than three to four amino acids, DNA or RNA of more than two nucleotides, oligosaccharides of more than three sugars, etc. The method can also be extended to determination of the number of other atoms for which heavy isotopes are available (e.g., 15N, 34S, 18O, etc.).