Investigation of enzyme kinetics using quench-flow techniques with MALDI-TOF mass spectrometry

Subsecond Time-Resolved Mass Spectrometry in Dynamic Structural Biology

Life at the molecular level is a dynamic world, where the key players-proteins, oligonucleotides, lipids, and carbohydrates-are in a perpetual state of structural flux, shifting rapidly between local minima on their conformational free energy landscapes. The techniques of classical structural biology, X-ray crystallography, structural NMR, and cryo-electron microscopy (cryo-EM), while capable of extraordinary structural resolution, are innately ill-suited to characterize biomolecules in their dynamically active states. Subsecond time-resolved mass spectrometry (MS) provides a unique window into the dynamic world of biological macromolecules, offering the capacity to directly monitor biochemical processes and conformational shifts with a structural dimension provided by the electrospray charge-state distribution, ion mobility, covalent labeling, or hydrogen-deuterium exchange. Over the past two decades, this suite of techniques has provided important insights into the inherently dynamic processes that drive function and pathogenesis in biological macromolecules, including (mis)folding, complexation, aggregation, ligand binding, and enzyme catalysis, among others. This Review provides a comprehensive account of subsecond time-resolved MS and the advances it has enabled in dynamic structural biology, with an emphasis on insights into the dynamic drivers of protein function.

Droplet-based optofluidic systems for measuring enzyme kinetics

The study of enzyme kinetics is of high significance in understanding metabolic networks in living cells and using enzymes in industrial applications. To gain insight into the catalytic mechanisms of enzymes, it is necessary to screen an enormous number of reaction conditions, a process that is typically laborious, time-consuming, and costly when using conventional measurement techniques. In recent times, droplet-based microfluidic systems have proved themselves to be of great utility in large-scale biological experimentation, since they consume a minimal sample, operate at high analytical throughput, are characterized by efficient mass and heat transfer, and offer high levels of integration and automation. The primary goal of this review is the introduction of novel microfluidic tools and detection methods for use in high-throughput and sensitive analysis of enzyme kinetics. The first part of this review focuses on introducing basic concepts of enzyme kinetics and describing most common microfluidic approaches, with a particular focus on segmented flow. Herein, the key advantages include accurate control over the flow behavior, efficient mass and heat transfer, multiplexing, and high-level integration with detection modalities. The second part describes the current state-of-the-art platforms for high-throughput and sensitive analysis of enzyme kinetics. In addition to our categorization of recent advances in measuring enzyme kinetics, we have endeavored to critically assess the limitations of each of these detection approaches and propose strategies to improve measurements in droplet-based microfluidics. Graphical abstract.

Molecular system identification for enzyme directed evolution and design

The rational design of chemical catalysts requires methods for the measurement of free energy differences in the catalytic mechanism for any given catalyst Hamiltonian. The scope of experimental learning algorithms that can be applied to catalyst design would also be expanded by the availability of such methods. Methods for catalyst characterization typically either estimate apparent kinetic parameters that do not necessarily correspond to free energy differences in the catalytic mechanism or measure individual free energy differences that are not sufficient for establishing the relationship between the potential energy surface and catalytic activity. Moreover, in order to enhance the duty cycle of catalyst design, statistically efficient methods for the estimation of the complete set of free energy differences relevant to the catalytic activity based on high-throughput measurements are preferred. In this paper, we present a theoretical and algorithmic system identification framework for the optimal estimation of free energy differences in solution phase catalysts, with a focus on one- and two-substrate enzymes. This framework, which can be automated using programmable logic, prescribes a choice of feasible experimental measurements and manipulated input variables that identify the complete set of free energy differences relevant to the catalytic activity and minimize the uncertainty in these free energy estimates for each successive Hamiltonian design. The framework also employs decision-theoretic logic to determine when model reduction can be applied to improve the duty cycle of high-throughput catalyst design. Automation of the algorithm using fluidic control systems is proposed, and applications of the framework to the problem of enzyme design are discussed.

Unravelling the mysteries of sub-second biochemical processes using time-resolved mass spectrometry

Many important chemical and biochemical phenomena proceed on sub-second time scales.

Mass spectrometry: towards in vivo analysis of biological systems

In vivo analysis is of paramount importance in monitoring physiological processes that take place in living organisms. Mass spectrometry, an analytical technique with high speed, sensitivity and specificity, is indispensable in biochemical studies nowadays. However, traditional mass spectrometric techniques are of limited applicability in direct analysis of living organisms due to various constraints, e.g., the necessity of ionization of analytes under vacuum and perturbation of physiological functions of living organisms during analysis. Recent development of mass spectrometry, particularly the development of ambient ionization techniques, has opened the door for direct analysis of living organisms. These new mass spectrometric techniques have the features that the ionization processes take place under atmospheric pressure and no or only little sample preparation is required, thus are well suited for analysis of living specimens without significantly perturbing their physiological states. The role of these mass spectrometric techniques in in vivo analysis has been increasingly important in recent years and is expected to be further expanded in the future. In this review, the use of various mass spectrometric techniques in in vivo analysis of biological systems is summarized and the prospects are discussed.

Studies on the regioselectivity and kinetics of the action of trypsin on proinsulin and its derivatives using mass spectrometry

Human M-proinsulin was cleaved by trypsin at the R(31)R(32)-E(33) and K(64)R(65)-G(66) bonds (B/C and C/A junctions), showing the same cleavage specificity as exhibited by prohormone convertases 1 and 2 respectively. Buffalo/bovine M-proinsulin was also cleaved by trypsin at the K(59)R(60)-G(61) bond but at the B/C junction cleavage occurred at the R(31)R(32)-E(33) as well as the R(31)-R(32)E(33) bond. Thus, the human isoform in the native state, with a 31 residue connecting C-peptide, seems to have a unique structure around the B/C and C/A junctions and cleavage at these sites is predominantly governed by the structure of the proinsulin itself. In the case of both the proinsulin species the cleavage at the B/C junction was preferred (65%) over that at the C/A junction (35%) supporting the earlier suggestion of the presence of some form of secondary structure at the C/A junction. Proinsulin and its derivatives, as natural substrates for trypsin, were used and mass spectrometric analysis showed that the k(cat.)/K(m) values for the cleavage were most favourable for the scission of the bonds at the two junctions (1.02±0.08×10(5)s(-1)M(-1)) and the cleavage of the K(29)-T(30) bond of M-insulin-RR (1.3±0.07×10(5)s(-1)M(-1)). However, the K(29)-T(30) bond in M-insulin, insulin as well as M-proinsulin was shielded from attack by trypsin (k(cat.)/K(m) values around 1000s(-1)M(-1)). Hence, as the biosynthetic path follows the sequence; proinsulin→insulin-RR→insulin, the K(29)-T(30) bond becomes shielded, exposed then shielded again respectively.

Time-resolved mass spectrometry for monitoring millisecond time-scale solution-phase processes

Many chemical and biochemical reactions equilibrate within a few seconds of initiation under "native" conditions. To understand the microscopic processes underlying these reactions, the most direct approach is to monitor the reaction as equilibrium is established (i.e. the reaction kinetics). However, this requires the ability to characterize the reaction mixture on the millisecond time-scale. In this review, we survey the contributions of time-resolved mass spectrometry (TR-MS) to the characterization of millisecond time-scale (bio)chemical processes, with a focus on biochemical applications. Compared to conventional time-resolved techniques, which use optical detection, the primary advantage of TR-MS is the ability to detect virtually all reactive species simultaneously. This provides a singularly high detail account of the reaction without the need for a chromophoric change on turnover or artificial chromophoric probes. To provide millisecond time-resolution, TR-MS set-ups usually employ continuous-flow rapid mixers, corresponding either to a fixed "tee" that provides a single reaction time-point or an adjustable position mixer that allows continuous reaction monitoring. TR-MS has been used to monitor processes with rates up to 500 s(-1) and to provide a detailed account of complex reactions involving 10 co- populated species. This corresponds to significantly lower time-resolution than optical methods, which can measure rates in excess of 900 s(-1) under ideal conditions, but substantially more detail (optical studies are typically limited to one or two analytes). TR-MS has been implemented on a number of platforms, including capillary and microfluidic set-ups. Capillary-based implementations are straightforward to fabricate and provide the most efficient rapid mixing. Microfluidic implementations have also been devised to enable multi-step experimental workflows that incorporate TR-MS. As a general method for time-resolved measurements, the applications for TR-MS are wide ranging. TR-MS has been used to identify intermediates in organic reactions, reveal protein (un)folding mechanisms, monitor enzyme catalysis in the pre-steady-state and, in conjunction with hydrogen-deuterium exchange, characterize protein conformational dynamics. While not without limitations, TR-MS represents a powerful alternative for measuring solution phase processes on the millisecond time-scale and a new, promising approach for revealing mechanistic details in (bio)chemical reactions.

Observation of a chemically labile, noncovalent enzyme intermediate in the reaction of metal-dependent Aquifex pyrophilus KDO8PS by time-resolved mass spectrometry

The direct detection of intermediates in enzymatic reactions can yield important mechanistic insights but may be difficult due to short intermediate lifetimes and chemical instability. Using a rapid-mixing device coupled with electrospray ionization time-of-flight mass spectrometry, the noncovalent hemiketal intermediate in the reaction of metal-dependent 3-deoxy-D-manno-octulosonate-8-phosphate (KDO8P) synthase from Aquifex pyrophilus was observed in the millisecond time range. Using single turnover conditions, the noncovalent complexes of enzyme with Cd(2+):phosphoenolpyruvate, Cd(2+):phosphate, Cd(2+):KDO8P, and Cd(2+):intermediate complexes were resolved. The intermediate complex is present during times ranging from 50-630 ms, indicating that the intermediate builds up at the ambient temperatures of the experiment. This represents the first direct detection of the intermediate with a native metal-dependent KDO8PS, and further demonstrates that time-resolved mass spectrometry is a useful tool in mechanistic studies of enzymatic reactions.

Online quench-flow electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry for elucidating kinetic and chemical enzymatic reaction mechanisms

We have developed an automated quench-flow microreactor which interfaces directly to an electrospray ionization (ESI) mass spectrometer. We have used this device in conjunction with ESI Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) to demonstrate the potential of this approach for studying the mechanistic details of enzyme reactions. For the model system chosen to test this device, namely, the pre-steady-state hydrolysis of p-nitrophenyl acetate by the enzyme chymotrypsin, the kinetic parameters obtained are in good agreement with those in the literature. To our knowledge, this is the first reported use of online quench-flow coupled with FTICR MS. Furthermore, we have exploited the power of FTICR MS to interrogate the quenched covalently bound enzyme intermediate using top-down fragmentation. The accurate mass capabilities of FTICR MS permitted the nature of the intermediate to be assigned with high confidence. Electron capture dissociation (ECD) fragmentation allowed us to locate the intermediate to a five amino acid section of the protein--which includes the known catalytic residue, Ser(195). This experimental approach, which uniquely can provide both kinetic and chemical details of enzyme mechanisms, is a potentially powerful tool for studies of enzyme catalysis.

On the mechanism of multiple lysine methylation by the human mixed lineage leukemia protein-1 (MLL1) core complex

Transcription in eukaryotic genomes depends on enzymes that regulate the degree of histone H3 lysine 4 (H3K4) methylation. The mixed lineage leukemia protein-1 (MLL1) is a member of the SET1 family of H3K4 methyltransferases and is frequently rearranged in acute leukemias. Despite sequence comparisons that predict that SET1 family enzymes should only monomethylate their substrates, mono-, di-, and trimethylation of H3K4 has been attributed to SET1 family complexes in vivo and in vitro. To better understand this paradox, we have biochemically reconstituted and characterized a five-component 200-kDa MLL1 core complex containing human MLL1, WDR5, RbBP5, Ash2L, and DPY-30. We demonstrate that the isolated MLL1 SET domain is a slow monomethyltransferase and that tyrosine 3942 of MLL1 prevents di- and trimethylation of H3K4. In contrast, a complex containing the MLL1 SET domain, WDR5, RbBP5, Ash2L, and DPY-30, displays a marked approximately 600-fold increase in enzymatic activity but only to the dimethyl form of H3K4. Single turnover kinetic experiments reveal that the reaction leading to H3K4 dimethylation involves the transient accumulation of a monomethylated species, suggesting that the MLL1 core complex uses a non-processive mechanism to catalyze multiple lysine methylation. We have also discovered that the non-SET domain components of the MLL1 core complex possess a previously unrecognized methyltransferase activity that catalyzes H3K4 dimethylation within the MLL1 core complex. Our results suggest that the mechanism of multiple lysine methylation by the MLL1 core complex involves the sequential addition of two methyl groups at two distinct active sites within the complex.

Monitoring enzyme reaction and screening of inhibitors of acetylcholinesterase by quantitative matrix-assisted laser desorption/ionization Fourier transform mass spectrometry

A matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS)-based assay was developed for kinetic measurements and inhibitor screening of acetylcholinesterase. Here, FTMS coupled to MALDI was applied to quantitative analysis of choline using the ratio of choline/acetylcholine without the use of additional internal standard, which simplified the experiment. The Michaelis constant (K(m)) of acetylcholinesterase (AChE) was determined to be 73.9 micromol L(-1) by this approach. For Huperzine A, the linear mixed inhibition of AChE reflected the presence of competitive and noncompetitive components. The half maximal inhibitory concentration (IC(50)) value of galantamine obtained for AChE was 2.39 micromol L(-1). Inhibitory potentials of Rhizoma Coptidis extracts were identified with the present method. In light of the results the referred extracts as a whole showed inhibitory action against AChE. The use of high-resolution FTMS largely eliminated the interference with the determination of ACh and Ch, produced by the low-mass compounds of chemical libraries for inhibitor screening. The excellent correlation with the reported kinetic parameters confirms that the MS-based assay is both accurate and precise for determining kinetic constants and for identifying enzyme inhibitors. The obvious advantages were demonstrated for quantitative analysis and also high-throughput characterization. This study offers a perspective into the utility of MALDI-FTMS as an alternate quantitative tool for inhibitor screening of AChE.

Letter: correlation between phosphorylation ratios by matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis and enzyme kinetics

To identify the correlation between the phosphorylation ratios by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-ToF MS) analysis and enzyme kinetics (Km, Vmax, and Vmax/Km) is important to understand whether MALDI-TOF MS can be applied for monitoring the properties of peptides that are substrates of protein kinases. The correlation between phosphorylation ratios and enzyme kinetics was examined using peptides for protein kinase C (PKC) and for 60 kDa phosphoprotein, encoded by the cellular sarcoma gene (c-Src). Phosphorylation ratios, analyzed by MALDI-ToF MS, showed higher correlation coefficient (r = or > +0.7) for Vmax/Km compared with that (r = or < -/+0.6) for Km or Vmax. For ion modes, a higher correlation coefficient between phosphorylation ratios and Vmax/Km was identified in the positive mode (r = or > +0.7) compared to that in the negative mode (r = or < +0.5). These results suggest that MALDI-ToF MS is a useful tool to evaluate Vmax/Km of peptides for protein kinases.

Determination of phosphorylated and O-glycosylated sites by chemical targeting (CTID) at ambient temperature

In the analytical approach called chemically targeted identification (CTID), peptides containing phosphorylated or glycosylated serine and threonine underwent beta-elimination to produce an unsaturated double bond. Nucleophilic addition of 2-aminoethanethiol to this bond occurred, yielding aminoethylcysteine. Thus, sites containing posttranslational modifications were made susceptible to lysine endopeptidase. Structural information could then be obtained by mass analysis of the proteolytic products. The method was demonstrated by the analysis of beta-casein tryptic digest peptides and an O-glycosylated peptide. Contrary to an earlier report, the glycopeptide was found to react with essentially the same kinetics as phosphopeptides. Conversion of all five phosphoserines in residues 15, 17, 18, 19, and 35 in N-terminal tryptic phosphopeptides from bovine beta-casein were followed by monitoring the time course of the addition reaction. The chemistry proceeded rapidly at room temperature with a half-reaction time of 15 min. No side reaction products were observed. However, care had to be taken to minimize all counterions, which either precipitate barium or neutralize the base. In the case of 2-aminoethanethiol, excess Ba(OH)2 was needed to offset the effect of the hydrochloride. Alternatively, pre-incubation with base followed by nucleophilic addition was found to work satisfactorily. The use of water-soluble thiol allowed the procedure to be carried out in the solid phase, with a micro pipet greatly facilitating sample cleanup.

How to make big molecules fly out of liquid water: applications, features and physics of laser assisted liquid phase dispersion mass spectrometry

Applications, features, and mechanistic details of laser assisted liquid phase dispersion mass spectrometry are highlighted and discussed. It has been used in the past to directly isolate charged molecular aggregates from the liquid phase and to determine their molecular weight employing sensitive time-of-flight mass spectrometry. The liquid matrix in this MALDI (matrix assisted laser desorption and ionization) type approach consists of a 10 microm diameter free liquid filament in vacuum (or a free droplet) which is excited with a focused infrared laser pulse tuned to match the absorption frequency of the OH-stretch vibration of bulk water near 2.8 microm. Due to these features we will refer to the approach as free liquid matrix assisted laser dispersion of ions or ionic aggregates (IR-FL-MALDI), although also LILBID ("laser induced liquid beam (bead) desorption and ionization") has been proposed early as a descriptive acronym for the technique and may be used alternatively. Low-charge-state macromolecular adducts are isolated in the gas phase from solution via a yet poorly characterized mechanism which sensitively depends upon the laser intensity and wavelength, and after the gentle liquid-to-vacuum transfer the aggregates are analyzed via time-of-flight (TOF) mass spectrometry (MS). Possible mechanisms for the isolation and charging of biomolecules directly from liquid solution are discussed in the present contribution. Recent technical advances such as minimizing the sample consumption, strategies for high throughput mass spectrometry, and coupling of liquid beam MS with HPLC will be highlighted as well. An interesting feature of IR-FL-MALDI is what we call the linear response, i.e., a surprising linearity of the gas phase mass signal on the solution concentration over many orders of magnitude for a large number of biomolecular systems as well as ions. Due to these features the approach may be regarded as a true solution probing spectroscopy, which enables elegant biokinetic studies. Several experiments in which time resolved IR-FL-MALDI-MS has recently been employed successfully are given. A particular highlight is the possibility to quantitatively detect oxidation states in solution, which clearly distinguishes the present approach from other established MS source concepts. Due to the good matrix tolerance also proteins in complex mixtures can be monitored quantitatively.

Exosite interactions contribute to tension-induced cleavage of von Willebrand factor by the antithrombotic ADAMTS13 metalloprotease

Von Willebrand factor (VWF) is a multimeric protein that mediates platelet adhesion at sites of vascular injury, and ADAMTS13 (a disintegrin and metalloprotease with thrombospondin)is a multidomain metalloprotease that limits platelet adhesion by a feedback mechanism in which fluid shear stress induces proteolysis of VWF and prevents disseminated microvascular thrombosis. Cleavage of the Tyr(1605)-Met(1606) scissile bond in the VWF A2 domain depends on a Glu(1660)-Arg(1668) segment in the same domain and on the noncatalytic spacer domain of ADAMTS13, suggesting that extensive enzyme-substrate interactions facilitate substrate recognition. Based on mutagenesis and kinetic analysis, we find that the ADAMTS13 spacer domain binds to an exosite near the C terminus of the VWF A2 domain. Deleting the spacer domain from ADAMTS13 or deleting the exosite from the VWF substrate reduced the rate of cleavage approximately 20-fold. A cleavage product containing the exosite was a hyperbolic mixed-type inhibitor of ADAMTS13 proteolysis of either VWF multimers or model peptide substrates but only if the ADAMTS13 enzyme contained the spacer domain. The specificity of this unique mechanism depends on tension-induced unfolding of the VWF A2 domain, which exposes the scissile bond and exosite for interaction with complementary sites on ADAMTS13.

Applications of mass spectrometry in early stages of target based drug discovery

Mass spectrometry (MS) has been applied to drug discovery for many years. With the advent of new ionization techniques, MS has emerged as an important analytical tool in identification and characterization of protein targets, structure elucidation of synthetic compounds, and early drug metabolism and pharmacokinetics studies. Two MS-based strategies, function-based and affinity-based, have been employed in recent years for screening and evaluation of compounds. In the function-based approach, the effects of compounds on the biological activity of a target molecule are measured. In the affinity-based approach, compounds are screened based on their binding affinities to target molecules. The interaction between targets and compounds can be directly evaluated by monitoring the formation of non-covalent target-ligand complexes (direct detection) or indirectly evaluated by detecting the compounds after separating bound compounds from unbound (indirect detection). Various techniques including high performance liquid chromatography (HPLC)-MS, size exclusion chromatography (SEC)-MS, frontal affinity chromatography (FAC)-MS and desorption/ionization on silicon (DIOS)-MS can be applied. The recent advances, relative advantages, and limitations of each MS-based method as a tool in compound screening and compound evaluation in the early stages of drug discovery are discussed in this review.

Time-Resolved Micro Liquid Desorption Mass Spectrometry: Mechanism, Features, and Kinetic Applications

Liquid water beam desorption mass spectrometry is an intriguing technique to isolate charged molecular aggregates directly from the liquid phase and to analyze them employing sensitive mass spectrometry. The liquid phase in this approach consists of a 10 µm diameter free liquid filament in vacuum which is irradiated by a focussed infrared laser pulse resonant with the OH-stretch vibration of bulk water. Depending upon the laser wavelength, charged (e.g. protonated) macromolecules are isolated from solution through a still poorly characterized mechanism. After the gentle liquid-to-vacuum transfer the low-charge-state aggregates are analyzed using time-of-flight mass spectrometry. A recent variant of the technique uses high performance liquid chromatography valves for local liquid injections of samples in the liquid carrier beam, which enables very low sample consumption and high speed sample analysis. In this review we summarize recent work to characterize the ‘desorption’ or ion isolation mechanism in this type of experiment. A decisive and interesting feature of micro liquid beam desorption mass spectrometry is that — under certain conditions — the gas-phase mass signal for a large number of small as well as supramolecular systems displays a surprisingly linear response on the solution concentration over many orders of magnitude, even for mixtures and complex body fluids. This feature and the all-liquid state nature of the technique makes this technique a solution-type spectroscopy that enables real kinetic studies involving (bio)polymers in solution without the need for internal standards. Two applications of the technique monitoring enzyme digestion of proteins and protein aggregation of an amyloid model system are highlighted, both displaying its potential for monitoring biokinetics in solution.

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.

Monitoring enzymatic conversions by mass spectrometry: a critical review

This review highlights recent advances in the application of electrospray ionisation and matrix-assisted laser desorption/ionisation mass spectrometry (MS) to study enzymatic reactions. Several assay schemes for different fields of application are presented. The employment of MS as a means of detection in pre-steady-state kinetic studies by rapid-mixing direct analysis and rapid-mixing quench flow techniques is discussed. Several steady-state kinetic studies of a broad range of different enzymatic systems are presented as well as enzyme inhibition studies for various target enzymes. As a promising new development multiplex assays, which monitor the conversion of several substrates simultaneously in one experiment, are described. This assay type has been used for competition studies, enzymatic activity screenings and for diagnostic purposes in clinical chemistry. Generally, it can be concluded that mass spectrometry offers an intriguing alternative as detection methodology in enzymatic bioassays. Its applicability for the monitoring the conversion of naturally occurring substrates and its overall versatility make MS an especially promising tool for the study of enzyme-catalysed processes.

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.

A method for connecting solution-phase enzyme activity assays with immobilized format analysis by mass spectrometry

This paper reports an enzyme activity assay that combines the assets of both homogeneous and solid-phase formats. In this method, enzyme reactions are carried out in solution using substrates that are tagged with an immobilization reagent that allows the substrates to be selectively immobilized to self-assembled monolayers (SAMs), for direct analysis by matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). As a model enzyme reaction, this work examined the transfer of a methyl group from S-adenosyl-l-methionine (AdoMet) to an arginine side chain of a peptide substrate by the enzyme protein arginine methyltransferase 1 (RMT1). A cysteine-terminated peptide substrate was methylated by RMT1 in solution and then applied to a maleimide-presenting SAM to give selective immobilization of the peptide. Time-dependent analysis of methylation using MALDI-TOFMS clearly showed that both the presence and relative amount of the two reaction products-the mono- and dimethylated peptides-can be conveniently evaluated. This assay strategy is rapid, takes advantage of solution-phase assay conditions, avoids the use of labels and complicated purification steps, and is applicable to multianalyte analyses.

Mechanistic studies on enzymatic reactions by electrospray ionization MS using a capillary mixer with adjustable reaction chamber volume for time-resolved measurements

Mass spectrometry (MS)-based techniques have enormous potential for kinetic studies on enzyme-catalyzed processes. In particular, the use of electrospray ionization (ESI) MS for steady-state measurements is well established. However, there are very few reports of MS-based studies in the pre-steady-state regime, because it is difficult to achieve the time resolution required for this type of experiment. We have recently developed a capillary mixer with adjustable reaction chamber volume for kinetic studies by ESI-MS with millisecond time resolution (Wilson, D. J.; Konermann, L. Anal. Chem. 2003, 75, 6408-6414). Data can be acquired in kinetic mode, where the concentrations of selected reactive species are monitored as a function of time, or in spectral mode, where entire mass spectra are obtained for selected reaction times. Here, we describe the application of this technique to study the kinetics of enzyme reactions. The hydrolysis of p-nitrophenyl acetate by chymotrypsin was chosen as a simple chromophoric model system. On-line addition of a "makeup solvent" immediately prior to ionization allowed the pre-steady-state accumulation of acetylated chymotrypsin to be monitored. The rate constant for acetylation, as well as the dissociation constant of the enzyme-substrate complex obtained from these data, is in excellent agreement with results obtained by conventional stopped-flow methods. Bradykinin was chosen to illustrate the performance of the ESI-MS-based method with a nonchromophoric substrate. In this case, the unfavorable rate constant ratio for acylation and deacylation of the enzyme precluded measurements in the pre-steady-state regime. Steady-state experiments were carried out to determine the turnover number and the Michaelis constant for bradykinin. The methodologies used in this work open a wide range of possibilities for future ESI-MS-based kinetic assays in enzymology.

General assay for sugar nucleotidyltransferases using electrospray ionization mass spectrometry

An electrospray ionization mass spectrometry-based assay has been developed to study the class of enzymes called sugar nucleotidyltransferases that couple sugar-1-phosphates and nucleotide triphosphates to form Leloir pathway glycosyl donors. The recombinant Escherichia coli and the commercially available yeast uridine-diphosphoglucose pyrophosphorylases were used as model systems. This technique allows the simultaneous and direct detection of the substrates and products without separation and, as described, is as sensitive as traditional coupled techniques. More importantly, the assay is capable of easily measuring kinetic values and inhibition constants for a range of natural and nonnatural substrates. This new assay was used to show for the first time that the reaction of the commercially available yeast uridine-diphosphoglucose pyrophosphorylase preparation is competitively inhibited by adenosine 5'-triphosphate (ATP), an observation that indicates a single active site that accepts both uridine 5'-triphosphate and ATP substrates.

Mass spectrometric approaches for the investigation of dynamic processes in condensed phase

Mass spectrometry (MS) offers many advantages over other established spectroscopic techniques employed for the investigation of processes in condensed phase. The sensitivity, specificity, and speed afforded by MS-based methods enable to obtain very valuable insights into the mechanism of complex dynamic processes. Off-line methods rely on quenching to halt the progress of the reaction of interest and allow for the implementation of a broad range of analytical procedures for sample fractionation, isolation, or desalting. On the contrary, on-line methods are designed to carry out the real-time monitoring of dynamic processes through a continuous uninterrupted analysis of reaction mixtures, with the only caveat that the sample solutions be directly amenable to the available ionization technique. The utilization of rapid mixing devices in direct connection with a mass spectrometer or included in off-line schemes provides access to the initial moments of a reaction, which can offer very important information about the reaction mechanism. This report summarizes the different off- and on-line strategies developed to study chemical and biochemical reactions in solution and obtain kinetic/mechanistic information. The merits of the various experimental designs, the characteristics of the different instrumental setups, and the factors affecting time resolution are discussed with the aid of specific examples, which highlight the contributions of MS to the different facets of the investigation of dynamic processes in condensed phase.

Detection of novel enzyme intermediates in PEP-utilizing enzymes

This review will focus on established and newly emerging strategies for identifying and characterizing enzyme intermediates using a rapid transient kinetic approach. The merits of this methodology as well as the basics of experimental design are described. Several illustrative examples of PEP-utilizing enzymes have been chosen as they all perform unique, novel chemistries involving enzyme intermediates and have proven to be exciting pharmaceutical targets for antibiotics and herbicides. A novel application of this approach using time-resolved electrospray mass spectrometry to detect chemically labile enzyme intermediates is also discussed.

Kinetic and substrate binding analysis of phosphorylase b via electrospray ionization mass spectrometry: a model for chemical proteomics of sugar phosphorylases

As a general strategy for determining the chemical function of the class of enzymes that cleaves glycosidic linkages with phosphate, the first mass spectrometry and direct detection assay for sugar phosphorylases has been developed and used to study the inhibition and minimal binding requirements of rabbit muscle phosphorylase b. In contrast to the currently employed assays for these enzymes that measure the nonphysiologically relevant reverse reaction of glycosidic bond synthesis and thereby require prior knowledge of not just one but two sugar components, this new method has the potential to greatly reduce the complexity in discovering the substrate specificity of a new enzyme. Certain phosphorylases can catalyze the degradation of glycogen into alpha-D-glucose-1-phosphate and are targets for the development of antidiabetic therapeutics. By electrospray ionization mass spectrometry analysis, the kinetic parameters K(m), V(max), and K(i) (for alpha/beta-D-glucose) have been determined for the rabbit muscle phosphorylase b. This enzyme accepts maltoheptaose, maltohexaose, and maltopentaose as substrates in the direction of glycogen degradation, but the tetrasaccharide maltotetraose cannot serve as a substrate for this phosphorylysis reaction.

Enzymatic reaction of the immobilized enzyme on porous silicon studied by matrix-assisted laser desorption/ionization-time of flight-mass spectrometry

Desorption/ionization on silicon mass spectrometry (DIOS-MS) is a matrix-free technique that allows for the direct desorption/ionization of low-molecular-weight compounds with little or no fragmentation of analytes. This technique has a relatively high tolerance for contaminants commonly found in biological samples. DIOS-MS has been applied to determine the activity of immobilized enzymes on the porous silicon surface. Enzyme activities were also monitored with the addition of a competitive inhibitor in the substrate solution. It is demonstrated that this method can be applied to the screening of enzyme inhibitors. Furthermore, a method for peptide mapping analysis by in situ digestion of proteins on the porous silicon surface modified by trypsin, combined with matrix-assisted laser desorption/ionization-time of flight-MS has been developed.

Mass spectrometric characterization of a three-enzyme tandem reaction for assembly and modification of the novobiocin skeleton

The tripartite scaffold of the natural product antibiotic novobiocin is assembled by the tandem action of novobiocin ligase (NovL) and novobiocic acid noviosyl transferase (NovM). The noviosyl ring of the tripartite scaffold is further decorated by a methyltransferase (NovP) and a carbamoyltransferase (NovN), resulting in the formation of novobiocin. To facilitate kinetic evaluation of alternate substrate usage by NovL and NovM toward the creation of variant antibiotic scaffolds, an electrospray ionization/MS assay for obtaining kinetic measurements is presented for NovL and NovM separately, in each case with natural substrate and the 3-methyl-4-hydroxybenzoic acid analog. Additionally, assays of tandem two-enzyme (NovL/NovM) and three-enzyme (NovL/NovM/NovP) incubations were developed. The development of these assays allows for the direct detection of each intermediate followed by its utilization as substrate for the next enzyme, as well as the subsequent formation of final product as a function of time. This MS tandem assay is useful for optimization of conditions for chemoenzymatic generation of novobiocin and is also suitable for evaluation of competitive usage of variant substrate analogs by multiple enzymes. The studies presented here serve as a platform for the subsequent expansion of the repertoire of coumarin-based antibiotics.

Development of an LC–MS based enzyme activity assay for MurC: application to evaluation of inhibitors and kinetic analysis

An enzyme activity assay, based on mass spectrometric (MS) detection of specific reaction product following HPLC separation, has been developed to evaluate pharmaceutical hits identified from primary high throughput screening (HTS) against target enzyme Escherichia coli UDP-N-acetyl-muramyl-L-alanine ligase (MurC), an essential enzyme in the bacterial peptidoglycan biosynthetic pathway, and to study the kinetics of the enzyme. A comparative analysis of this new liquid chromatographic-MS (LC-MS) based assay with a conventional spectrophotometric Malachite Green (MG) assay, which detects phosphate produced in the reaction, was performed. The results demonstrated that the LC-MS assay, which determines specific ligase activity of MurC, offers several advantages including a lower background (0.2% versus 26%), higher sensitivity (> or = 10 fold), lower limit of quantitation (LOQ) (0.02 microM versus 1 microM) and wider linear dynamic range (> or = 4 fold) than the MG assay. Good precision for the LC-MS assay was demonstrated by the low intraday and interday coefficient of variation (CV) values (3 and 6%, respectively). The LC-MS assay, free of the artifacts often seen in the Malachite Green assay, offers a valuable secondary assay for hit evaluation in which the false positives from the primary high throughput screening can be eliminated. In addition, the applicability of this assay to the study of enzyme kinetics has also been demonstrated.

Quantitative matrix-assisted laser desorption/ionization mass spectrometry for the determination of enzyme activities

Quantitative matrix-assisted laser desorption/ionization (MALDI) time-of-flight (ToF) mass spectrometry (MS) was applied for the determination of concentrations of low-molecular-weight (< 400Da) substrates and products of enzyme-catalyzed reactions. Isotope-labeled and fluorinated internal standards were used for the quantification. Automated quantitative MALDI-ToF MS analysis of quenched samples allowed the direct and simultaneous observation of time-dependent decrease of substrate concentration and increase of product concentration without any need for prepurification or desalting steps. The results showed good agreement with established but more elaborate analytical methods. MALDI-ToF MS thus is an interesting alternative tool for the determination of enzyme activities. Due to automated and miniaturized measurement it is especially suitable for the screening of biocatalysts.

Determination of enzyme/substrate specificity constants using a multiple substrate ESI-MS assay

The traditional method used to investigate the reaction specificity of an enzyme with different substrates is to perform individual kinetic measurements. In this case, a series of varied concentrations are required to study each substrate and a non-regression analysis program is used several times to obtain all the specificity constants for comparison. To avoid the large amount of experimental materials, long analysis time, and redundant data processing procedures involved in the traditional method, we have developed a novel strategy for rapid determination of enzyme substrate specificity using one reaction system containing multiple competing substrates. In this multiplex assay method, the electrospray ionization mass spectrometry (ESI-MS) technique was used for simultaneous quantification of multiple products and a steady-state kinetics model was established for efficient specificity constant calculation. The system investigated was the bacterial sulfotransferase NodH (NodST), which is a host specific nod gene product that catalyzes the sulfate group transfer from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to natural Nod factors or synthetic chitooligosaccharides. Herein, the reaction specificity of NodST for four chitooligosaccharide acceptor substrates of different chain length (chitobiose, chitotriose, chitotetraose, and chitopentaose) was determined by both individual kinetic measurements and the new multiplex ESI-MS assay. The results obtained from the two methods were compared and found to be consistent. The multiplex ESI-MS assay is an accurate and valid method for substrate specificity evaluation, in which multiple substrates can be evaluated in one assay.

Discovery of the archaeal chemical link between glycogen (starch) synthase families using a new mass spectrometry assay

Starch and its analogue glycogen are biosynthesized by enzymes that have been classified by sequence similarities into two families that have no significant sequence overlap: the animal/fungal glycogen synthases and the plant/bacterial glycogen (starch) synthases. Recent gene sequence analysis of putative archaea enzymes implicates them as a third family that links the structural and functional features of the other two classes. Herein, we present the first rapid electrospray ionization mass spectrometry-based assay to quantify any carbohydrate-polymerizing activity, the first cloning and recombinant expression as well as verification of the putative function of a glycogen synthase from the hyperthermophilic archaea Pyrococcus furiosus, and the characterization of a variety of glycogen synthases with the new assay. The new assay allowed the determination of Km and Vmax values for the rabbit, yeast, and P. furiosus glycogen synthases. Most surprisingly, unlike the synthases from rabbit or yeast and in contradiction to what would be expected from structural studies of other nucleotide-sugar binding proteins, the synthase from the archaea source accepts both uridine- and adenine-diphosphate activated glucose competitively and with comparable affinities to form a glucose polymer. This loose substrate specificity implicates this protein as the chemical link between the two branches of glycogen synthases that have evolved to accept primarily one or the other nucleotide as well as a good source enzyme for polymer bioengineering efforts.

A Capillary Mixer with Adjustable Reaction Chamber Volume for Millisecond Time-Resolved Studies by Electrospray Mass Spectrometry

A novel continuous-flow apparatus for on-line kinetic studies of (bio)chemical solution-phase processes by electrospray ionization mass spectrometry (ESI-MS) is described. The device is based on two concentric capillaries. Fluid is released from the inner capillary into the intercapillary space, where it mixes with solution flowing through the outer capillary, thus initiating the reaction of interest. Gas-phase analyte ions are formed near the tip of the outer capillary by pneumatically assisted ESI. This setup allows the mixer to be placed directly within the ion source, thus providing a minimal dead volume of ~8 nL. Time-resolved data can be recorded in both "spectral" and "kinetic" modes. In the former case, the position of the inner capillary is fixed at various points, such that entire mass spectra can be recorded for selected reaction times. For experiments in kinetic mode, the mass spectrometer monitors the signal intensity at selected m/z values, while the inner capillary is continuously pulled back, thus providing intensity-time profiles for specific reactive species. A theoretical framework is developed that allows the measured kinetics to be analyzed by taking into account the effects of laminar flow within the reaction capillary. Failure to take these effects into account results in erroneous rate constants. Studies on the demetalation kinetics of chlorophyll reveal that the apparatus can reliably measure rate constants up to at least 100 s-1. This represents a substantial improvement over previous ESI-MS-based kinetic methods. Spectral mode experiments on the refolding of ubiquitin show the changing proportions of denatured and tightly folded protein subpopulations in solution. When monitored in kinetic mode, the refolding process was found to proceed with a rate constant of 5.2 s-1.

A snapshot of enzyme catalysis using electrospray ionization mass spectrometry

Insights into the early molecular events involving protein-ligand/substrate interactions such as protein signaling and enzyme catalysis can be obtained by examining these processes on a very short, millisecond time scale. We have used time-resolved electrospray mass spectrometry to delineate the catalytic mechanism of a key enzyme in bacterial lipopolysaccharide biosynthesis, 3-deoxy-d-manno-2-octulosonate-8-phosphate synthase (KDO8PS). Direct real-time monitoring of the catalytic reaction under single enzyme turnover conditions reveals a novel hemiketal phosphate intermediate bound to the enzyme in a noncovalent complex that establishes the reaction pathway. This study illustrates the successful application of mass spectrometry to reveal transient biochemical processes and opens a new time domain that can provide detailed structural information of short-lived protein-ligand complexes.

Determination of protein phosphorylation by extracellular signal-regulated kinase using capillary electrophoresis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Extracellular signal-regulated kinase (ERK) is a key regulatory enzyme mediating cell responses to mitogenic stimulation and is one of the key components in linking growth factor receptor activation to serine/threonine protein phosphorylation processes. Phosphorylation reaction by ERK plays an important role in many signal transduction pathways. ERK phosphorylates numerous substrates such as MBP, microtubule-associated protein 2 (MAP2) and nuclear protein. In particular, MBP is a substrate commonly employed for the detection of ERK activity and contains the consensus primary sequence PRT97P. In this paper, we compared the degree of the phosphorylation reaction of MBP substrate peptides by ERK with the three different MBP substrate peptides, MBP1(KNIVTPRTPPPSQGK), MBP2(VPRTPGGRR) and MBP3(APRTPGGRR) in order to select an efficient substrate peptide for phosphorylation reaction by ERK. The results showed that the MBP3 peptide is the most efficient substrate for phosphorylation reaction by ERK. Using MBP3 peptide, the phosphorylation reaction of MBP by ERK was monitored with both matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and capillary electrophoresis (CE). Our results demonstrate the feasibility of the CE method, the method being a simple and reliable technique in determining and characterizing various kinds of enzyme reaction especially including kinase enzymes.

Strategic use of affinity-based mass spectrometry techniques in the drug discovery process

Advances in biomolecular mass spectrometry (Bio-MS) have made this technique an invaluable tool for analytical chemists and biochemists alike. The applicability of Bio-MS approaches in drug discovery now encompasses in vitro, cellular, and in vivo pharmacological and clinical applications in an unprecedented expansion of utility. As a result, the role of Bio-MS in pharmaceutical discovery continues to proliferate for both structural and functional characterization of biomolecules. From target characterization to lead optimization, affinity techniques have been used to purify, probe, and enrich analytes of interest. Affinity selection employed prior to MS analysis can "edit" out extraneous noise and enable the researcher to examine only what is important. These affinity-based methods can be used as an alternative strategy when classical biochemical techniques are insufficient in advancing difficult projects. We have applied various affinity techniques in conjunction with mass spectrometry throughout the drug discovery process. This perspective will describe affinity-based mass spectrometry methodologies and related concepts, illustrated with original results.

Kinetic characterization of enzyme inhibitors using electrospray-ionization mass spectrometry coupled with multiple reaction monitoring

Electrospray ionization mass spectrometry coupled to multiple reaction monitoring (ESI-MS/MRM) has been applied for the first time to analyze enzyme inhibitor kinetics. Specifically, a known competitive inhibitor, guanosine 5'-monophosphate (GMP), and a synthetic, transition-state analogue inhibitor, guanosine 5'-[1D-(1,3,4/2)-5-methyl-5-cyclohexene-1,2,3,4-tetrol 1-diphosphate] (1) have been characterized against recombinant fucosyltransferase (Fuc-T) V using ESI-MS/MRM. Dixon analysis with GMP yielded a signature plot for competitive inhibition. Nonlinear regression analysis gave a Ki of 211.8+/-24.7 microM. The conventional analysis using GDP-[U-14C]-Fuc yielded a similar Ki value of 235.6+/-59.4 microM, confirming the validity of the MS-based method. The synthetic inhibitor 1 showed potent competitive inhibition with a Ki of 25.6+/-2.8 microM. Although 1 possesses a chemically reactive allyl phosphate group, ESI-MS/MRM showed that there was no reduction in the concentration of 1 and no production of a predicted metabolite GDP during the assay. MS/MS also confirmed the absence of a possible pseudo-trisaccharide product. The results clearly show that 1 is neither a slow-reacting donor nor does it act as a suicide-type inhibitor toward Fuc-T V. ESI-MS/MRM is therefore a powerful tool for the kinetic characterization of enzyme inhibitors, providing complete disclosure of the mechanism of action of 1 as an inhibitor.

A strategy for the determination of enzyme kinetics using electrospray ionization with an ion trap mass spectrometer

A simple and rapid means of enzyme kinetic analysis was achieved using electrospray ionization mass spectrometry and a one-point normalization factor. The model system used, glutathione S-transferase from porcine liver, is a two-substrate enzyme catalyzing the conjugation of glutathione with a variety of compounds containing an electrophilic center. An internal standard that is structurally similar to the product was added to the reaction quench solution, and a single-point normalization factor was used to determine the product concentration without the need of a calibration curve. Kinetic parameters, such as Km, Vmax and Ki (for thyroxine), obtained by electrospray mass spectrometry agreed with those obtained from traditional UV-vis spectroscopy, and competitive vs noncompetitive inhibition reactions could be delineated via mass spectrometry. These results suggest that our method can be applied to enzymatic processes in which spectrophotometric or spectrofluorometric assays are not feasible or when the relevant substrates do not incorporate chromophores or fluorophores. This new method is competitive with traditional UV assays in that it is facile and it involves very little analysis time.

Characterization of a multi-functional metal-mediated nuclease by MALDI-TOF mass spectrometry

Mass spectrometric analysis of reaction products allows simultaneous characterization of activities mediated by multifunctional enzymes. By use of MALDI-TOF mass spectrometry, the relative influence of magnesium and manganese promoted exonuclease and phosphatase activities of Esherichia coli exonuclease III have been quantitatively measured, offering a rapid and sensitive alternative to radioactivity quantification and gel electrophoresis procedures for determination of reaction rate constants. Manganese is found to promote higher levels of exonuclease activity, which could be a source of mutagenic effects if this ion were selected as the natural cofactor. Several potential applications of these methods to quantitative studies of DNA repair chemistry are also described.

Assay of protein tyrosine phosphatases by using matrix-assisted laser desorption ionization time-of-flight mass spectrometry

A nonradioactive assay for protein tyrosine phosphatases (PTPs), employing a tyrosine-phosphorylated peptide as a substrate, has been developed and applied to analyze purified enzymes, cell extracts, and immunoprecipitates. The reaction was followed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) in a linear and positive ion mode with delayed extraction. MALDI-TOF MS detects a loss of peptide mass by 80 Da as a result of dephosphorylation and, more importantly, it yields phospho-peptide to dephosphorylated product peak intensity ratios proportional to their concentration ratios. A strong bias of the MALDI-TOF MS toward detection of the non-phospho-peptide allows accurate detection of small fractions of dephosphorylation. The method is highly sensitive and reproducible. It can be applied to general assays of protein phosphatases with various phospho-peptides as substrates.

From small-molecule reactions to protein folding: studying biochemical kinetics by stopped-flow electrospray mass spectrometry

This work introduces stopped-flow electrospray ionization (ESI) mass spectrometry (MS) as a method for studying fast biochemical reaction kinetics. After initiating a reaction by rapid mixing of two solutions, the mixture is transferred to a reaction vessel and a steady liquid flow to the ESI source of the mass spectrometer is established. The kinetics are studied in real time by monitoring selected ion intensities as a function of time. In order to characterize the performance of this setup the acid-induced demetallation of chlorophyll a was studied. It was found that the reaction is second order in acid concentration and that pseudo-first-order rate constants of up to roughly 7 s(-1) can be measured reliably. Stopped-flow ESI MS was also applied to study the acid-induced denaturation of myoglobin. The data presented here confirm the occurrence of a short-lived unfolding intermediate during this reaction. Stopped-flow ESI MS can provide information that is not accessible by optical rapid-mixing experiments. Therefore it appears that this novel technique has the potential to become a standard tool for kinetic studies in a number of different fields.