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The application of multiplexed isobaric tandem mass tag (TMT) labeling and an LTQ Orbitrap XL ETD (electron transfer dissociation) hybrid mass spectrometer as a direct approach for qualitative and quantitative characterization of glycoproteins is reported. Bovine fetuin was used as a model glycoprotein in this study. For online liquid chromatography-mass spectrometry (LC-MS) analysis, high-resolution, mass accurate full scan MS spectra were acquired in the Orbitrap mass analyzer followed by data-dependent tandem mass spectrometry (MS/MS) with alternating collision-induced dissociation (CID), ETD, and higher-energy collisional dissociation (HCD) scans. An additional in-source dissociation scan was used as a highly sensitive and selective detection method for eluting glycosylated peptides. By alternatively using three different dissociation methods, 23 glycoforms from all 5 corresponding glycopeptides were identified from a trypsin digest of bovine fetuin. With ETD, labile glycans were retained without any signs of carbohydrate cleavage with concurrent fragmentation of the peptide backbone. Glycosylation sites were clearly localized from the ETD fragmentation data. Glycan structure elucidation was accomplished using CID. The CID experiments generated fragment ions predominantly from cleavage of glycosidic bonds without breaking the peptide bond. Novel to this method, the TMT labeling protocol was modified and adapted for higher labeling efficiency, and a TriVersa NanoMate was used to reinfuse samples to improve ETD and HCD spectra of glycopeptides. Quantification with TMT was verified based on the HCD spectra from multiple nonglycopeptides and glycopeptides. This method can be used as a qualitative and quantitative technique for direct characterization of glycoproteins and has applicability for detection of counterfeit glycoprotein drug products. [ABSTRACT FROM AUTHOR]

Implementation of modern analytical techniques, such as intact mass spectrometry, may allow for more detailed quality assessments of protein therapeutics. The complexity of the protein therapeutic manufacturing process as well as the sensitivity of these drugs to different storage conditions can lead to the presence of several undesired products, including truncations, degradation products, byproducts, and differentially modified protein variants that are difficult to detect by peptide mapping. Intact mass spectrometry can be used to identify the intact protein composition, inclusive of post-translational modifications (PTMs) but can also generate a chemical fingerprint of the different protein species present in a given sample. In this work, we systematically evaluated the influence of multiple charge states, multiple isotopes per charge state, and operating resolution on the suitability of intact mass spectrometry for quantitative analysis using insulin and somatotropin as model systems. Standard curves could be generated using absolute intensity data or using the relative ratio between the analyte and internal standard. These methods demonstrate the validity of quantitative intact mass spectrometry for the analysis of protein therapeutic drugs, thus providing a foundation for future comparative methods. [ABSTRACT FROM AUTHOR]

For automated production of tandem mass spectrometric data for proteins and peptides >3 kDa at >50 000 resolution, a dual online-offline approach is presented here that improves upon standard liquid chromatography-tandem mass spectrometry (LC-MS/MS) strategies. An integrated hardware and software infrastructure analyzes online LC-MS data and intelligently determines which targets to interrogate offline using a posteriori knowledge such as prior observation, identification, and degree of characterization. This platform represents a way to implement accurate mass inclusion and exclusion lists in the context of a proteome project, automating collection of high-resolution MS/MS data that cannot currently be acquired on a chromatographic time scale at equivalent spectral quality. For intact proteins from an acid extract of human nuclei fractionated by reversed-phase liquid chromatography (RPLC), the automated offline system generated 57 successful identifications of protein forms arising from 30 distinct genes, a substantial improvement over online LC-MS/MS using the same 12 T LTQ FT Ultra instrument. Analysis of human nuclei subjected to a shotgun Lys-C digest using the same RPLC/automated offline sampling identified 147 unique peptides containing 29 co- and post-translational modifications. Expectation values ranged from 10-5 to 10-99, allowing routine multiplexed identifications. [ABSTRACT FROM AUTHOR]

Combinatorial biosynthesis of type I polyketide synthases is a promising approach for the generation of new structural derivatives of polyketide-containing natural products. A target of this approach has been to change the extender units incorporated into a polyketide backbone to alter the structure and activity of the natural product. One limitation to these efforts is that only four extender units were known: malonyl-CoA, methylmalonyl-CoA, ethylmalonyl-CoA, and methoxymalonyl-acyl carrier protein (ACP). The chemical attributes of these extender units are quite similar, with the exception of the potential hydrogen bonding interactions by the oxygen of the methoxy moiety. Furthermore, the incorporated extender units are not easily modified by using simple chemical approaches when combinatorial biosynthesis is coupled to semisynthetic chemistry. We recently proposed the existence of two additional extender units, hydroxymalonyl-ACP and aminomalonyl-ACP, involved in the biosynthesis of zwittermicin A. These extender units offer unique possibilities for combinatorial biosynthesis and semisynthetic chemistry because of the introduction of free hydroxyl and amino moieties into a polyketide structure. Here, we present the biochemical and mass spectral evidence for the formation of these extender units. This evidence shows the formation of ACP-linked extender units for polyketide synthesis. Interestingly, aminomalonyl-ACP formation involves enzymology typically found in nonribosomal peptide synthesis. [ABSTRACT FROM AUTHOR]

Rapamycin, FK506, and FK520 are immunosuppressant macrolactone natural products comprised of predominantly polyketide-based core structures. A single nonproteinogenic pipecolic acid residue is installed into the scaffold by a nonribosomal peptide synthetase that also performs the subsequent macrocyclization step at the carbonyl group of this amino acid. It has been assumed that pipecolic acid is generated from lysine by the cyclodeaminases RapL/FkbL. Herein we report the heterologous overexpression and purification of RapL and validate its ability to convert L-lysine to L-pipecolic acid by a cyclodeamination reaction that involves redox catalysis. RapL also accepts L-ornithine as a substrate, albeit with a significantly reduced catalytic efficiency. Turnover is presumed to encompass a reversible oxidation at the α-amine, internal cyclization, and subsequent re-reduction of the cyclic Δ¹-piperideine-2-carboxylate intermediate. As isolated, RapL has about 0.17 equiv of tightly bound NAD+, suggesting that the enzyme is incompletely loaded when overproduced in E. coli. In the presence of exogenous NAD+, the initial rate is elevated 8-fold with a Km of 2.3 μM for the cofactor, consistent with some release and rebinding of NAD+ during catalytic cycles. Through the use of isotopically labeled substrates, we have confirmed mechanistic details of the cyclodeaminase reaction, including loss of the α-amine and retention of the hydrogen atom at the α-carbon. In addition to the characterization of a critical enzyme in the biosynthesis of a medically important class of natural products, this work represents the first in vitro characterization of a lysine cyclodeaminase, a member of a unique group of enzymes which utilize the nicotinamide cofactor in a catalytic manner. [ABSTRACT FROM AUTHOR]

Mass spectrometry has gained widespread acceptance for the characterization of protein therapeutics as a part of the regulatory approval process. Improvements in mass spectrometer sensitivity, resolution, and mass accuracy have enabled more detailed and confident analysis of larger biomolecules for confirming amino acid sequences, assessing sequence variants, and characterizing post translational modifications. This work demonstrates the suitability of a combined approach using intact MS and multistage top down MS/MS analyses for the characterization of a protein therapeutic drug. The protein therapeutic granulocyte-colony stimulating factor was analyzed using a Thermo Fusion Tribrid mass spectrometer using a multistage top down MS approach. Intact mass analysis identified the presence of two disulfide bonds based on exact mass shifts while a combined collision induced dissociation (CID), higher-energy collisional dissociation (HCD), and electron transfer dissociation (ETD) MS/MS approach obtained 80% protein sequence coverage. Isolating MS/MS fragments for MS3 analysis using HCD or CID increased the sequence coverage to 89%. 95% sequence coverage was obtained by reducing human granulocyte-colony stimulating factor (G-CSF) prior to MS/MS and MS3 analysis to specifically target the residues between the disulfide bonds. The use of this combined intact MS and multistage top down MS approach allows for rapid and accurate determination of the primary sequence of a protein therapeutic drug product. [ABSTRACT FROM AUTHOR]

Characterizing combinations of coding polymorphisms (cSNPs), alternative splicing and post-translational modifications (PTMs) on a single protein by standard peptide-based proteomics is challenging owing to <100% sequence coverage and the uncoupling effect of proteolysis on such variations >10-20 residues apart. Because top down MS measures the whole protein, combinations of all the variations affecting primary sequence can be detected as they occur in combination. The protein form generated by all types of variation is here termed the "proteotype", akin to a haplotype at the DNA level. Analysis of proteins from human primary leukocytes harvested from leukoreduction filters using a dual on-line/off-line top down MS strategy produced >600 unique intact masses, 133 of which were identified from 67 unique genes. Utilizing a two-dimensional platform, termed multidimensional protein characterization by automated top down (MudCAT), 108 of the above protein forms were subsequently identified in the absence of MS/MS in 4 days. Additionally, MudCAT enables the quantitation of allele ratios for heterozygotes and PTM occupancies for phosphorylated species. The diversity of the human proteome is embodied in the fact that 32 of the identified proteins harbored cSNPs, PTMs, or were detected as proteolysis products. Among the information were three partially phosphorylated proteins and three proteins heterozygous at known cSNP loci, with evidence for non-1:1 expression ratios obtained for different alleles. [ABSTRACT FROM AUTHOR]

Proteomics has grown significantly with the aid of new technologies that consistently are becoming more streamlined. While processing of proteins from a whole cell lysate is typically done in a bottom-up fashion utilizing MS/MS of peptides from enzymatically digested proteins, top-down proteomics is becoming a viable alternative that until recently has been limited largely to offline analysis by tandem mass spectrometry. Here we describe a method for high-resolution tandem mass spectrometery of intact proteins on a chromatographic time scale. In a single liquid chromatography-tandem mass spectrometry (LC- MS/MS) run, we have identified 22 yeast proteins with molecular weights from 14 to 35 kDa. Using anion exchange chromatography to fractionate a whole cell lysate before online LC-MS/MS, we have detected 231 metabolically labeled (14N/15N) protein pairs from Saccharomyces cerevisiae. Thirty-nine additional proteins were identified and characterized from LC-MS/MS of selected anion exchange fractions. Automated localization of multiple acetylations on Histone H4 was also accomplished on an LC time scale from a complex protein mixture. To our knowledge, this is the first demonstration of top-down proteomics (i.e., many identifications) on linear ion trap Fourier transform (LTQ FT) systems using high-resolution MS/MS data obtained on a chromatographic time scale. [ABSTRACT FROM AUTHOR]

Previously, the FDA validated a method to assess the structure and composition of heparin products by separating and quantifying disaccharide level digests by reverse-phase-ion-pairing liquid chromatography (RPIP-HPLC) coupled to a low resolution and low sensitivity ion trap mass-spectrometer. Here, improved separation, information content and sensitivity were obtained through the use of reverse phase ion-pairing ultra-high pressure liquid chromatography (RPIP-UHPLC) coupled with a quadrupole time-of-flight (Q-TOF) mass spectrometer. Thus, with the new method, improved structural characterization of the same 20 lots of heparin sodium active pharmaceutical ingredients (APIs) as were analyzed in the previous work were obtained. In addition, for the first time, 10 low molecular weight heparin (LMWH) lots were characterized representing multiple lots manufactured by three different processes (dalteparin, tinzaparin or enoxaparin). In this study, UHPLC separation conditions and the enzymatic digesting protocol were optimized for analysis of disaccharide level digests of heparin and positive and negative electrospray ionization (ESI) modes were tested. The negative ion mode ESI analysis was found to be superior to the positive ion mode for these measurements, and a combination of heparin lyase II and III were optimal for heparin digestion. The data obtained establishes the normal variation in the composition of heparin sodium or LMWHs in this assay. These values are useful as possible product benchmarks and for surveillance of the heparin products being imported into the US market., (Published by Elsevier B.V.)