Design and chemical synthesis of a homogeneous polymer-modified erythropoiesis protein

New agents that stimulate erythropoiesis

Recombinant human erythropoietin (rhEpo) has proven to be remarkably safe and effective for treatment of anemias, primarily those secondary to renal disease and malignancy. Despite the worldwide use of rhEpo, concerns about its cost, the need for frequent parenteral administration, and the development of anti-Epo antibodies have prompted development of improved agents to stimulate erythropoiesis. Three strategies appear to be particularly promising. The half-life of Epo in the circulation can be prolonged by the addition of N-linked carbohydrate groups, by formation of adducts with polyethylene glycol, and by preparation of Epo multimers. Second, mimetic peptides can effectively trigger signal transduction at the Epo receptor, thereby boosting red-cell production. Finally, the hypoxia inducible transcription factor (HIF) can be pharmacologically induced by oral agents, resulting in enhanced expression not only of endogenous Epo but also of other genes important in the regulation of erythropoiesis.

Novel strategies for stimulating erythropoiesis and potential new treatments for anaemia

As with many other therapeutic areas in modern-day medicine, scientific advances in drug development (using such techniques as recombinant DNA technology, site-directed mutagenesis, pegylation of molecules, peptide library screening, and gene transfer) have resulted in the development of potential new agents and strategies for stimulating erythropoiesis. These advances are of possible benefit in treating anaemia due to various causes, including chronic renal failure. Several new treatments will soon become clinically available, while others are at present at an early stage of development but are nevertheless of scientific interest. We review these new therapeutic strategies, and discuss at what stage some of the newer products are in relation to their clinical development programme.

Polymer conjugates as anticancer nanomedicines

The transfer of polymer-protein conjugates into routine clinical use, and the clinical development of polymer-anticancer-drug conjugates, both as single agents and as components of combination therapy, is establishing polymer therapeutics as one of the first classes of anticancer nanomedicines. There is growing optimism that ever more sophisticated polymer-based vectors will be a significant addition to the armoury currently used for cancer therapy.

Mis-translation of a computationally designed protein yields an exceptionally stable homodimer: implications for protein engineering and evolution

We recently used computational protein design to create an extremely stable, globular protein, Top7, with a sequence and fold not observed previously in nature. Since Top7 was created in the absence of genetic selection, it provides a rare opportunity to investigate aspects of the cellular protein production and surveillance machinery that are subject to natural selection. Here we show that a portion of the Top7 protein corresponding to the final 49 C-terminal residues is efficiently mis-translated and accumulates at high levels in Escherichia coli. We used circular dichroism, size-exclusion chromatography, small-angle X-ray scattering, analytical ultra-centrifugation, and NMR spectroscopy to show that the resulting C-terminal fragment (CFr) protein adopts a compact, extremely stable, homo-dimeric structure. Based on the solution structure, we engineered an even more stable variant of CFr by disulfide-induced covalent circularisation that should be an excellent platform for design of novel functions. The accumulation of high levels of CFr exposes the high error rate of the protein translation machinery. The rarity of correspondingly stable fragments in natural proteins coupled with the observation that high quality ribosome binding sites are found to occur within E. coli protein-coding regions significantly less often than expected by random chance implies a stringent evolutionary pressure against protein sub-fragments that can independently fold into stable structures. The symmetric self-association between two identical mis-translated CFr sub-domains to generate an extremely stable structure parallels a mechanism for natural protein-fold evolution by modular recombination of protein sub-structures.

Design of homogeneous, monopegylated erythropoietin analogs with preserved in vitro bioactivity

OBJECTIVE

Erythropoietin (Epo) bioactivity is significantly reduced by modification of lysine residues with amine-reactive reagents, which are the most commonly used reagents for attaching polyethylene glycols (PEGs) to proteins to improve protein half-life in vivo. The aims of this study were to determine whether Epo bioactivity can be preserved by targeting attachment of maleimide-PEGs to engineered cysteine analogs of Epo, and to determine whether the pegylated Epo cysteine analogs have improved pharmacokinetic properties in vivo.

MATERIALS AND METHODS

Thirty-four Epo cysteine analogs were constructed by site-directed mutagenesis and expressed as secreted proteins in baculovirus-infected insect cells. Following purification, monopegylated derivatives of 12 cysteine analogs were prepared using 20-kDa maleimide-PEGs. In vitro biological activities of the proteins were measured in an Epo-dependent cell proliferation assay. Plasma levels of insect cell-expressed wild-type Epo (BV Epo) and a pegylated Epo cysteine analog were quantitated by ELISA following intravenous administration to rats.

RESULTS

Biological activities of 17 purified Epo cysteine analogs and 10 purified pegylated Epo cysteine analogs were comparable to that of BV Epo in the in vitro bioassay. The only pegylated cysteine analogs that displayed consistently reduced in vitro bioactivities were substitutions for lysine residues, PEG-K45C and PEG-K154C. The pegylated Epo cysteine analog had a slower initial distribution phase and a longer terminal half-life than BV Epo in rats, but the majority of both proteins were cleared rapidly from the circulation.

CONCLUSIONS

Targeted attachment of maleimide-PEGs to engineered Epo cysteine analogs permits rational design of monopegylated Epo analogs with minimal loss of in vitro biological activity. Insect cell-expressed Epo proteins are cleared rapidly from the circulation in rats, possibly due to improper glycosylation. Site-specific pegylation appears to improve the pharmacokinetic properties of Epo.

Glycopeptides as versatile tools for glycobiology

This review describes the recent advances in the field of glycopeptide and small glycoprotein synthesis. The strategies covered include chemical and chemoenzymatic synthesis, native chemical ligation (NCL), and expressed chemical ligation. The importance of glycopeptide synthesis is exemplified by giving the reader an overview of how versatile and important these well-defined glycopeptides are as tools in glycobiology.

A designed well-folded monomeric four-helix bundle protein prepared by Fmoc solid-phase peptide synthesis and native chemical ligation

The design and total chemical synthesis of a monomeric native-like four-helix bundle protein is presented. The designed protein, GTD-Lig, consists of 90 amino acids and is based on the dimeric structure of the de novo designed helix-loop-helix GTD-43. GTD-Lig was prepared by the native chemical ligation strategy and the fragments (45 residues long) were synthesized by applying standard fluorenylmethoxycarbonyl (Fmoc) chemistry. The required peptide-thioester fragment was prepared by anchoring the free gamma-carboxy group of Fmoc-Glu-allyl to the solid phase. After chain elongation the allyl moiety was orthogonally removed and the resulting carboxy group was functionalized with a glycine-thioester followed by standard trifluoroacetic acid (TFA) cleavage to produce the unprotected peptide-thioester. The structure of the synthetic protein was examined by far- and near-UV circular dichroism (CD), sedimentation equilibrium ultracentrifugation, and NMR and fluorescence spectroscopy. The spectroscopic methods show a highly helical and native-like monomeric protein consistent with the design. Heat-induced unfolding was studied by tryptophan absorbance and far-UV CD. The thermal unfolding of GTD-Lig occurs in two steps; a cooperative transition from the native state to an intermediate state and thereafter by noncooperative melting to the unfolded state. The intermediate exhibits the properties of a molten globule such as a retained native secondary structure and a compact hydrophobic core. The thermodynamics of GuHCl-induced unfolding were evaluated by far-UV CD monitoring and the unfolding exhibited a cooperative transition that is well-fitted by a two-state mechanism from the native to the unfolded state. GTD-Lig clearly shows the characteristics of a native protein with a well-defined structure and typical unfolding transitions. The design and synthesis presented herein is of general applicability for the construction of large monomeric proteins.

Erythropoietin abuse and erythropoietin gene doping: detection strategies in the genomic era

The administration of recombinant human erythropoietin (rhEPO) increases the maximum oxygen consumption capacity, and is therefore abused as a doping method in endurance sports. The detection of erythropoietin (EPO) abuse is based on direct pharmacological and indirect haematological approaches, both of which have several limitations. In addition, current detection methods cannot cope with the emerging doping strategies of EPO mimicry, analogues and gene doping, and thus novel detection strategies are urgently needed. Direct detection methods for EPO misuse can be either pharmacological approaches that identify exogenous substances based on their physicochemical properties, or molecular methods that recognise EPO transgenes or gene transfer vectors. Since direct detection with molecular methods requires invasive procedures, it is not appropriate for routine screening of large numbers of athletes. In contrast, novel indirect methods based on haematological and/or molecular profiling could be better suited as screening tools, and athletes who are suspect of doping would then be submitted to direct pharmacological and molecular tests. This article reviews the current state of the EPO doping field, discusses available detection methods and their shortcomings, outlines emerging pharmaceutical and genetic technologies in EPO misuse, and proposes potential directions for the development of novel detection strategies.

Protein semi-synthesis: New proteins for functional and structural studies

Our ability to alter and control the structure and function of biomolecules, and of proteins in particular, will be of utmost importance in order to understand their respective biological roles in complex systems such as living organisms. This challenge has prompted the development of powerful modern techniques in the fields of molecular biology, physical biochemistry and chemical biology. These fields complement each other and their successful combination has provided unique insights into protein structure and function at the level of isolated molecules, cells and organisms. Chemistry is without doubt most suited for introducing subtle changes into biomolecules down to the atomic level, but often struggles when it comes to large targets, such as proteins. In this review, we attempt to give an overview of modern and broadly applicable techniques that permit chemical synthesis to be applied to complex protein targets in order to gain control over their structure and function. As will be demonstrated, these approaches offer unique possibilities in our efforts to understand the molecular basis of protein functioning in vitro and in vivo. We will discuss modern synthetic reactions that can be applied to proteins and give examples of recent highlights. Another focus of this review will be the application of inteins as versatile protein engineering tools.

In Situ Preparation of Protein−“Smart” Polymer Conjugates with Retention of Bioactivity

Protein-polymer conjugates are widely used in biotechnology and medicine, and new methods to prepare the bioconjugates would be advantageous for these applications. In this report, we demonstrate that bioactive "smart" polymer conjugates can be synthesized by polymerizing from defined initiation sites on proteins, thus preparing the polymer conjugates in situ. In particular, free cysteines, Cys-34 of bovine serum albumin (BSA) and Cys-131 of T4 lysozyme V131C, were modified with initiators for atom transfer radical polymerization (ATRP) either through a reversible disulfide linkage or irreversible bond by reaction with pyridyl disulfide- and maleimide-functionalized initiators, respectively. Initiator conjugation was verified by electrospray-ionization mass spectroscopy (ESI-MS), and the location of the modification was confirmed by muLC-MSMS (tandem mass spectrometry) analysis of the trypsin-digested protein macroinitiators. Polymerization of N-isopropylacrylamide (NIPAAm) from the protein macroinitiators resulted in thermosensitive BSA-polyNIPAAm and lysozyme-polyNIPAAm in greater than 65% yield. The resultant conjugates were characterized by gel electrophoresis and size exclusion chromatography (SEC) and easily purified by preparative SEC. The identity of polymer isolated from the BSA conjugate was confirmed by (1)H NMR, and the polydispersity index was determined by gel permeation chromatography (GPC) to be as low as 1.34. Lytic activities of the lysozyme conjugates were determined by two standard assays and compared to that of the unmodified enzyme prior to polymerization; no statistical differences in bioactivity were observed.

Site-specific polymer modification of therapeutic proteins

Recent advances in chemoselective ligation technology have made possible the modification of proteins with polymers in a site-specific and controlled manner. These approaches rely on the incorporation of chemoselective anchors into the protein backbone by either chemical or recombinant means, and subsequent modification with a polymer carrying a complementary linker. As a result, the assembly process and the covalent structure of the resulting protein-polymer conjugate are completely controlled, enabling the rational optimization of drug properties, in particular efficacy and pharmacokinetic properties. Application of chemoselective ligation technologies to cytokines and chemokines has led to the generation of new lead proteins for use as erythropoietic agents and HIV fusion inhibitors.

Incorporation of spin-labelled amino acids into proteins

The elucidation of structure and function of proteins and membrane proteins by EPR spectroscopy has become increasingly important in recent years as technological advances have been made in the design of spectrometers and in the chemistry of the nitroxide group. These new developments have increased the demand for tailor-made amino acids carrying a spin label on the one hand and for reliable methods for their incorporation into proteins on the other. Here we describe methods for site-specific spin labelling of proteins. It is shown that a combination of recombinant synthesis of proteins with chemically produced peptides (expressed protein ligation) allows the preparation of site-specifically spin-labelled proteins.

Synthesis of Uniform Protein−Polymer Conjugates

We have developed a novel technique to synthesize near-uniform protein-polymer conjugates by initiating atom transfer radical polymerization of monomethoxy poly(ethylene glycol)-methacrylate from 2-bromoisobutyramide derivatives of chymotrypsin (a protein-initiator). Polymerization initiated from the monosubstituted protein-initiator resulted in the conjugate containing a single, near-monodisperse polymer chain per protein molecule with polydispersity index 1.05. Increasing the number of conjugated 2-bromoisobutyramide initiators per molecule of protein increased the molecular weights and polydispersity indices of the final protein-polymer conjugates. The generic nature of this technique was demonstrated by initiating polymerization of nonionic, cationic, and anionic monomers from the protein-initiator. Protein-polymer conjugates synthesized by this novel technique retained 50-86% of the original enzyme activity. The technique described herein should be useful in synthesizing well-defined protein-polymer conjugates exhibiting a wide range of physical and chemical properties.

Peptide Ligation by a Reversible and Reusable C-Terminal Thiol Handle

[reaction: see text] Peptide ligation of noncysteinyl residues can be achieved conveniently by a reversible C-terminal thiol handle together with a Ag(+) ion-assisted S,N-acyl migration. The regenerated C-terminal handle permits tandem ligation of multiple segment.

Chemical synthesis of proteins

Proteins have become accessible targets for chemical synthesis. The basic strategy is to use native chemical ligation, Staudinger ligation, or other orthogonal chemical reactions to couple synthetic peptides. The ligation reactions are compatible with a variety of solvents and proceed in solution or on a solid support. Chemical synthesis enables a level of control on protein composition that greatly exceeds that attainable with ribosome-mediated biosynthesis. Accordingly, the chemical synthesis of proteins is providing previously unattainable insight into the structure and function of proteins.

Comparison of epoetin alfa and darbepoetin alfa biological activity under different administration schedules in normal mice

The unit of erythropoietic activity has long been the standard by which erythropoietic agents are judged, but the development of long-acting agents such as darbepoetin alfa has highlighted the shortcomings of this approach. To this point, we compared the in vivo activity of Epoetin alfa and darbepoetin alfa per microgram of protein core. Using the established mass-to-unit conversion for Epoetin alfa (1 microg congruent with 200 U), we then calculated darbepoetin alfa activity in units. Activity varied with treatment regimen (1 microg darbepoetin alfa congruent with 800 U for 3 times weekly dosing to 8,000 U for a single injection). This analysis reveals the inadequacy of evaluating darbepoetin alfa activity in terms of standard erythropoietic units. We therefore propose that for molecules with heightened biological activity, a more legitimate basis for comparison is the protein mass.

His6 tag-assisted chemical protein synthesis

To make more practical the total chemical synthesis of proteins by the ligation of unprotected peptide building blocks, we have developed a method to facilitate the isolation and handling of intermediate products. The synthetic technique makes use of a His6 tag at the C terminus of the target polypeptide chain, introduced during the synthesis of the C-terminal peptide segment building block. The presence of a His6 tag enables the isolation of peptide or protein products directly from ligation reaction mixtures by Ni-NTA affinity column purification. This simple approach enables facile buffer exchange to alternate reaction conditions and is compatible with direct analytical control by protein MS of the multiple ligation steps involved in protein synthesis. We used syntheses of crambin and a modular tetratricopeptide repeat protein of 17 kDa as models to examine the utility of this affinity purification approach. The results show that His6 tag-assisted chemical protein synthesis is a useful method that substantially reduces handling losses and provides for rapid chemical protein syntheses.

Chemical and biological properties of polymer-modified proteins

Modification with polymers such as polyethylene glycol (PEG) can increase circulating lifetime, reduce immunogenicity and simplify the handling of pharmaceutical proteins. These benefits are currently exploited in six marketed polymer-modified protein therapeutics and about a dozen product candidates in clinical trials. However, traditional protein modification techniques are restricted by the limited control over polymer structure and the location and number of polymer attachment sites. New technology, in the form of chemical protein synthesis and the generation of precision polymers, has been applied to generate synthetic erythropoiesis protein (SEP). This promising treatment for anaemia has been synthesised with precision polymer modification methodology to improve the target protein pharmaceutical. The chemical and biological properties of proteins prepared by both traditional and novel approaches are contrasted in this discussion of chemical protein synthesis and polymer modification in protein drug discovery.

Synthetic Erythropoietic Proteins: Tuning Biological Performance by Site-Specific Polymer Attachment

Chemical synthesis in combination with precision polymer modification allows the systematic exploration of the effect of protein properties, such as charge and hydrodynamic radius, on potency using defined, homogeneous conjugates. A series of polymer-modified synthetic erythropoiesis proteins were constructed that had a polypeptide chain similar to the amino acid sequence of human erythropoietin but differed significantly in the number and type of attached polymers. The analogs differed in charge from +5 to -26 at neutral pH and varied in molecular weight from 30 to 54 kDa. All were active in an in vitro cell proliferation assay. However, in vivo potency was found to be strongly dependent on overall charge and size. The trends observed in this study may serve as starting points for the construction of more potent synthetic EPO analogs in the future.

Tailor-made glycoproteins

Posttranslational modifications are a fundamental mechanism for the regulation of cellular physiology and function. A recent paper by Zhang et al. provides a novel strategy for the generation of homogeneous glycoproteins. The ability to install covalent modifications site-specifically into proteins holds tremendous promise for deciphering the role of posttranslational modifications and has exciting implications for the development of protein therapeutics.

Site-Specific Polymer Attachment to a CCL-5 (RANTES) Analogue by Oxime Exchange

A synthetic strategy that allows for the site-specific attachment of polymers such as poly(ethylene glycol) (PEG) to protein pharmaceuticals is described. PEG was attached to a 67-amino acid fully synthetic CCL-5 (RANTES) analogue at its GAG binding site both to reduce aggregation and to increase the circulating lifetime. Effective protection of an Aoaa chemoselective linker during peptide assembly, total chemical protein synthesis, and protein folding was achieved with an isopropylidene group. Mild deprotection of the resulting folded synthetic protein and subsequent polymer attachment occur without interference with the native folded structure and activity.

Expanding the Genetic Code

Although chemists can synthesize virtually any small organic molecule, our ability to rationally manipulate the structures of proteins is quite limited, despite their involvement in virtually every life process. For most proteins, modifications are largely restricted to substitutions among the common 20 amino acids. Herein we describe recent advances that make it possible to add new building blocks to the genetic codes of both prokaryotic and eukaryotic organisms. Over 30 novel amino acids have been genetically encoded in response to unique triplet and quadruplet codons including fluorescent, photoreactive, and redox-active amino acids, glycosylated amino acids, and amino acids with keto, azido, acetylenic, and heavy-atom-containing side chains. By removing the limitations imposed by the existing 20 amino acid code, it should be possible to generate proteins and perhaps entire organisms with new or enhanced properties.

Novel forms of chemical protein diversity — in nature and in the laboratory

Novel chemical variants of proteins have been found in nature, including potent 'microprotein' natural products and folded protein molecules that contain a cyclic polypeptide chain. Researchers have used chemical synthesis and genetic methods to make these proteins and more: protein catenanes, neoglycoproteins, and artificial protein molecules with novel architectures or made from novel building blocks. De novo design has taken a big step forward with the accurate design and construction of proteins with complex molecular structure. A variety of non-coded amino acids and other building blocks has been used to make increasingly sophisticated protein molecular devices for use as biosensors and for the study of signal transduction inside living cells.

Native Chemical Ligation through in Situ O to S Acyl Shift

[reaction: see text] A novel strategy to generate thioester peptides compatible with Fmoc chemistry is presented. Peptide-C(alpha)oxy-(2-mercapto-1-carboxyamide)ethyl ester undergoes an O to S acyl shift during ligation and the newly formed thioester intermediate reacts with an N-terminal cysteine fragment generating a product with native amide bond at the ligation site.

Cysteine-Reactive Polymers Synthesized by Atom Transfer Radical Polymerization for Conjugation to Proteins

In this communication we report a strategy for the synthesis of semitelechelic polymers reactive to cysteines. An initiator modified with a pyridyl disulfide was prepared and used for the CuBr/2,2'-bipyridine-mediated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate. Polydispersity indices (M(w)/M(n)) of the polymers with different molecular weights were 1.25 or less. The pyridyl disulfide end group was preserved during the polymerization and allowed direct conjugation of the polymer to cysteine residues of bovine serum albumin. The described method provides a general way for the preparation of protein-polymer conjugates through a reversible disulfide bond without the need for postsynthesis modification of the polymers.

Conversion of a mechanosensitive channel protein from a membrane-embedded to a water-soluble form by covalent modification with amphiphiles

Covalent modification of integral membrane proteins with amphiphiles may provide a general approach to the conversion of membrane proteins into water-soluble forms for biophysical and high-resolution structural studies. To test this approach, we mutated four surface residues of the pentameric Mycobacterium tuberculosis mechanosensitive channel of large conductance (MscL) to cysteine residues as anchors for amphiphile attachment. A series of modified ion channels with four amphiphile groups attached per channel subunit was prepared. One construct showed the highest water solubility to a concentration of up to 4mg/ml in the absence of detergent. This analog also formed native-like, alpha-helical homo-pentamers in the absence of detergent as judged by circular dichroism spectroscopy, size-exclusion chromatography and various light-scattering techniques. Proteins with longer, or shorter polymers attached, or proteins modified exclusively with polar cysteine-reactive small molecules, exhibited reduced to no solubility and higher-order aggregation. Electron microscopy revealed a homogeneous population of particles consistent with a pentameric channel. Solubilization of membrane proteins by covalent attachment of amphiphiles results in homogeneous particles that may prove useful for crystallization, solution NMR spectroscopy, and electron microscopy.

Cyanogen Bromide Cleavage Generates Fragments Suitable for Expressed Protein and Glycoprotein Ligation

Herein cyanogen bromide is employed for the efficient production of N-terminal cysteine containing protein fragments for expressed protein ligation (EPL) from polyhistidine-tagged precursors. We provide three examples of efficient CNBr cleavage of fragments of the glycoprotein erythropoietin that can be ligated with peptides or glycopeptide mimetics potentially giving rise to semisynthetic glycoprotein therapeutics.

Developments in peptide and amide synthesis

The solid-phase methodology is key for an effective synthesis of peptides, from a milligram scale for research to a multi-kilo scale for drug production. Indeed, small peptides containing up to 20-30 amino acids are most readily synthesized by a solid-phase strategy. Larger peptides (up to 60 amino acids) should be synthesized by a convergent approach (i.e. synthesis of protected constituent peptides in solid-phase and combination of these units in solution). Larger peptides and proteins are prepared by chemical ligation, where unprotected segments have been prepared in solid-phase.

Versatile protein biotinylation strategies for potential high-throughput proteomics

We present intein-mediated approaches for efficient biotinylation of proteins site-specifically. The reactive C-terminal thioester generated from intein-assisted protein splicing (either in vitro or in live cells) served as an attractive and exclusive site for attaching cysteine-containing biotin. Using these novel biotinylation strategies, we were able to efficiently biotinylate many proteins from different biological sources in a potentially high-throughput, high-content fashion. Some of these proteins were subsequently immobilized, in a very simple manner, onto different avidin-functionalized solid surfaces for applications such as protein microarray and surface plasmon resonance (SPR) spectroscopy, highlighting the numerous advantages of using biotin over other tags (e.g., GST, His-tag, etc.) as the method of choice in protein purification/immobilization. In addition, our intein-mediated strategies provided critical advantages over other protein biotinylation strategies in a number of ways. For the first time, we also successfully demonstrated that intein-mediated protein biotinylation proceeded adequately inside both bacterial and mammalian living cells, as well as in a cell-free protein synthesis system. Taken together, our results indicate the versatility of these intein-mediated strategies for potential high-throughput proteomics applications. They may also serve as useful tools for various biochemical and biophysical studies of proteins both in vitro and in vivo.

Total chemical synthesis of crambin

Crambin is a small (46 amino acids) protein isolated from the seeds of the plant Crambe abyssinica. Crambin has been extensively used as a model protein for the development of advanced crystallography and NMR techniques and for computational folding studies. We set out to establish synthetic access to crambin. Initially, we synthesized the 46 amino acid polypeptide by native chemical ligation of two distinct sets of peptide segments (15 + 31 and 31 + 15 residues). The synthetic polypeptide chain folded in good yield to give native crambin containing three disulfide bonds. The chemically synthesized crambin was characterized by LC-MS and by 2D-NMR. However, the 31-residue peptide segments were difficult to purify, and this caused an overall low yield for the synthesis. To overcome this problem, we synthesized crambin by the native chemical ligation of three segments (15 + 16 + 15 residues). Total synthesis using the ligation of three segments gave more than a 10-fold increase in yield and a protein product of exceptionally high purity. This work demonstrates the efficacy of chemical protein synthesis by the native chemical ligation of three segments and establishes efficient synthetic access to the important model protein crambin for experimental studies of protein folding and stability.

Sortase-mediated protein ligation: a new method for protein engineering

Sortase (SrtA), a transpeptidase from Staphylococcus aureus, catalyzes a cell-wall sorting reaction at an LPXTG motif by cleaving between threonine and glycine and subsequently joining the carboxyl group of threonine to an amino group of pentaglycine on the cell wall peptidoglycan. We have applied this transpeptidyl activity of sortase to in vitro protein ligation. We found that in the presence of sortase, protein/peptide with an LPXTG motif can be specifically ligated to an aminoglycine protein/peptide via an amide bond. Additionally, sortase can even conjugate substrates such as (d)-peptides, synthetic branched peptides, and aminoglycine-derivatized small molecules to the C terminus of a recombinant protein. The sortase-mediate protein ligation is robust, specific, and easy to perform, and can be widely applied to specific protein conjugation with polypeptides or molecules of unique biochemical and biophysical properties.

Recombinant Erythropoietin and Analogues

Erythropoietin (EPO) increases the number of circulating erythrocytes and thus muscle oxygenation. The availability of the recombinant protein (rEPO) has increased the risk of its illegal use in sports, its detection being a difficult challenge. Five different hematopoietic parameters were initially chosen as indirect markers of rEPO abuse: concentration of serum EPO, concentration of serum-soluble transferrin receptors (sTFr), hematocrit, percentage of reticulocytes, and percentage of macrocytes. New models considering only hemoglobin, serum EPO concentration, and percentage of reticulocytes are simpler and seem to be more sensitive when low doses of rEPO are used. A more direct method of urine analysis (isoelectrofocusing, double blotting, and chemiluminescent detection) based on the charge differences between rEPO and endogenous EPO, related to their carbohydrate composition, provides proof of rEPO use. Furthermore, this approach permits the detection of darbepoetin, a direct analogue of EPO also known as NESP ("new erythropoiesis stimulating protein"). Recently a protein conjugate, "synthetic erythropoiesis protein" (SEP), containing precision-length, monodisperse, negatively charged polymers instead of oligosaccharides has been synthesized. Finally, EPO-mimetics are molecules capable of acting as EPO in dimerizing the EPO receptor. Two kinds of EPO-mimetics have been described: peptides and nonpeptides. The enhancement of oxygen availability to muscles by rEPO, analogues, and mimetics constitutes one of the main challenges to doping control. Major steps have already been developed for detection ofrEPO and some analogues. In the near future, the transfection to an athlete's body of genes that code for erythropoietin might be an emerging doping issue, and sports authorities have incorporated "gene doping" among the prohibited practices.

Solution- and Solid-Phase Syntheses of Guanidine-Bridged, Water-Soluble Linkers for Multivalent Ligand Design

[reaction: see text] Efficient syntheses of guanidine-bridged poly(ethylene glycol) linkers of various lengths in fully protected form are reported for both solution- and solid-phase protocols. The application of such linkers in the construction of water-soluble and high-affinity multivalent ligands against cholera toxin is demonstrated. Synthetic intermediates for multivalent ligands as large as 20 kDa in molecular weight have been assembled using presynthesized linkers. The final ligands are highly water-soluble, thus enabling proper biophysical characterization.

Total chemical synthesis and electrophysiological characterization of mechanosensitive channels from Escherichia coli and Mycobacterium tuberculosis

Total chemical protein synthesis was used to generate multimilligram quantities of the mechanosensitive channel of large conductance from Escherichia coli (Ec-MscL) and Mycobacterium tuberculosis (Tb-MscL). Cysteine residues introduced to allow chemical ligation were masked with cysteine-reactive molecules, resulting in side chain functional groups similar to those of the wild-type protein. Synthetic channel proteins were transferred to 2,2,2-trifluoroethanol and reconstituted into vesicle membranes. Fluorescent imaging of vesicles showed that channel proteins were membrane-localized. Single-channel recordings showed that reconstituted synthetic Ec-MscL has conductance, pressure dependence, and substate distribution similar to those of the recombinant channel. Reconstituted synthetic Tb-MscL also displayed conductance and pressure dependence similar to that of the recombinant protein. Possibilities for the incorporation of unnatural amino acids and biophysical probes, and applications of such synthetic ion channel analogs, are discussed.

Effect of Copper Salts on Peptide Bond Formation Using Peptide Thioesters

[reaction: see text] In the present paper, systematic studies revealed that Cu(I) salts in general and Cu(II) salts under certain circumstances promote effective reaction between peptide thiol esters and the N-terminal amino function of a second peptide segment to give the native amide bond for both solution- and solid-phase syntheses. Chiral integrity was retained. Reaction conditions were optimized and applied to the synthesis of a small protein, the identity of which was confirmed by NMR analysis.

A New Approach to the Chemical Synthesis of Keto-Proteins

To increase the versatility of protein-conjugation, an orthogonal protection strategy is described, which enables the efficient synthesis of keto-proteins bearing a reactive ketone functionality using Boc, Fmoc, and chemical ligation methodologies. A 1,3-dithiolane group was used to protect the ketone function of levulinate- and pyruvate-derivatized peptides during solid-phase synthesis, acidolytic cleavage, and purification. When required, the 1,3-dithiolane group could be cleanly removed using aqueous silver or mercuric solutions to regenerate the reactive keto-protein at ambient temperature. The liberated keto-protein was chemoselectively conjugated in situ to an aminooxy-derivatized monodisperse polymer.

Separation of Recombinant Human Erythropoietin Glycoforms by Capillary Electrophoresis Using Volatile Electrolytes. Assessment of Mass Spectrometry for the Characterization of Erythropoietin Glycoforms

The separation of the glycoforms of erythropoietin (EPO) by capillary electrophoresis (CE) was recently published as a monograph by the European Pharmacopoeia (European Pharmacopoeia 4 2002, 1316, 1123-1128). Although the experimental CE conditions employed a background electrolyte containing additives suitable for on-line UV-absorption detection, they were not appropriate for on-line mass spectrometry (MS) detection. In this work, an attempt was made to investigate experimental conditions employing volatile electrolyte systems to achieve the separation and characterization of EPO glycoforms using CE and ESI-MS methodologies. The influence of several operating conditions, such as the coating of the internal walls of the capillary as well as the composition, concentration, and the pH of the separation buffer were investigated. The results demonstrated that when the internal walls of the capillaries were permanently coated with Polybrene and a buffer electrolyte containing 400 mM of HAc-NH4Ac (acetic acid-ammonium acetate), pH 4.75, was used, a significantly reproducible separation was achieved for EPO glycoforms. Intact EPO was characterized by two mass spectrometry techniques: electrospray ionization (ESI-MS) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF-MS). The data demonstrated that MALDI-TOF-MS provided a good approximation to an average molecular mass of the EPO molecule. However, it was still necessary to carry out further separation of the intact EPO glycoforms in order to obtain molecular mass information when ESI-MS was used.

Designing proteins for therapeutic applications

Protein design is becoming an increasingly useful tool for optimizing protein drugs and creating novel biotherapeutics. Recent progress includes the engineering of monoclonal antibodies, cytokines, enzymes and viral fusion inhibitors.