The present application relates to the field of protein sequencing. In particular, the present application relates to a system and apparatus for protein sequencing allowing high throughput of samples with high sensitivity.
Protein sequencing is used in many biochemical, pharmaceutical, and biomedical research fields to determine the amino acid composition of a sample protein, as well as the sequence in which those amino acids take within a given protein. By determining the amino acid sequence of a new protein, its structural conformation can be better known. Further, an unknown sample protein can be readily identified as a previously known protein through the use of protein sequencing.
Protein sequencing can be performed in a number of different manners, from the use of the Edman degradation reaction, thioacylation, or the use of mass spectrometry, matrix assisted laser desorption ionization, or electrospray ionization (ESI). A brief description of each of these sequencing methods follows, with a:
I. The Edman Degradation Process
The Edman degradation process, first described by P. Edman in 1957, is the basis for modem chemical peptide sequencing. The Edman degradation process operates by removing and identifying each amino acid from the N-terminal end of a protein, thereby allowing a practitioner to identify the composition and sequence of a particular protein. P. Edman, ACTA. CHEM SCAND. 10,761 (1957). More specifically, three reactions are used in the Edman degradation process to remove each N-terminal amino acid: (1) coupling, (2) cleavage, and (3) conversion.
The first reaction, often referred to as coupling, modifies the N-terminal amino acid by adding phenylisothiocyanate (“PITC”.) to the amino group, typically in a base-catalyzed reaction. The result of coupling is a phenylthiocarbamyl (“PTC”) protein with the PTC-coupled amino acid occurring at the N-terminal end of the protein. This PTC-coupled amino acid can then be subjected to the second reaction, cleavage, to remove the PTC-coupled amino acid from the protein. The cleavage reaction is typically performed by treating the PTC protein with an anhydrous acid, thereby allowing the sulfur from the PTC group to react with the first carbonyl carbon in the protein chain. As such, this cyclization reaction results in the removal of the first amino acid as an 2-anilino-5(4)-thiozolinone (“ATZ”) derivative, thereby exposing the next N-terminal amino acid on the protein. At this point the, cleaved amino acid, as an ATZ derivative, can be extracted from the residual polypeptide. The cleaved amino acid is then subjected to the third reaction, conversion, wherein the ATZ derivative is converted to a phenylthiohydantoin (“PTH”) amino acid (the “converted amino acid”) by exposing the ATZ derivative to heat and an aqueous, methanolic, or anhydrous acid. The PTH amino acid is more stable and allows for analyzing and identification of the amino acid.
Identification of the PTH amino acid derivative may be performed by either using fluorescent reagents that attach to the cleaved PTH amino acid derivative, or by using fluorescent reagents in the earlier steps of the Edman process to cause a fluorescent-coupled PTH amino acid derivative. However, such reactions are slow, and may result in low percentages of fluorescent coupled amino acid derivatives due to the fact that fluorescent reagents tend to have unfavorable electron configuration. As a result, other methods of identification, including the use of gas liquid chromatography such as high pressure liquid chromatography (“HPLC”), surface phase microextraction chromatography, or mass spectrometry may be used to identify the PTH amino acid derivative.
According to the Edman degradation process, the process of coupling, cleaving, and then converting and identifying the amino acid from the remaining polypeptide is then continued in an iterative fashion until each of the amino acids comprising the original protein have been removed from the N-terminal end and identified.
II. Thioacylation Protein Sequencing
As an improvement on the Edman degradation process, thioacylation allows the use of relatively mild conditions and faster reactions than the original Edman degradation. Typical thioacylation sequencing involves three steps, similar to the Edman degradation, but the coupling step results in attaching the N-terminal amino acid to an insoluble support, allowing for solid phase chemistry to be utilized.
A more complete discussion of thioacylation degradation can be found in U.S. Pat. No. 5,246,865 to Stolowitz et al. (the “Stolowitz Patent”), which is incorporated by reference herein. The Stolowitz Patent indicates that the most of the proposed compounds used for thioacylation have a lower reactivity than the PITC utilized in the Edman degradation process. The Stolowitz Patent discloses the use of more generally available reagents that display better reactivity than the previous thioacylating compounds and allow for better deposition of the cleaved amino acid complexes on a hydrophobic membrane. Thus, the method disclosed in the Stolowitz Patent allows for a more sensitive sequencing system due to the increased reactivity and better retention on a hydrophobic film layer. Further, gas chromatography, mass spectrometry, or chemical ionization mass spectrometry can be used to identify each amino acid complex that is removed from the polypeptide or protein in each iteration of the degradation reaction by the thioacylation protein sequencing process. However, the method disclosed in the Stolowitz Patent utilizes reactants that may modify the side chains of amino acids, making proper sequencing difficult.
III. Mass Spectrometry
Protein sequencing through the use of mass spectrometry is used in many chemical identification applications by measuring the ratio between the mass and charge of a sample. While it is possible to sequence larger polypeptides using mass spectrometry, substantial computing power is required to perform such an analysis. Further, mass spectrometry protein sequencing cannot accurately identify large proteins without modification of the proteins, either through ionization of the proteins (usually performed through electrospray ionization), or chemical or enzymatic digestion of the proteins into smaller polypeptides, each of which may cause the transformation of certain amino acids.
Variations of protein sequencing using mass spectrometry include ladder sequencing. Ladder sequencing utilizes mass spectrometry to compare the resultant peptides that are given off after sequential digesting of proteins. The digestion process may be performed using enzymatic techniques that cleave a protein into multiple polypeptides, or as a modified Edman chemical degradation.
Several methods for performing ladder sequencing may be used, including the use of exopeptidases to cleave off terminal amino acids or dipeptides. This technique has limited application due to the variability of reactivity with respect to the target protein. Alternatively, PITC with a low percentage of phenylisocyanate (“PIC”) has been used to generate several peptide fragments that can be compared to statistically determine the sequence of the protein in a mixture. This PITC/PIC method has the disadvantage of resulting in a substantial loss of peptides during washing cycles, and reducing the effectiveness of ionization of the products, which can significantly alter the effectiveness of sequencing when small protein sample sizes are utilized.
As will be appreciated, the multiple approaches taken to protein sequencing have been made in an attempt to produce a protein sequencing system that: can be used with high sensitivity so that small samples can be accurately sequenced, can be used on a broad range of proteins without selectivity issues; and which allows a higher throughput of samples to allow protein sequencing to be used on a larger and more efficient scale. However, the several approaches noted above do not allow large sample sizes to be run in short time periods due to the multiple iterations of cycling required under the Edman process and its related methods, and due to the focus on obtaining high sensitivity in sequencing results. Conversely a reliable, high throughput system would be greatly appreciated in the art to allow qualitative identification of protein or peptide sequences.
IV. Automated Sequencing
As will be appreciated from the above discussion of protein sequencing, the processes involved in any degradation or enzymatic digestion sequencing is repetitive and can be time consuming-particularly when small sample sizes are involved and care must be taken not to lose a substantial amount of the sample during processing. As such, automated chemical systems have been developed to perform such tasks. For example, U.S. Pat. No. 6,813,568 to Powell et al. (the “Powell Patent”), incorporated by reference herein, claims that chemical analysis results in more easily interpreted sequences than mass spectrometry, but that chemical analysis is “at least ten to twenty times less sensitive than most mass [spectrometry] analyzers.” Col. 1, Ins. 50-54. As such, the Powell Patent discloses a microfluidics-based automated protein sequencing system that includes several rotary selector valves and switching valves attached to a processor to direct delivery of reagents and high pressure reactants to use with high pressure or gas chromatography, a cleavage reactor vessel for each protein being analyzed, a conversion reactor for each protein being analyzed, and a restraining means for restraining unprocessed polymer in a reaction vessel.
As such, the automated apparatus disclosed in the Powell Patent includes several valves that must operate under high pressure, and includes a conversion reactor for each protein analyzed. Because conversion can be a rate limiting step in protein sequencing, and the additional conversion vessels required for each sample necessarily adds to the mass of a protein sequencer, a system and apparatus that would allow for reduced cost in manufacturing and increased throughput of samples would be greatly appreciated.
Further, a system and method that would allow for greater flexibility in the type of processing and detection used in protein sequencing would be greatly appreciated. Finally, a system and method that allows for modular increases in the number of samples that can be processed at any given time while still retaining reliability would be greatly appreciated.
A. Overview
The present application relates to a system and apparatus for performing high throughput protein sequencing with reliable identification of proteins or polypeptides. Contrary to prior sequencing methods that require iterative sequencing of each of the N-terminal amino acids and subsequently identify each amino acid cleaved from the protein or polypeptide to reconstruct a sequence, the present application relates to a system of identifying proteins or polypeptides from an organism or other protein source utilizing a minimal number of cleavage cycles and with no requirement for conversion of the cleaved samples to identify the sample protein or polypeptide.
One embodiment of the present application relates to utilizing a known DNA, RNA, or protein library to act as a known set of sequences against which unknown proteins may be compared for identification. In particular, the identification of proteins or polypeptides from a particular organism or group of organisms (such as populations, subspecies, species, genera, etc.) is used to narrow the universe of potential proteins that are being tested to a discrete protein population. By way of nonlimiting example, a DNA sample, RNA sample, or array of proteins from the organism or groups of organisms may be used to form or extrapolate a library of protein sequences of the relevant protein population for later identification of an unknown protein sample or samples. It will be appreciated that protein population libraries can be identified by using previous methods of DNA or RNA sequencing, mass spectroscopy, or in depth protein sequencing. Because the libraries of genomes for various organisms are now available from many different sources, a protein population for an organism or group of organisms may be readily available, and relevant proteins may be identified without any sequencing performed prior to testing unknown samples.
In one embodiment, a system for identifying proteins comprises the cleavage of about five or fewer N-terminal or C-terminal amino acids from an unknown protein or amino acid using chemistries used in the ladder process, Edman process, thioacylation process, or other known processes. For example, five or fewer coupling and cleavage reactions may be cyclically performed to remove the first five or fewer amino acids from the unknown protein. After each cycle, the cleaved amino acid may be washed off from the reaction vessel and identified. The identification of the first five or fewer amino acids is then recorded as a partial sequence, and that partial sequence is compared to the protein sequence library previously discussed.
It will be appreciated that several proteins from the protein population represented in the protein sequence library may have identical amino acid sequences to that of the unknown sample. This is one reason why the Edman process and other processes previously used require sequencing by identifying each amino acid in an unknown sample to identify the polypeptide or protein. However, according to one aspect of the present application, the molecular weight of each unknown sample is also taken and compared against the molecular weight of the population proteins identified by comparing the first five or fewer amino acids of the unknown sample with the protein sequence library. In this manner, when both the first five or fewer amino acids of an unknown sample and its molecular weight are compared to the first five amino acids and molecular weight of the protein population accumulated for the protein sequence library, nearly all unknown samples from a particular organism can be identified simply by comparing the discovered sequence and molecular weight. As such, identification through limited sequencing can be accomplished without exhaustive and iterative sequencing. In the event that the first five or fewer amino acids and molecular weight cannot positively identify the unknown sample as a single protein or peptide from the identified protein population, additional sequencing may be performed on the unknown sample. In the event that several proteins with identical sequences of the first five amino acids are identified in a protein sequencing library, the first 10 or fewer, or the first 20 or fewer amino acids for each sample could be taken. However, a significant reduction in time taken to identify the samples would be appreciated even if only half of the unknown samples were immediately identifiable through the comparison method, or if.
It will be appreciated that alternative embodiments in which the first 6 or fewer amino acids from the N-terminal or C-terminal end are identified along with the molecular weight of the unidentified protein may be compared to the protein population to identify the unknown protein. Alternatively, embodiments in which the first 10 or fewer amino acids from the N-terminal or C-terminal end are identified along with the molecular weight of the unidentified protein are compared to the protein population to identify the unknown protein. Alternatively, embodiments in which the first 20 or fewer amino acids from the N-terminal or C-terminal end are identified along with the molecular weight of the unidentified protein are compared to the protein population to identify the unknown protein. Alternatively, embodiments in which the first 30 or fewer amino acids from the N-terminal or C-terminal end are identified along with the molecular weight of the unidentified protein are compared to the protein population to identify the unknown protein.
In an alternative embodiment, a protein population or protein sequence library may not be created prior to the sequencing of unknown samples. For example, one or more samples may be processed in a manner that identifies the first 5 or fewer amino acids in sequence, the first 6 or fewer amino acids in sequence, the first 10 or fewer amino acids in sequence, or the first 20 or fewer amino acids in sequence, along with the molecular weight of the one or more unknown protein samples. Once the initial amino acid sequence is identified, a mapped genome may be analyzed to identify all potential proteins that may be produced by the organism in question, or an RNA, DNA, or known protein samples may be probed to identify a protein population that has an identical initial sequence by, for example, a blast search.
According to yet another embodiment of the present application, a short series of ladder sequencing may be utilized to cleave an unknown sample that has been bonded or attached to a solid surface (such as a membrane) into several different sized polypeptide fragments. Once the free fragments are washed, the solid surface may be subjected to mass spectrometry to identify the sequence of a certain number of amino acids within the protein. The location of these identified amino acids, along with the molecular weight of the sample, can then be compared against a previously generated protein sequence library as discussed above, or may be used to probe a genome, RNA, or DNA as previously discussed to identify an unknown protein sample.
It will be appreciated that each of the above embodiments can be performed by obtaining a relatively pure protein sample from a mixed protein sample by utilizing a 2D separation, such as gel electrophorisis or chromatography, to separate out the various proteins in an unknown sample into its individual protein samples. Alternatively, a 1D separation may be performed, with the mass differences of the proteins in a mixed sample may be utilized to identify the multiple proteins in a mixed sample, although such a mixture will complicate analysis of the sample.
An exemplary embodiment of one aspect of the present application would involve the use of an unknown mixed protein sample. The unknown mixed protein sample is subjected to a 2D separation, and a purified protein sample is obtained by pulling out one of the samples from the 2D separation—which should hold several molecules of a particular unknown protein. The sample is then adhered to a membrane attached to a reaction vessel and run through an automated system as described below. Reagents are selected to perform a ladder process sequencing by utilizing the PITC/PIC reagents discussed above in a manner to obtain the sequence of the first 6 N-terminal amino acids. After washing the reagents and optionally saving the eluted cleaved peptides and amino acids from the reactor vessel, the remaining fragments still attached to the film are subjected to mass spectrometry to determine the sequence of the first 6 N-terminal amino acids and the molecular weight of the fragments, from which the molecular weight of the entire sample can be derived, if necessary.
In this example, the amino acid sequence for the first 6 amino acids is GDPGGV. A search of a known protein database for the 6 amino acid sequence, in this instance, the database maintained for proteins at the National Center for Biotechnology Information, is searched for the GDPGGV sequence. A total of 46 possible proteins are identified when a search of this 6 amino acid sequence is performed. Additionally, the 46 possible proteins identified include proteins from several different organisms. This list can be substantially reduced by removing all but the known organism from which the sample was taken, if known. Additionally, the number in the protein sequence where the GDPGGV sequence is found is identified in the database, so it can be determined whether the unknown protein was later phosphorylated or otherwise changed from its original state in the organism from which it came.
In the event that no such results are present for a given sequence, a DNA or RNA probe corresponding to the amino acid sequence can be created to identify the sequence in the organism's DNA that codes for the protein, thereby allowing the identification of the protein.
B. Automated System Embodiments
Turning now to
Protein sequencing system 20 further comprises a reaction plate 300 having multiple reaction vessels 310. Reaction vessels 310 are sized and shaped to allow deposition of a sample onto a membrane (not shown) attached to reaction vessel 310 or onto the edges of reaction vessel 310. Further, reaction vessels 310 may comprise several holes drilled through reaction plate 300 such that each reaction vessel 310 corresponds with one of the manifold branches 210 of manifold branch assembly 200 sealably attached to its bottom end. On the top end of reaction plate 300 is sealably attached to an exit channel assembly 350 comprising several exit channels 352 corresponding to each reaction vessel.
As shown in
According to one embodiment, each exit channel 352 has a selection valve 451 placed to allow diversion of any fluids flowing from the reaction vessel 310 to be diverted either to a waste pathway 452 or to a collection port 500. As shown in
It will be appreciated that the system described above does not include a conversion chamber as is typically dictated by protein sequencing processes. While conversion of any cleaved amino acid can be performed after collection in this system, such conversion is not required, thereby allowing substantially higher throughput than previous systems. In one embodiment, if the cleaved amino acid product is collected, the product is optionally sent through a mass spectrometer for identification of the cleaved amino acid. Alternatively, if a ladder process is used to identify key amino acids within the unknown sample, the cleaved peptides may optionally be channeled through waste pathway 452, with the remaining peptides bonded to a solid surface in the reaction vessel being collected and used to establish the sequence of the unknown protein.
C. Alternate Embodiments.
Yet another embodiment according to the present application is shown in
With reference to
Optionally, as shown in
It will be appreciated that several optional features may be present. For example, supply terminals may be connected to the housing for supplying power to an electrical motor and/or a heating device located inside the reactor chamber. Further, a controlling unit is optionally located inside a housing for controlling the operation of the electrical motor. In another embodiment, a communication device is optionally located inside the housing for communicating with a remotely located controlling unit thereby enabling controlling the operation of the electrical motor, and the communication device is a wireless communication device.
In yet another embodiment of the present invention, the controlling unit is programmed by a processor or a controller to determine the sequence in which the circumferential manifold channel located on the inner most pipe is brought in fluid flow communication with one of the fluid inlet ports/channels of the outer pipe and the time period for which the circumferential manifold channel located on the inner most pipe is brought in fluid flow communication with one of the fluid inlet ports/channels of the outer pipe.
In addition, according to one embodiment shown in
Optionally, the controlling unit is programmed by the user to determine the sequence in which the circumferential manifold channel located on the inner most pipe is brought in fluid flow communication with one of the fluid inlet ports/channels of the outer pipe and the time period for which the circumferential manifold channel located on the inner most pipe is brought in fluid flow communication with one of the fluid inlet ports/channels of the outer pipe. Further, the temperature of the reactor and/or the product chamber(s) can be also so controlled by providing power to the heaters provided for the reactor and/or product chamber(s) for variable periods of time.
D. Slider Valve
Turning now to
Turning now to
Although the embodiments above have been described in detail with reference to preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.
The present nonprovisional application claims priority to U.S. Provisional Patent Application Ser. No. 60/744,760, filed Apr. 13, 2006, and titled AUTOMATED CHEMICAL ANALYZER, Provisional Patent Application Ser. No. 60/807344, filed Jul. 14, 2006, and titled AN AUTOMATED REACTOR DEVICE, and U.S. Provisional Patent Application Ser. No. 60/863,570 and titled EFFICIENT METHOD FOR PARTIAL SEQUENCING OF PEPTIDE/PROTEIN USING ACID OR BASE XANTHATES, filed Oct. 30, 2006.
Number | Date | Country | |
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60744760 | Apr 2006 | US | |
60807344 | Jul 2006 | US | |
60863570 | Oct 2006 | US |