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Uncaging approach, native membrane dynamics and lipidic cubic phases in biomolecular solid-state NMR
(2019)
It was previously shown for the Escherichia coli diacylglycerol kinase (DgkA) that enzyme-reactions at the membrane interface can be monitored by solid-state NMR. However, such studies can face problems due to limited accessibility of the active sites: Natural substrates for membrane enzymes, but also ligands for membrane proteins or lipid mediators, are either partitioning into the membrane and cannot be added easily, or if soluble exhibit accessibility restrictions, as they cannot freely pass through lipid bilayers. This situation complicates quantitative kinetic analysis of biochemical processes such as enzyme activity, ligand binding, but also oligomerization or folding reactions in the membrane or at its interface under MAS NMR conditions.
To overcome these limitations the feasibility and possible advantages of the uncaging approach as a new tool for biomolecular solid-state NMR to trigger reactions by light have been explored. DgkA’s enzymatic activity, exemplary of a biochemical process on the membrane interface, was thereby triggered in situ during MAS by light-induced release of its substrates that were rendered inactive with photolabile protecting groups. To be capable of uncaging sufficient amounts of substrate during MAS to follow the enzymatic reaction via 31P real-time NMR measurements, several illumination variants including an existing illumination setup to study retinal proteins under cryogenic conditions via DNP enhanced NMR were tested. As uncaging of micromole amounts of substrates requires a higher flux compared to initiation of a photocycle in retinal proteins, a new illumination setup was built with Bruker Biospin and Leoni Fibertech. It consists of a modified MAS probe and a suitable fiber bundle, allowing to efficiently couple light from high power LEDs into a sapphire rotor containing the sample, without disturbing the magnetic field homogeneity or sample rotation. By reducing the sample volume to the illuminated area up to 60 mM ATP were released by uncaging NPE ATP to initiate DgkA’s activity in several tested membrane mimetics. These mimetics included liposomes and bicelles, which are well established in the field of biomolecular solid state NMR as well as the optically transparent lipidic cubic phase of monoolein, widely used in membrane protein crystallography, but not yet well characterized as membrane mimetic under MAS conditions. A unique and powerful but compared to time and spatial resolution often underrepresented advantage of the uncaging approach for biophysical studies has been demonstrated by successful uncaging of a non-miscible lipid substrate to trigger DgkA’s kinase reaction: Initiation of processes that cannot easily be triggered by mixing. Examples of these are reactions involving highly hydrophobic, membrane partitioning compounds including lipid substrates, ligands or interaction partners, but also oligomerization or folding of biomacromolecules. The herein performed experiments therefore serve as a first demonstration of the uncaging approach’s feasibility and compatibility with a wide variety of membrane mimetics and give a first indication of its potential for a variety of biomolecular solid state NMR experiments.
As high accessibility for solutes has been a second focus for the choice of membrane mimetics, DgkA’s activity in the lipidic cubic phases of monoacylglycerols with its two continuous networks of water channels has been further characterized. Kinetic parameters obtained from 31P real time solid state NMR experiments revealed that DgkA’s activity is similar to activities obtained in swollen cubic phases in a bath solution with wider water channels. Diffusion of ATP in a non swollen cubic phase was however strongly reduced compared to ATP in solution as diffusion measurements showed. Therefore, saturation of the enzyme required distinctly higher ATP concentrations. These results thereby underline the advantage of a non invasive and label free method like NMR to directly gain information about enzymatic reactions of immobilized enzymes in porous materials. The obtained wealth of information from 31P real time NMR experiments and biochemical assays in different membrane mimetics in presence and absence of lipid substrates and activators also provided further insight into DgkA’s enzymatic activity. It confirms ATP binding and hydrolysis in the absence of a lipid substrate, in agreement with the proposed mode of substrate binding, and allowed to estimate the in vivo relevance of previously observed ATPase activity in liposomes.
Further exploration of the cubic phase as membrane mimetic for protein solid state NMR revealed its high stability under MAS at elevated temperatures and capacity to reconstitute sufficient amounts of DgkA. Unlike monoolein, DgkA was cross-polarizable in a cubic phase and exhibited similar dynamics compared to DgkA reconstituted into liposomes, allowing to acquire the herein shown dipolar coupling based 2D protein spectra. As lipidic cubic phases are not containing phospholipids, monoacylglycerols could be especially useful as membrane mimetics for 31P correlation spectra. Initial experiments under DNP conditions, where in liposomes line broadening causes severe overlap of phospholipid signals and unspecific cross polarization highlight this aspect.
In summary, herein reported results of the experiments performed with lipidic cubic phases demonstrate that they are robust and versatile membrane mimetics. They could be of advantage for a variety of solid-state NMR experiments where either optical transparency for efficient illumination is desired, accessibility for solutes and membrane components under MAS is required, or interference of phosphorous signals of other membrane mimetics must be avoided.
In the second chapter of this thesis 1H solid-state NMR as a label free method to probe membrane order and dynamics directly within a cellular and disease relevant context was used to observe the effects of soluble epoxide hydrolase (sEH) encoding gene knock-outs on membrane dynamics. Knock-out of the sEH encoding gene changed the overall membrane dynamics in the physiological temperature range of native membranes derived from mouse brains, making the bulk membrane more dynamic. To confirm that these effects are related to the enzymatic activity of sEH, substrates and products of sEH were added to evaluate their effects on membrane dynamics. 19,20 dihydroxydocosapentaenoic acid (DHDP), a product of sEH, partially reversed the knock out phenotype in a concentration dependent manner whereas the substrate 19,20 epoxydocosapentaenoic acid did not cause any effects. As both polyunsaturated fatty acids did not show differences in phase behavior in a simple phospholipid bilayer these results provide evidence that the previously observed concentration dependent DHDP induced relocation of cholesterol away from detergent resistant lipid raft fractions is associated with alteration of membrane dynamics. Therefore, also the effect of cholesterol removal via cyclodextrin on membrane dynamics was analyzed. Removal of cholesterol led to a similar temperature profile of wild type and knock out membranes thereby supporting the hypothesis that DHDP induced relocation of cholesterol is causing altered membrane dynamics. These alterations have been shown by the lead authors of the collaborative research project to induce relocation of various membrane proteins and are involved in the development of diabetic retinopathy. Furthermore, in this context inhibition of sEH has been shown to inhibit diabetic retinopathy and proposed as target for prevention of one of the leading causes of blindness in the developed world.
Transport mechanism of a multidrug resistance protein investigated by pulsed EPR spectroscopy
(2019)
In human several diseases result from malfunctions of ATP-binding cassette (ABC) systems, which form one of the largest transport system superfamily. Many ABC exporters contain asymmetric nucleotide-binding sites (NBSs) and some of them are inhibited by the transported substrate.1 For the active transport of diverse chemically substrates across biological membranes, ABC transport complexes use the energy of ATP binding and subsequent hydrolysis. In this thesis, the heterodimeric ABC exporter TmrAB2,3 from Thermus thermophilus, a functional homolog of the human antigen translocation complex TAP, was investigated by using pulsed electron-electron double resonance (PELDOR/DEER) spectroscopy. In the presence of ATP, TmrAB exists in an equilibrium between inward- and outward-facing conformations. This equilibrium can be modulated by changing the ATP concentration, showing asymmetric behaviour in the open-to-close equilibrium between the consensus and the degenerate NBSs. At the degenerate NBS the closed conformation is more preferred and closure of one of the NBSs is sufficient to open the periplasmic gate at the transmembrane domain (TMD).3 By determining the temperature dependence of this conformational equilibrium, the thermodynamics of the energy coupling during ATP-induced conformational changes in TmrAB were investigated. The results demonstrate that ATP-binding alone drives the global conformational switching to the outward-facing state and allows the determination of the entropy and enthalpy changes for this step. With this knowledge, the Gibbs free energy of this ATP induced transition was calculated. Furthermore, an excess of substrate, meaning trans-inhibition of the transporter is resulting mechanistically in a reverse transition from the outward-facing state to an occluded conformation predominantly.3 This work unravels the central role of the reversible conformational equilibrium in the function and regulation of an ABC exporter. For the first time it is shown that the conformational thermodynamics of a large membrane protein complex can be investigated. The presented experiments give new possibilities to investigate other related medically important transporters with asymmetric NBSs or other similar protein complexes.
This doctoral thesis deals with the structural and dynamical NMR characterization of biomolecules, covering a broad range of proteins, from small peptides to large GPCRs proteins. This work consists of two projects, which are presented in chapter II and III. Chapter II is focused on the structural screening of peptides and small proteins ranging from 14 to 71 amino acids, while chapter III describes the structure and light dynamics of the disease relevant rhodopsin G90D mutant. The main method used to investigate both types of proteins is NMR spectroscopy. Both chapters comprise individual general introduction, materials and methods, results and discussion sections, and a final conclusion paragraph.
‘Chapter I: Methodological aspects of protein NMR spectroscopy’ presents an overview of different NMR methods developed for the rapid characterization of protein structure and dynamics. Multidimensional NMR, which is routinely used in structural biology, is indispensable for protein structure determination in solution. However, detailed information with resolution at the atomic level is time consuming and requires weeks of expensive measurement time, followed by the manual data analysis. Therefore, the development of time-saving NMR techniques is highly required for screening studies of a large amount of proteins, and can be also helpful for studying unstable biomolecules, as their short lifetime often restricts the experimental procedure.
This chapter covers the two main approaches to accelerate a multidimensional NMR experiment: fast-pulsing techniques that aim to reduce the duration of an individual measurement, and non-uniform sampling technique (NUS), which was developed to reduce the overall number of increments in virtual time domains. A combination of both approaches, fast-pulsing and non-uniform sampling, allows speeding up the measurement time by 2-3 orders of magnitude. Furthermore, recently developed software called TA (targeted acquisition) combines various time-saving approaches, including fast-pulsing, non-uniform sampling and targeted acquisition. Targeted acquisition algorithm records a set of multidimensional NMR spectra in semi-interleaved incremental mode. This provides the ability to monitor the quality of the recorded spectra in real-time and therefore enables the completion of the experiments after the desired quality is achieved. Using this approach will greatly reduce the measurement time without losing important structural information. The implemented automated FLYA assignment further contributes to the rapid and simplified readout of the chemical shift assignment progress of the TA program. During this doctoral dissertation, the scientific collaboration with the TA software developer Prof. Vladislav Orekhov (Sweden) took place, and resulted in the successful establishing of this new NMR technology in the Schwalbe laboratory. TA is now routinely applied in Prof. Schwalbe group for the structure elucidation of small proteins.
‘Chapter II: Rapid NMR and biophysical characterization of small proteins’ describes the structural analysis of peptides and small proteins, which were recently identified within the framework of the Priority Program (SPP 2002). Due to technical limitations in detections of small systems and strict assumptions concerning the smallest size of the gene that can be translated, small open reading frames (sORFs) were excluded from the automated gene annotation for a very long time. Thanks to the newly developed computational and experimental approaches, the ability to identify and detect the small proteins consisting of less than approximately 70 amino acids sparked a growing scientific interest by microbiologist. In the past years, hundreds of new short protein sequences were discovered. Although some peptides were found to be involved in diverse essential biological processes, the functional elucidation of a large number of recently discovered peptides and small proteins remains a challenging task. It is well established that the structure of proteins is often linked to their function. However, the size of small constructs often restricts the possible diversity of secondary structure elements that might be adopted by a protein. Furthermore, as was shown for intrinsic discorded proteins (IDPs), the absence of a well-defined three-dimensional structure does not necessarily mean lack of function. Moreover, peptides, which are initially unstructured in the isolated form can fold in a stable structured conformation upon interaction with their biological partners. Solution state NMR spectroscopy is perfectly amenable for the structural characterization of systems of this size. It provides a rapid readout about the conformational state of small peptides unambiguously, distinguishing between folded, molten globule and unstructured conformations.
During this doctoral thesis the workflow protocol for fast screening of peptides and small proteins was established and applied to 20 candidates ranging from 14 to 71 amino acids, which were identified and selected by six microbiological groups, all members of the Priority Program on small proteins (SPP2002) funded by the German research foundation (DFG). The screening protocol includes sample preparation and biochemical characterization. Peptides containing less than 30 amino acids were synthesized by solid phase synthesis (SPPS), while small proteins containing more than 30 amino acids were heterologously expressed in E. coli.
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