Electrostatic interaction has long been proposed to be an important factor for stabilizing protein. Charge-charge interaction may especially be important to the thermostability of protein, as having more surface electrostatic interactions is one of the common structural features found in thermophilic proteins when compared to their mesophilic homologues. In order to quantitatively investigate the electrostatic contribution to protein stability, two complementary approaches, namely the double mutant cycle approach and pKa shift approach, were carried out. / In the double mutant cycle approach, the coupling free energies of two salt bridges (E6/R92 and K46/E62) and one a long range ion pair (E90/R92) were estimated by using circular dichroism, to find out the thermodynamic parameters of the protein model Thermococcus celer L30e and its charge-to-neutral mutants. It was found that the coupling free energy was temperature independent and was about 3 kJ mol-1 per salt bridge. By using a novel analysis of double mutant cycle of DeltaC p, it was also found that the interaction of salt bridge plays an important role in the reduction of DeltaCp. The temperature independency of coupling free energy and the effect of reducing DeltaCp could explain the general observation very well that thermophilic proteins have highly up-shifted protein stability curves is due to its elevated electrostatic interactions when compared with their mesophilic homologs. / In the pKa shift approach, the native state pKa values of acidic residues were obtained by fitting the side chain carboxyl 13C chemical shifts to microscopic model or global fitting of titrational event (GloFTE), whereas the denatured state pKa values were obtained by conventional pH titration of terminal protected 5-residue glycine-based model peptide. It was found that the surface charge-charge interactions, either attractive or repulsive, were strong and complicated because of the high surface charge density of T. celer L30e. However, the fact that most of the acidic residues have significantly downshifted native state pK a values indicated the surface charge distribution of T. celer L30e is optimized for stabilizing the protein. In addition, we have shown that temperature has negligible effect on pKa values in both native state and denatured state, therefore temperature can only marginally amplify the stabilizing effect in linear manner. / To overcome the unwanted crystallization problem of wild-type T. celer L30e in the low ionic strength neutral pH NMR conditions, which were essential for the pKa shift approach, a quintuple Arg-to-Lys variant was designed to dramatically improve the crystalline solubility, while the surface charges, as well as the structural, thermodynamic, and electrostatic properties, were conserved. It has also shown that electrostatic interaction played a critical role in crystallization at low ionic strength conditions, and arginine residue was especially important in crystal packing because of its high ability of forming salt bridges and hydrogen bonds. / Wild-type T. celer L30e has also shown to have no observable residual structure in the guanidine HC1-induced denatured state, indicating that denatured state of T. celer L30e should not have large effect on the overall protein stability. / Chan, Chi Ho. / Adviser: Kam Bo Wong. / Source: Dissertation Abstracts International, Volume: 73-01, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 202-218). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

A L-asparaginase (EC 3.5.1.1) é uma enzima importante para o tratamento da leucemia linfoblástica aguda (LLA), neoplasia mais frequente em crianças e adolescentes. A L-asparaginase hidrolisa a L-asparagina resultando em ácido aspártico e amônio, impedindo que as células tumorais utilizem esse aminoácido para síntese proteica, ocasionando a morte celular apoptótica. Atualmente a enzima é obtida a partir de Escherichia coli e Erwinia chrysanthemi; no entanto, ambas as formulações estão associadas a um alto índice de efeitos adversos que comprometem a evolução e eficácia do tratamento. A levedura Saccharomyces cerevisiae tem o gene ASP1 responsável pela produção de L-asparaginase 1 (Sc_ASPase1) que tem sido pouco estudada. Para elucidar as características de Sc_ASPase1 nós expressamos a proteína em E. coli BL21(DE3) e a purificamos por cromatografia de afinidade. Sc_ASPase1 tem uma atividade especifica de 195,4 U/mg para L-asparagina e de 0,36 U/mg para L-glutamina, e um comportamento alostérico com um K0.5 de 75 µM para L-asparagina. Por meio de mutação sitio dirigida demonstramos a importância dos resíduos Thr64-Thy78-Th141-Lys215 para a catálise. As isoformas mutantes da proteína A331D, K335E, Y243S, S301N e ΔG77 não apresentaram melhoria nos parâmetros cinéticos ou atividade específica. Construímos e clonamos Sc_ASPase1 com a deleção dos primeiros 52 aminoácidos, porém nas condições testadas a proteína foi expressa na forma insolúvel. Demonstramos que Sc_ASPase1 possui potencial antineoplásico, pois com 10 U/mL de enzima foi capaz causar a 85% de mortalidade da linhagem leucêmica MOLT-4. Na mesma concentração, a enzima de E. coli é capaz de matar 95% de células dessa mesma linhagem. / L-Asparaginase (EC 3.5.1.1) is an important enzyme for the treatment of acute lymphoblastic leukemia (ALL), the most common malignancy in children and adolescents. L-asparaginase hydrolyzes L-asparagine resulting in ammonium and aspartic acid, preventing tumor cells of using such amino acid for protein synthesis, leading to apoptotic cell death. Currently, the enzyme is obtained from Escherichia coli and Erwinia chrysanthemi; however, both formulations are associated with a high incidence of side effects that compromise the progress and effectiveness of treatment. The yeast Saccharomyces cerevisiae has ASP1 gene responsible for the production of L-asparaginase 1 (Sc_ASPase1) that has been poor studied. To elucidate the characteristics of Sc_ASPase1, we expressed the protein in E. coli BL21 (DE3) cells and purified it by affinity chromatography. Sc_ASPase1 has a specific activity of 195.4 U/mg for L-asparagine and 0.36 U/mg for L-glutamine, and an allosteric behavior with a K0.5 of 75 µM for L-asparagine. Through site directed mutation, we demonstrated the importance of Thr64-Thy78-Th141-Lys215 residues for catalysis. The mutant protein isoforms A331D, K335E, Y243S, S301N and ΔG77 showed no improvement in kinetic parameters or specific activity. We build and cloned Sc_ASPase1 with the deletion of the first 52 amino acids, but under the conditions tested the protein was expressed in insoluble form. Sc_ASPase1 have demonstrated potential antineoplastic activityc, since 10 U/mL of enzyme lead to 85% of mortality in leukemia cell line MOLT-4. At the same concentration, the E. coli enzyme kills 95% of the cells of the same line.

PSD-95/Dlg/ZO-1 (PDZ) domain - PDZ binding motif (PBM) interactions have been one of the most well studied protein-protein interaction systems through biochemical, biophysical and high-throughput screening (HTS) strategies. This has allowed us to understand the mechanism of individual PDZ-PBM interactions and the re-engineering of PBMs to bind tighter or to gain or lose certain specificity. However, there are several thousand native PDZ domains whose biological ligands remain unknown. Because of the low sequence identity among PDZ domain homologues, promiscuous binding profiles (defined as a PDZ domain that can accommodate a set of PBMs or a PBM that can be recognized by many PDZ domains), and context-dependent interaction mechanism, we have an inadequate understanding of the general molecular mechanisms that determine the PDZ-PBM specificity. Therefore, predicting PDZ specificity has been elusive. In addition, no de novo PBM ligand or artificial non-native PDZ domain have been successfully designed. This reflects the general challenges in understanding the general principles of PDZ-ligand interactions, namely that they are context-dependent, exhibit weak binding affinity, narrow binding energy range, and larger interaction surface than other protein-ligand interactions. Together, PDZ domains make good model systems to investigate the fundamental principles of protein-protein interactions with a wide spectrum of biomedical implications. My studies suggest that understanding PBM specificity with the set of structural positions forming the binding pocket can connect sequence, structure and function of a PDZ domain in a general context. They also suggest that this way of understanding the specificity will shed light on prediction and engineering of specificity rationally. Structural analysis on most of the available PDZ domain structures was established to support the principle (Chapter I). The principle was tested against two different types of PBM; C-terminal PBM (Chapter II) and internal PBM (Chapter III), and shown to support better understanding and design of PDZ domain specificity. We further applied the principle to design de novo PDZ domains, and the preliminary data hints that it is optimistic to engineer PDZ domain specificity (Appendix A and B).

Proteins are large macromolecules that play important roles in nature. With the development of modern molecular biology techniques, protein engineering has emerged as a useful tool and found many applications in areas ranging from food industry, environmental protection, to medical and life science. Biomimetic membrane incorporates biological elements, such as proteins, to form membranes that mimic the high specificity and conductance of natural biological membranes. For any application involving the usage of proteins, the first barrier is always the production of proteins with sufficient stability, and the incorporation of proteins into the artificial matrix. This thesis contains two major parts, the first part is focused on the development and testing of method to immobilize active enzymes. The second part is devoted to study the degradation of membrane proteins in E. coli cells. In the immobilization study, Pyrophosphatase (PpaC) was chose as a model enzyme. A dual functional tag consist of histidine and methionine has been developed, in which histidine is used for purification while methionine is metabolically replaced with azidohomoalanine (AHA) for immobilization. We found that the addition of the tag and the incorporation of AHA did not significantly impair the properties of proteins, and the histidine–AHA tag can facilitate protein purification, immobilization, and labeling. This tag is expected to be useful in general for many proteins. Degradation of soluble protein has been well characterized, but the membrane protein degradation process remains elusive. SsrA tag is a well-known recognition sequence for soluble protein degradation, which marks prematurely terminated protein products translated from damaged mRNA. SsrA tagged membrane proteins was found to be substrate of a cytosolic protease complex ClpXP, which mediated complete degradation.

Naturally occurring enzymatic pathways enable highly specific, rapid thiophenic sulfur cleavage occurring at ambient temperature and pressure, which may be harnessed for the desulfurization of petroleum-based fuel. One pathway found in bacteria is a four-step catabolic pathway (the 4S pathway) converting dibenzothiophene (DBT), a common crude oil contaminant, into 2-hydroxybiphenyl (HBP) without disrupting the carbon-carbon bonds. 2’-Hydroxybiphenyl-2-sulfinate desulfinase (DszB), the rate-limiting enzyme in the enzyme cascade, is capable of selectively cleaving carbon-sulfur bonds. Accordingly, understanding the molecular mechanisms of DszB activity may enable development of the cascade as industrial biotechnology. Based on crystallographic evidence, we hypothesized that DszB undergoes an active site conformational change associated with the catalytic mechanism. Moreover, we anticipated this conformational change is responsible, in part, for enhancing product inhibition. Rhodococcus erythropolis IGTS8 DszB was recombinantly produced in Escherichia coli BL21 and purified to test these hypotheses. Activity and the resulting conformational change of DszB in the presence of HBP were evaluated. The activity of recombinant DszB was comparable to the natively expressed enzyme and was competitively inhibited by the product, HBP. Using circular dichroism, global changes in DszB conformation were monitored in response to HBP concentration, which indicated that both product and substrate produced similar structural changes. Molecular dynamics (MD) simulations and free energy perturbation with Hamiltonian replica exchange molecular dynamics (FEP/λ-REMD) calculations were used to investigate the molecular-level phenomena underlying the connection between conformation change and kinetic inhibition. In addition to the HBP, MD simulations of DszB bound to common, yet structurally diverse, crude oil contaminates 2’2-biphenol (BIPH), 1,8-naphthosultam (NTAM), 2-biphenyl carboxylic acid (BCA), and 1,8-naphthosultone (NAPO) were performed. Analysis of the simulation trajectories, including root mean square fluctuation (RMSF), center of mass (COM) distances, and strength of nonbonded interactions, when compared with FEP/λ-REMD calculations of ligand binding free energy, showed excellent agreement with experimentally determined inhibition constants. Together, the results show that a combination of a molecule’s hydrophobicity and nonspecific interactions with nearby functional groups contribute to a competitively inhibitive mechanism that locks DszB in a closed conformation and precludes substrate access to the active site. Limitations in DszB’s potential applications in industrial sulfur fixation are not limited to turnover rate. To better characterize DszB stability and to gain insight into ways by which to extend lifetime, as well as to pave the way for future studies in inhibition regulation, we evaluated the basic thermal and kinetic stability of DszB in a variety of solvation environments. Thermal stability of DszB was measured in a wide range of different commercially available buffer additives using differential scanning fluorimetry (DSF) to quickly identify favorable changes in protein melting point. Additionally, a fluorescent kinetic assay was employed to investigate DszB reaction rate over a 48 hr time period in a more focused group of buffer to link thermal stability to DszB life-time. Results indicate a concerningly poor short-term stability of DszB, with an extreme preference for select osmolyte buffer additives that only moderately curbed this effect. This necessitates a means of stability improvement beyond alteration of solvation environment. To this end, a more general investigation of glycosylation and its impact on protein stability was performed. Post-translational modification of proteins occurs in organisms from all kingdoms life, with glycosylation being among the most prevalent of amendments. The types of glycans attached differ greatly by organism but can be generally described as protein-attached carbohydrate chains of variable lengths and degrees of branching. With great diversity in structure, glycosylation serves numerous biological functions, including signaling, recognition, folding, and stability. While it is understood that glycans fulfill a variety of important roles, structural and biochemical characterization of even common motifs and preferred rotamers is incomplete. To better understand glycan structure, particularly their relevance to protein stability, we modeled and computed the solvation free energy of 13 common N- and O-linked glycans in a variety of conformations using thermodynamic integration. N-linked glycans were modeled in the β-1,4-linked conformation, attached to an asparagine analog, while O-linked glycans were each modeled in both the α-1,4 and β-1,4-linked conformations attached to both serine and threonine analogs. Results indicate a strong preference for the β conformation and show a synergistic effect of branching on glycan solubility. Our results serve as a library of solvation free energies for fundamental glycan building blocks to enhance understanding of more complex protein-carbohydrate structures moving forward.

Many histidine-rich sites in proteins bind transition metal ions such as Zn2+, natively. Such sites can encourage proper protein folding or allow access to enzymatic capabilities such as hydrolysis. Ru(II) and Tc(I) also bind to aromatic amines providing access to unique chemistries not observed in biology. Ru(II) complexes have shown efficacy in fighting cancer and catalysis, while 99mTc complexes are used in radio-imaging. To incorporate such metal-ions' activities into proteins, several mutants have been designed to bind Zn2+, Ru2+, and Tc1+ by introducing three histidines onto their surfaces. The first design, Z0, utilized a chimeric approach by substituting a turn in engrailed homeodomain for the superimposable Zn2+-binding loop of astacin. In the second design, 3HT-C, three histidine residues were incorporated into the N-terminus of the Trp-cage. The final scaffold, ubiquitin, was used to make two mutants: 3HIU with a 3-histidine containing loop inserted between residues 9 and 10, and 3HPU with 3-histidine point mutations near residues 35-38. Z0 proved unstable due to incorporation of a hydrophobic patch onto its surface and was not able to be isolated in sufficient quantities for study. However, the other proteins were stable and soluble. Zn2+-binding by 3HT-C was investigated by intrinsic tryptophan fluorescence quenching, circular dichroism, and RP-HPLC. Binding by 3HIU and 3HPU was studied by CD. All designed proteins bind to Zn2+ with Kd values in the micromolar range. 3HIU and 3HPU were further studied for their ability to bind Ru(tacn)2+ complexes. While addition of Ru-complexes caused oligomerization to various extents depending upon reaction conditions, homogeneous Ru-protein monomers were purified by a combination of size-exclusion, cation-exchange and immobilized-metal affinity chromatographies. Ru-binding was confirmed by ESI-MS, and structural integrity was investigated by CD. Results indicate that Ru(tacn)2+ complexes can be bound to surface binding sites in proteins without disruption of structure, opening the door for the study of catalysis in a protein context. 99mTc(CO)3+-binding studies were performed with 3HIU by RP-HPLC. This protein binds Tc+ and resists substitution by free L-histidine, suggesting that peptidic Tc-binding tags could be designed with this approach to readily incorporate the radionuclide into any protein expression system.

PHAs are a class of biopolymers consisting of (R)-3-hydroxy-fatty acids and are produced by the majority of eubacteria and some archaeal bacteria as carbon storage material. In general, PHA is synthesised when a carbon source is available in excess while another essential nutrient is limited. The key enzyme of PHA biosynthesis, the PHA synthase, catalyses the polymerisation of the substrate (R)-3-hydroxyacyl-CoA to PHA accompanied by the release of coenzyme A. PHA is stored intracellularly as inclusions, the so-called PHA granules. When the external carbon source becomes exhausted, bacteria can metabolise these carbon inclusions by degradation of the polymer. PHA granules are water-insoluble, spherical inclusions of approximately 50-500 nm in diameter which consist of a hydrophobic polyester core surrounded by a phospholipid layer with embedded and attached proteins. One could consider isolated PHA granules as bio-beads due to their structure and size. In this study we tested if the PHA synthase can be used as an anchor molecule in order to display proteins of interest at the PHA granule surface. Furthermore, these modified PHA granules were analysed for their potential applicability as bio-beads in biotechnological procedures. The concept of using the PHA synthase as granule-anchoring molecule for display of proteins of interest was established by the functional display of the ß- galactosidase at PHA granules. This “proof of concept” was followed by the display of biotechnologically more interesting proteins. The IgG binding domain of protein A as well as streptavidin, which is known for its biotin binding ability, were fused to the PHA synthase, respectively, and therefore localised at the PHA granule surfaces during PHA granule assembly, resulting in functional bio-protein A -beads and bio-streptavidin-beads. Moreover, their applicability in biotechnological assays was demonstrated. Recently, we fused the green fluorescent protein (GFP) to the PHA synthase and demonstrated that the PHA granule assembly does not start randomly distributed in the cytoplasm but occurred localised at or near the cell poles. To further investigate if the localisation of the PHA granule formation process is due to polar positional information inherent to the PHA synthase, different mutated versions of the PHA synthase of Cupriavidus necator were created and analysed for a potential alteration in localisation. Furthermore, the phasin protein PhaP1 of C. necator was fused to HcRed, a far-red fluorescent protein, and localisation studies were accomplished when the fusion protein was expressed under different conditions in Escherichia coli.

Environmental signals are sensed by membrane-spanning receptors that communicate with the cell interior. Bacterial chemoreceptors modulate the activity of the CheA kinase in response to binding of small ligands or upon interaction with substrate-bound periplasmic-binding proteins. The mechanism of signal transduction across the membrane is a displacement of the second transmembrane domain (TM2) a few angstroms toward the cytoplasm. This movement repositions a dynamic transmembrane helix relative to the plane of the cell membrane. The research presented in this dissertation investigated the contribution of TM2-membrane interactions to signaling by the aspartate chemoreceptor (Tar) of Escherichia coli. Aromatic residues that reside at the cytoplasmic polar-hydrophobic membrane interface (Trp-209 and Tyr-210) were found to play a significant role in regulating signaling by Tar. These interactions were subsequently manipulated to modulate the signaling properties of Tar. The baseline signaling state was shown to be incrementally altered by repositioning the Trp-209/Tyr-210 pair. To our knowledge, this is the first example of harnessing membrane-protein interactions to modulate the signal output of a transmembrane receptor in a controlled and predictable manner. Potential long-term applications include the use of analogous mutations to elucidate two-component signaling pathways, to engineer the signaling parameters of biosensors that incorporate chemoreceptors, and to predict the movement of dynamic transmembrane helices in silico.

Enzymes are attractive catalysts in organic synthesis since they are efficient, selective and environmentally friendly. A large number of enzyme-catalyzed transformations have been described in the literature. If no natural enzyme can carry out a desirable reaction, one possibility is to modify an existing enzyme by protein engineering and thereby obtain a catalyst with the desired properties. In this thesis, the development of enantioselective enzymes and their use in synthetic applications is described.  In the first part of this thesis, enantioselective variants of Candida antarctica lipase A (CALA) towards α-substituted p-nitrophenyl esters were developed by directed evolution. A highly selective variant of CALA towards p-nitrophenyl 2-phenylpropanoate was developed by pairwise randomization of amino acid residues close to the active site. The E value of this variant was 276 compared to 3 for the wild type. An approach where nine residues were altered simultaneously was used to discover another highly enantioselective CALA variant (E = 100) towards an ibuprofen ester. The sterical demands of this substrate made it necessary to vary several residues at the same time in order to reach a variant with improved properties. In the second part of the thesis, a designed variant of Candida antarctica lipase B (CALB) was employed in kinetic resolution (KR) and dynamic kinetic resolution (DKR) of secondary alcohols. The designed CALB variant (W104A) accepts larger substrates compared to the wild type, and by the application of CALB W104A, the scope of these resolutions was extended. First, a DKR of phenylalkanols was developed using CALB W104A. An enzymatic resolution was combined with in situ racemization of the substrate, to yield the products in up to 97% ee. Secondly, the KR of diarylmethanols with CALB W104A was developed. By the use of diarylmethanols with two different aryl groups, highly enantioselective transformations were achieved. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows:<strong>  </strong>Paper 5: Submitted.

Engineered affinity proteins have together with naturally derived antibodies becomeindispensable tools in many areas of life-science and with the increasing number ofapplications, the need for high-throughput methods for generation of such different affinityproteins is evident. Today, combinatorial protein engineering is the most successful strategy toisolate novel non-immunoglobulin affinity proteins. In this approach, generally termed directedevolution, high-complexity combinatorial libraries are created from which affinity proteins areisolated using an appropriate selection method, thus circumventing the need for detailedknowledge of the protein structure or the binding mechanism, often necessary in more rationalapproaches. Since the introduction of the phage display technology that pioneered the field ofcombinatorial engineering, several alternative selection systems have been developed for thispurpose.This thesis describes the development of a novel selection system based onstaphylococcal surface display and its implementation in directed evolution approaches. In thefirst study, the transformation efficiency to the gram-positive bacteria Staphylococcus carnosus wassuccessfully improved around 10,000-fold to a level that would allow cell surface display ofcomplex combinatorial protein libraries. In two separate studies, the staphylococcal displaysystem was investigated for the applicability in both de novo selection and affinity maturation ofaffibody molecules. First, using a pre-selection strategy with one round of phage display, ahigh-complexity affibody library was displayed on staphylococcal cells. Using fluorescenceactivatedcell sorting, binders with sub-nanomolar affinity to tumor necrosis factor-alpha(TNF-α) were isolated. Second, a combined approach using phage display for de novo selectionof first-generation affibody binders and staphylococcal display in a subsequent affinitymaturation selection was applied to generate binders with low nanomolar affinity to the humanepidermal growth factor receptor-3 (ErbB3). Moreover, in an additional study, thestaphylococcal surface display system was improved by the introduction of a protease 3Ccleavage sequence in the displayed fusion products in order to facilitate straightforwardproduction of soluble proteins for further downstream characterization.Altogether, the presented studies demonstrate that the staphylococcal selection systemindeed is a powerful tool for selection and characterization of novel affinity proteins and couldbecome an attractive alternative to existing selection techniques. / <p>QC 20100726</p>