Recombinant expression and partial characterization of an active soluble histo-aspartic protease from Plasmodium falciparum

Molecular modification and biotechnological applications of microbial aspartic proteases

The growing preference for incorporating microbial aspartic proteases in industries is due to their high catalytic function and high degree of substrate selectivity. These properties, however, are attributable to molecular alterations in their structure and a variety of other characteristics. Molecular tools, functional genomics, and genome editing technologies coupled with other biotechnological approaches have aided in improving the potential of industrially important microbial proteases by addressing some of their major limitations, such as: low catalytic efficiency, low conversion rates, low thermostability, and less enzyme yield. However, the native folding within their full domain is dependent on a surrounding structure which challenges their functionality in substrate conversion, mainly due to their mutual interactions in the context of complex systems. Hence, manipulating their structure and controlling their expression systems could potentially produce enzymes with high selectivity and catalytic functions. The proteins produced by microbial aspartic proteases are industrially capable and far-reaching in regulating certain harmful distinctive industrial processes and the benefits of being eco-friendly. This review provides: an update on current trends and gaps in microbial protease biotechnology, exploring the relevant recombinant strategies and molecular technologies widely used in expression platforms for engineering microbial aspartic proteases, as well as their potential industrial and biotechnological applications.

Acid phosphatase-like proteins, a biogenic amine and leukotriene-binding salivary protein family from the flea Xenopsylla cheopis

The salivary glands of hematophagous arthropods contain pharmacologically active molecules that interfere with host hemostasis and immune responses, favoring blood acquisition and pathogen transmission. Exploration of the salivary gland composition of the rat flea, Xenopsylla cheopis, revealed several abundant acid phosphatase-like proteins whose sequences lacked one or two of their presumed catalytic residues. In this study, we undertook a comprehensive characterization of the tree most abundant X. cheopis salivary acid phosphatase-like proteins. Our findings indicate that the three recombinant proteins lacked the anticipated catalytic activity and instead, displayed the ability to bind different biogenic amines and leukotrienes with high affinity. Moreover, X-ray crystallography data from the XcAP-1 complexed with serotonin revealed insights into their binding mechanisms.

Methods Used to Investigate the Plasmodium falciparum Digestive Vacuole

Plasmodium falciparum malaria remains a global health problem as parasites continue to develop resistance to all antimalarials in use. Infection causes clinical symptoms during the intra-erythrocytic stage of the lifecycle where the parasite infects and replicates within red blood cells (RBC). During this stage, P. falciparum digests the main constituent of the RBC, hemoglobin, in a specialized acidic compartment termed the digestive vacuole (DV), a process essential for survival. Many therapeutics in use target one or multiple aspects of the DV, with chloroquine and its derivatives, as well as artemisinin, having mechanisms of action within this organelle. In order to better understand how current therapeutics and those under development target DV processes, techniques used to investigate the DV are paramount. This review outlines the involvement of the DV in therapeutics currently in use and focuses on the range of techniques that are currently utilized to study this organelle including microscopy, biochemical analysis, genetic approaches and metabolomic studies. Importantly, continued development and application of these techniques will aid in our understanding of the DV and in the development of new therapeutics or therapeutic partners for the future.

Malaria parasite plasmepsins: More than just plain old degradative pepsins

Plasmepsins are a group of diverse aspartic proteases in the malaria parasite Plasmodium Their functions are strikingly multifaceted, ranging from hemoglobin degradation to secretory organelle protein processing for egress, invasion, and effector export. Some, particularly the digestive vacuole plasmepsins, have been extensively characterized, whereas others, such as the transmission-stage plasmepsins, are minimally understood. Some (e.g. plasmepsin V) have exquisite cleavage sequence specificity; others are fairly promiscuous. Some have canonical pepsin-like aspartic protease features, whereas others have unusual attributes, including the nepenthesin loop of plasmepsin V and a histidine in place of a catalytic aspartate in plasmepsin III. We have learned much about the functioning of these enzymes, but more remains to be discovered about their cellular roles and even their mechanisms of action. Their importance in many key aspects of parasite biology makes them intriguing targets for antimalarial chemotherapy. Further consideration of their characteristics suggests that some are more viable drug targets than others. Indeed, inhibitors of invasion and egress offer hope for a desperately needed new drug to combat this nefarious organism.

Activation mechanism of plasmepsins, pepsin-like aspartic proteases from Plasmodium, follows a unique trans-activation pathway

Plasmodium parasites that cause malaria produce plasmepsins (PMs), pepsin-like aspartic proteases that are important antimalarial drug targets due to their role in host hemoglobin degradation. The enzymes are synthesized as inactive zymogens (pro-PMs), and the mechanism of their conversion to the active, mature forms has not been clearly elucidated. Our structural investigations of vacuolar pro-PMs with truncated prosegment (pro-tPMs) reveal that the formation of the S-shaped dimer is their innate property. Further structural studies, biochemical analysis, and molecular dynamics simulations indicate that disruption of the Tyr-Asp loop (121p-4), coordinated with the movement of the loop L1 (237-247) and helix H2 (101p-113p), is responsible for the extension of the pro-mature region (harboring the cleavage site). Consequently, under acidic pH conditions, these structural changes result in the dissociation of the dimers to monomers and the protonation of the residues in the prosegment prompts its unfolding. Subsequently, we demonstrated that the active site of the monomeric pro-tPMs with the unfolded prosegment is accessible for peptide substrate binding; in contrast, the active site is blocked in folded prosegment form of pro-tPMs. Thus, we propose a novel mechanism of auto-activation of vacuolar pro-tPMs that under acidic conditions can form a catalytically competent active site. One monomer cleaves the prosegment of the other one through a trans-activation process, resulting in formation of mature enzyme. As a result, once a mature enzyme is generated, it leads to the complete conversion of all the inactive pro-tPMs to their mature form. DATABASE: Atomic coordinates and structure factors have been submitted in the Protein Data Bank (PDB) under the PDB IDs 6KUB, 6KUC, and 6KUD.

Novel pseudo-aspartic peptidase from the midgut of the tick Rhipicephalus microplus

The characterization of Rhipicephalus microplus tick physiology can support efforts to develop and improve the efficiency of control methods. A sequence containing a domain with similarity to one derived from the aspartic peptidase family was isolated from the midgut of engorged female R. microplus. The lack of the second catalytic aspartic acid residue suggest that it may be a pseudo-aspartic peptidase, and it was named RmPAP. In this work we confirm the lack of proteolytic activity of RmPAP and investigate it’s non-proteolytic interaction with bovine hemoglobin by Surface Plasmon Resonance and phage display. Moreover we carried out RNAi interference and artificial feeding of ticks with anti-RmPAP antibodies to assess it’s possible biological role, although no changes were observed in the biological parameters evaluated. Overall, we hypothesize that RmPAP may act as a carrier of hemoglobin/heme between the tick midgut and the ovaries.

Biochemical characterization of plasmepsin V from Plasmodium vivax Thailand isolates: Substrate specificity and enzyme inhibition

Plasmepsin V (PMV) is a Plasmodium aspartic protease responsible for the cleavage of the Plasmodium export element (PEXEL) motif, which is an essential step for export of PEXEL containing proteins and crucial for parasite viability. Here we describe the genetic polymorphism of Plasmodium vivax PMV (PvPMV) Thailand isolates, followed by cloning, expression, purification and characterization of PvPMV-Thai, presenting the pro- and mature-form of PvPMV-Thai. With our refolding and purification method, approximately 1mg of PvPMV-Thai was obtained from 1g of washed inclusion bodies. Unlike PvPMV-Ind and PvPMV-Sal-1, PvPMV-Thai contains a four-amino acid insertion (SVSE) at residues 246-249. We have confirmed that this insertion did not interfere with the catalytic activity as it is located in the long loop (R241-E272) pointing away from the substrate-binding pocket. PvPMV-Thai exhibited similar activity to PfPMV counterparts in which PfEMP2 could be hydrolyzed more efficiently than HRPII. Substrate specificity studies at P1' showed that replacing Ser by Val or Glu of the PfEMP2 peptide markedly reduced the enzyme activity of PvPMV similar to that of PfPMV whereas replacing His by Val or Ser of the HRPII peptide increased the cleavage activity. However, the substitution of amino acids at the P2 position with Glu dramatically reduced the cleavage efficiency by 80% in PvPMV in contrast to 30% in PfPMV, indicating subtle differences around the S2 binding pocket of both PfPMV and PvPMV. Four inhibitors were also evaluated for PvPMV-Thai activity including PMSF, pepstatin A, nelfinavir, and menisporopsin A-a macrocyclic polylactone. We are the first to show that menisporopsin A partially inhibits the PvPMV-Thai activity at high concentration. Taken together, these findings provide insights into recombinant production, substrate specificity and inhibition of PvPMV-Thai.

The zymogen of plasmepsin V from Plasmodium falciparum is enzymatically active

Plasmepsin V, a membrane-bound aspartic protease present in Plasmodium falciparum, is involved in the export of malaria parasite effector proteins into host erythrocytes and therefore is a potential target for antimalarial drug development. The present study reports the bacterial recombinant expression and initial characterization of zymogenic and mature plasmepsin V. A 484-residue truncated form of proplasmepsin (Glu37-Asn521) was fused to a fragment of thioredoxin and expressed as inclusion bodies. Refolding conditions were optimized and zymogen was processed into a mature form via cleavage at the Asn80-Ala81 peptide bond. Mature plasmepsin V exhibited a pH optimum of 5.5-7.0 with Km and kcat of 4.6 μM and 0.24s(-1), respectively, at pH 6.0 using the substrate DABCYL-LNKRLLHETQ-E(EDANS). Furthermore, the prosegment of proplasmepsin V was shown to be nonessential for refolding and inhibition. Unexpectedly, unprocessed proplasmepsin V was enzymatically active with slightly reduced substrate affinity (∼ 2-fold), and similar pH optimum as well as turnover compared to the mature form. Both zymogenic and mature plasmepsin V were partially inhibited by pepstatin A as well as several KNI aspartic protease inhibitors while certain metals strongly inhibited activity. Overall, the present study provides the first report on the nonessentiality of the prosegment for plasmepsin V folding and activity, and therefore, subsequent characterization of its structure-function relationships of both zymogen and mature forms in the development of novel inhibitors with potential antimalarial activities is warranted.

Disulfide linkages in Plasmodium falciparum plasmepsin-i are essential elements for its processing activity and multi-milligram recombinant production yield

Plasmodium falciparum plasmepsin-I (PM-I) has been considered a potential drug target for the parasite that causes fatal malaria in human. Determination of PM-I structures for rational design of its inhibitors is hindered by the difficulty in obtaining large quantity of soluble enzyme. Nearly all attempts for its heterologous expression in Escherichia coli result in the production of insoluble proteins in both semi-pro-PM-I and its truncated form, and thus require protein refolding. Moreover, the yields of purified, soluble PM-I from all reported studies are very limited. Exclusion of truncated semi-pro-PM-I expression in E. coli C41(DE3) is herein reported. We also show that the low preparation yield of purified semi-pro-PM-I with autoprocessing ability is mainly a result of structural instability of the refolded enzyme in acidic conditions due to incomplete formation of disulfide linkages. Upon formation of at least one of the two natural disulfide bonds, nearly all of the refolded semi-pro-PM-I could be activated to its mature form. A significantly improved yield of 10 mg of semi-pro-PM-I per liter of culture, which resulted in 6-8 mg of the mature PM-I, was routinely obtained using this strategy.

Structural studies of vacuolar plasmepsins

Plasmepsins (PMs) are pepsin-like aspartic proteases present in different species of parasite Plasmodium. Four Plasmodium spp. (P. vivax, P. ovale, P. malariae, and the most lethal P. falciparum) are mainly responsible for causing human malaria that affects millions worldwide. Due to the complexity and rate of parasite mutation coupled with regional variations, and the emergence of P. falciparum strains which are resistant to antimalarial agents such as chloroquine and sulfadoxine/pyrimethamine, there is constant pressure to find new and lasting chemotherapeutic drug therapies. Since many proteases represent therapeutic targets and PMs have been shown to play an important role in the survival of parasite, these enzymes have recently been identified as promising targets for the development of novel antimalarial drugs. The genome of P. falciparum encodes 10 PMs (PMI, PMII, PMIV-X and histo-aspartic protease (HAP)), 4 of which (PMI, PMII, PMIV and HAP) reside within the food vacuole, are directly involved in degradation of human hemoglobin, and share 50-79% amino acid sequence identity. This review focuses on structural studies of only these four enzymes, including their orthologs in other Plasmodium spp.. Almost all original crystallographic studies were performed with PMII, but more recent work on PMIV, PMI, and HAP resulted in a more complete picture of the structure-function relationship of vacuolar PMs. Many structures of inhibitor complexes of vacuolar plasmepsins, as well as their zymogens, have been reported in the last 15 years. Information gained by such studies will be helpful for the development of better inhibitors that could become a new class of potent antimalarial drugs. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Structural insights into the activation and inhibition of histo-aspartic protease from Plasmodium falciparum

Histo-aspartic protease (HAP) from Plasmodium falciparum is a promising target for the development of novel antimalarial drugs. The sequence of HAP is highly similar to those of pepsin-like aspartic proteases, but one of the two catalytic aspartates, Asp32, is replaced with histidine. Crystal structures of the truncated zymogen of HAP and of the complex of the mature enzyme with inhibitor KNI-10395 have been determined at 2.1 and 2.5 Å resolution, respectively. As in other proplasmepsins, the propeptide of the zymogen interacts with the C-terminal domain of the enzyme, forcing the N- and C-terminal domains apart, thereby separating His32 and Asp215 and preventing formation of the mature active site. In the inhibitor complex, the enzyme forms a tight domain-swapped dimer, not previously seen in any aspartic proteases. The inhibitor is found in an unprecedented conformation resembling the letter U, stabilized by two intramolecular hydrogen bonds. Surprisingly, the location and conformation of the inhibitor are similar to those of the fragment of helix 2 comprising residues 34p-38p in the prosegments of the zymogens of gastric aspartic proteases; a corresponding helix assumes a vastly different orientation in proplasmepsins. Each inhibitor molecule is in contact with two molecules of HAP, interacting with the carboxylate group of the catalytic Asp215 of one HAP protomer through a water molecule, while also making a direct hydrogen bond to Glu278A' of the other protomer. A comparison of the shifts in the positions of the catalytic residues in the inhibitor complex presented here with those published previously gives further hints regarding the enzymatic mechanism of HAP.

The native conformation of plasmepsin II is kinetically trapped at neutral pH

Plasmepsin II (PMII), an aspartic protease from the malarial parasite Plasmodium falciparum, represents a model for understanding protease structure/function relationships due to its unique structure and properties. The present study undertook a thermodynamic and kinetic analysis of the PMII folding mechanism and a pH stability profile. Differential scanning calorimetry revealed that the native state of PMII (Np) was irreversibly unfolded, and in the pH range of 6.5-8.0, PMII refolds to a denatured state (Rp) with higher thermal stability than Np. Rp could also be formed upon partially unfolding PMII at pH 11.0 and 37 °C for 2h, followed by adjustment to a pH in the range of 6.5-8.0. While Rp could be folded/unfolded reversibly, Np was shown to exist as a kinetically trapped state. By examining the unfolding kinetics of Np and the kinetics of Rp folding to Np at 25 °C, it was found that Np is kinetically trapped by an unfolding barrier of 25.5 kcal/mol, and yet once unfolded, is prevented from folding by a comparable folding barrier. The folding mechanism of PMII is similar to that reported for pepsin. It is hypothesized that the PMII zymogen also utilizes a prosegment-catalyzed folding mechanism.

Crystal structures of the free and inhibited forms of plasmepsin I (PMI) from Plasmodium falciparum

Plasmepsin I (PMI) is one of the four vacuolar pepsin-like proteases responsible for hemoglobin degradation by the malarial parasite Plasmodium falciparum, and the only one with no crystal structure reported to date. Due to substantial functional redundancy of these enzymes, lack of inhibition of even a single plasmepsin can defeat efforts in creating effective antiparasitic agents. We have now solved crystal structures of the recombinant PMI as apoenzyme and in complex with the potent peptidic inhibitor, KNI-10006, at the resolution of 2.4 and 3.1Å, respectively. The apoenzyme crystallized in the orthorhombic space group P2(1)2(1)2(1) with two molecules in the asymmetric unit and the structure has been refined to the final R-factor of 20.7%. The KNI-10006 bound enzyme crystallized in the tetragonal space group P4(3) with four molecules in the asymmetric unit and the structure has been refined to the final R-factor of 21.1%. In the PMI-KNI-10006 complex, the inhibitors were bound identically to all four enzyme molecules, with the opposite directionality of the main chain of KNI-10006 relative to the direction of the enzyme substrates. Such a mode of binding of inhibitors containing an allophenylnorstatine-dimethylthioproline insert in the P1-P1' positions, previously reported in a complex with PMIV, demonstrates the importance of satisfying the requirements for the proper positioning of the functional groups in the mechanism-based inhibitors towards the catalytic machinery of aspartic proteases, as opposed to binding driven solely by the specificity of the individual enzymes. A comparison of the structure of the PMI-KNI-10006 complex with the structures of other vacuolar plasmepsins identified the important differences between them and may help in the design of specific inhibitors targeting the individual enzymes.

Rational redesign of porcine pepsinogen containing an antimicrobial peptide

A novel strategy for the controlled release and localization of bioactive peptides within digestive and immunity-related enzymes was developed. The N-terminus of porcine pepsinogen A was fused to the basic amino acid-rich region of bovine lactoferricin B termed 'tLfcB', a cationic antimicrobial/anticancer peptide. Recombinant tLfcB-porcine pepsinogen A was expressed in soluble form in Escherichia coli as a thioredoxin (Trx) fusion protein. Thioredoxin-tLfcB-porcine pepsinogen A was found to activate autocatalytically under acidic conditions. Recombinant pepsin A derived from the activation of the fusion protein had a catalytic rate and substrate affinity similar to that derived from the recombinant thioredoxin-porcine pepsinogen A control. Pepsin-treated thioredoxin-tLfcB-porcine pepsinogen A yielded increased antimicrobial activity against the Gram-negative bacteria E.coli relative to control suggesting that a second function (antimicrobial activity) was successfully engineered into a functional peptidase. The novel design strategy described herein presents a potential strategy for targeted delivery of antimicrobial or therapeutic peptides in transgenic organisms via re-engineering native proteins critical to plant and animal defense mechanisms.

Characterization of the monomer-dimer equilibrium of recombinant histo-aspartic protease from Plasmodium falciparum

Histo-aspartic protease (HAP) from Plasmodium falciparum is an intriguing aspartic protease due to its unique structure. Our previous study reported the first recombinant expression of soluble HAP, in its truncated form (lys77p-Leu328) (p denotes prosegment), as a thioredoxin (Trx) fusion protein Trx-tHAP. The present study found that the recombinant Trx-tHAP fusion protein aggregated during purification which could be prevented through the addition of 0.2% CHAPS. Trx-tHAP fusion protein was processed into a mature form of tHAP (mtHAP) by both autoactivation, and activation with either enterokinase or plasmepsin II. Using gel filtration chromatography as well as sedimentation velocity and equilibrium ultracentrifugation, it was shown that the recombinant mtHAP exists in a dynamic monomer-dimer equilibrium with an increasing dissociation constant in the presence of CHAPS. Enzymatic activity data indicated that HAP was most active as a monomer. The dominant monomeric form showed a K(m) of 2.0 microM and a turnover number, k(cat), of 0.036s(-1) using the internally quenched fluorescent synthetic peptide substrate EDANS-CO-CH(2)-CH(2)-CO-Ala-Leu-Glu-Arg-Met-Phe-Leu-Ser-Phe-Pro-Dap-(DABCYL)-OH (2837b) at pH 5.2.

Multifunctional aspartic peptidase prosegments

The structure-function relationships of aspartic peptidases (APs) (EC 3.4.23.X) have been extensively investigated, yet much remains to be elucidated regarding the various molecular mechanisms of these enzymes. Over the past years, APs have received considerable interest for food applications (e.g. cheese, fermented foods) and as potential targets for pharmaceutical intervention in human diseases including hypertension, cancer, Alzheimer's disease, AIDS (acquired immune deficiency syndrome), and malaria. A deeper understanding of the structure and function of APs, therefore, will have a direct impact on the design of peptidase inhibitors developed to treat such diseases. Most APs are synthesized as zymogens which contain an N-terminal prosegment (PS) domain that is removed at acidic pH by proteolytic cleavage resulting in the active enzyme. While the nature of the AP PS function is not entirely understood, the PS can be important in processes such as the initiation of correct folding, protein stability, blockage of the active site, pH-dependence of activation, and intracellular sorting of the zymogen. This review summarizes the current knowledge of AP PS function (especially within the A1 family), with particular emphasis on protein folding, cellular sorting, and inhibition.

Crystal structures of the histo-aspartic protease (HAP) from Plasmodium falciparum

The structures of recombinant histo-aspartic protease (HAP) from malaria-causing parasite Plasmodium falciparum as apoenzyme and in complex with two inhibitors, pepstatin A and KNI-10006, were solved at 2.5-, 3.3-, and 3.05-A resolutions, respectively. In the apoenzyme crystals, HAP forms a tight dimer not seen previously in any aspartic protease. The interactions between the monomers affect the conformation of two flexible loops, the functionally important "flap" (residues 70-83) and its structural equivalent in the C-terminal domain (residues 238-245), as well as the orientation of helix 225-235. The flap is found in an open conformation in the apoenzyme. Unexpectedly, the active site of the apoenzyme contains a zinc ion tightly bound to His32 and Asp215 from one monomer and to Glu278A from the other monomer, with the coordination of Zn resembling that seen in metalloproteases. The flap is closed in the structure of the pepstatin A complex, whereas it is open in the complex with KNI-10006. Although the binding mode of pepstatin A is significantly different from that in other pepsin-like aspartic proteases, its location in the active site makes unlikely the previously proposed hypothesis that HAP is a serine protease. The binding mode of KNI-10006 is unusual compared with the binding of other inhibitors from the KNI series to aspartic proteases. The novel features of the HAP active site could facilitate design of specific inhibitors used in the development of antimalarial drugs.

Expression and characterization of the recombinant aspartic proteinase A1 from Arabidopsis thaliana

The present study reports the recombinant expression, purification, and partial characterization of a typical aspartic proteinase from Arabidopsis thaliana (AtAP A1). The cDNA encoding the precursor of AtAP A1 was expressed as a functional protein using the yeast Pichia pastoris. The mature form of the rAtAP A1 was found to be a heterodimeric glycosylated protein with a molecular mass of 47kDa consisting of heavy and light chain components, approx. 32 and 16kDa, respectively, linked by disulfide bonds. Glycosylation occurred via the plant specific insert in the light chain. The catalytic properties of the rAtAP A1 were similar to other plant aspartic proteinases with activity in acid pH range, maximal activity at pH 4.0, K(m) of 44 microM, and k(cat) of 55 s(-1) using a synthetic substrate. The enzyme was inhibited by pepstatin A.

[Modeling of substrate and inhibitory complexes of histidine-aspartic protease]

A three-dimensional structure of histo-aspartic protease (HAP), a pepsin-like enzyme from the causative agent of malaria Plasmodium falciparum, is suggested on the basis of homologous modeling followed by equilibration by the method of molecular dynamics. The presence of a His residue in the catalytic site instead of an Asp residue, which is characteristic of pepsin-like enzymes, and replacement of some other conserved residues in the active site make it possible for the enzyme to function by the covalent mechanism inherent in serine proteases. The detailed structures of HAP complexes with pepstatin, a noncovalent inhibitor of aspartic proteases, and phenylmethylsulfonyl fluoride, a covalent inhibitor of serine proteases, as well as with a pentapeptide substrate are discussed.

The catalytic significance of the proposed active site residues in Plasmodium falciparum histoaspartic protease

Alanine mutations of the proposed catalytically essential residues in histoaspartic protease (HAP) (H34A, S37A and D214A) were generated to investigate whether: (a) HAP is a serine protease with a catalytic triad of His34, Ser37 and Asp214 [Andreeva N, Bogdanovich P, Kashparov I, Popov M & Stengach M (2004) Proteins55, 705-710]; or (b) HAP is a novel protease with Asp214 acting as both the acid and the base during substrate catalysis with His34 providing critical stabilization [Bjelic S & Aqvist J (2004) Biochemistry43, 14521-14528]. Our results indicated that recombinant wild-type HAP, S37A and H34A were capable of autoactivation, whereas D214A was not. The inability of D214A to autoactivate highlighted the importance of Asp214 for catalysis. H34A and S37A mutants hydrolyzed synthetic substrate indicating that neither His34 nor Ser37 was essential for substrate catalysis. Both mutants did, however, have reduced catalytic efficiency (P < or = 0.05) compared with wild-type HAP, which was attributed to the stabilizing role of His34 and Ser37 during catalysis. The mature forms of wild-type HAP, H34A and S37A all exhibited high activity over a broad pH range of 5.0-8.5 with maximum activity occurring between pH 7.5 and 8.0. Inhibition studies indicated that wild-type HAP, H34A and S37A were strongly inhibited by the serine protease inhibitor phenylmethanesulfonyl fluoride, but only weakly inhibited by pepstatin A. The data, in concert with molecular modeling, suggest a novel mode of catalysis with a single aspartic acid residue performing both the acid and base roles.

Purification and refolding optimization of recombinant bovine enterokinase light chain overexpressed in Escherichia coli

The nucleotide sequence encoding bovine enterokinase light chain (EK) from Chinese northern yellow bovine was isolated. Two single-nucleotide mutations, namely, C245G and A528T were identified. The gene encoding the Pro82Arg/Glu176Asp variant of known bovine EK was fused with glutathione S-transferase and overexpressed mainly as an inclusion body in Escherichia coli BL21 (DE3), upon induction with IPTG and glucose. Effective fusion protein purification, refolding, auto-catalytic cleavage and mature EK recovery were described. The specific activity of the purified EK was determined as 110+/- 10 U/mg, which was comparable to a specific activity of > or =20 U/mg of the E. coli expressed EK sample provided by Sigma (Cat. No. E4906). This procedure produced approximately 53 mg of EK per 500 mL of cell culture, which was much higher than previous reports, thus providing a basis for large-scale production of EK and for further applications in biotechnology.

Hemoglobin-degrading plasmepsin II is active as a monomer

A family of aspartic proteases called plasmepsins is important for hemoglobin degradation in intraerythrocytic Plasmodium parasites. Plasmepsin II (PM II) is the best studied member of this family. PM II and its close orthologs and paralogs form homodimers with extensive interfaces in all known crystal structures. This raised the question whether the homodimer is the functional subunit of plasmepsins in solution. We have used gel filtration chromatography, site-directed mutagenesis, and analytical ultracentrifugation to study the oligomeric status of PM II in solution. Our results reveal that PM II exists mainly as a monomer in solution and that the monomer is fully functional for catalysis. A hydrophobic loop at the PM II monomer surface, which would be buried in a PM II dimer, is shown to be essential for the hemoglobin degradation capability of PM II.

Expression and purification of a trivalent pertussis toxin-diphtheria toxin-tetanus toxin fusion protein in Escherichia coli

Pertussis toxoid, diphtheria toxoid, and tetanus toxoid are key components of diphtheria-tetanus-acellular pertussis vaccines. The efficacy of the vaccines is well documented, however, the vaccines are expensive partly because the antigens are derived from three different bacteria. In this study, a fusion protein (PDT) composed of the immunoprotective S1 fragment of pertussis toxin, the full-length non-toxic diphtheria toxin, and fragment C of tetanus toxin was constructed via genetic means. The correct fusion was verified by restriction endonuclease analysis and Western immunoblotting. Escherichia coli carrying the recombinant plasmid (pCoPDT) produced a 161kDa protein that was recognized by antibodies specific to the three toxins. The expression of the PDT protein was inducible by isopropyl-beta-d-thio-galactoside but the total amount of protein produced was relatively low. Attempts to improve the protein yield by expression in an E. coli strain (Rosetta-gami 2) that could alleviate rare-codon usage bias and by supplementation of the growth media with amino acids deemed to be a limiting factor in translation were not successful. The PDT protein remained in the insoluble fraction when the recombinant E. coli was grown at 37 degrees C but the protein became soluble when the bacteria were grown at 22 degrees C. The PDT protein was isolated via affinity chromatography on a NiCAM column. The protein was associated with five other proteins via disulfide bonds and non-covalent interactions. Following treatment with beta-mercaptoethanol, the PDT fusion was purified to homogeneity by preparative polyacrylamide gel electrophoresis with a yield of 45 microg/L of culture. Antisera generated against the purified PDT protein recognized the native toxins indicating that some, if not all, of the native epitopes were conserved.