Functional characterization of Asp-317 mutant of human renin expressed in COS cells

Discovery of Borosin Catalytic Strategies and Function through Bioinformatic Profiling

Borosins are ribosomally synthesized and post-translationally modified peptides (RiPPs) containing backbone α-N-methylations. These modifications confer favorable pharmacokinetic properties including increased membrane permeability and resistance to proteolytic degradation. Previous studies have biochemically and bioinformatically explored several borosins, revealing (1) numerous domain architectures and (2) diverse core regions lacking conserved sequence elements. Due to these characteristics, large-scale computational identification of borosin biosynthetic genes remains challenging and often requires additional, time-intensive manual inspection. This work builds upon previous findings and updates the genome-mining tool RODEO to automatically evaluate borosin biosynthetic gene clusters (BGCs) and identify putative precursor peptides. Using the new RODEO module, we provide an updated analysis of borosin BGCs identified in the NCBI database. From our data set, we bioinformatically predict and experimentally characterize a new fused borosin domain architecture, in which the modified natural product core is encoded N-terminal to the methyltransferase domain. Additionally, we demonstrate that a borosin precursor peptide is a native substrate of shewasin A, a reported aspartyl peptidase with no previously identified substrates. Shewasin A requires post-translational modification of the leader peptide for proteolytic maturation, a feature not previously observed in RiPPs. Overall, this work provides a user-friendly and open-access tool for the analysis of borosin BGCs and we demonstrate its utility to uncover additional biosynthetic strategies within the borosin class of RiPPs.

Computationally guided exploration of borosin biosynthetic strategies

Borosins are ribosomally synthesized and post-translationally modified peptides containing backbone α- N -methylations. Identification of borosin precursor peptides is difficult because (1) there are no conserved sequence elements among borosin precursor peptides and (2) the biosynthetic gene clusters contain numerous domain architectures and peptide fusions. To tackle this problem, we updated the genome mining tool RODEO to automatically evaluate putative borosin BGCs and identify precursor peptides. Enabled by the new borosin module, we analyzed all borosin BGCs found in available sequence data and assigned precursor peptides to previously orphan borosin methyltransferases. Additionally, we bioinformatically predict and experimentally characterize a new fused borosin domain architecture, in which the modified core is N-terminal to the methyltransferase domain. Finally, we demonstrate that a borosin precursor peptide is the native substrate of shewasin A, a previously characterized pepsin-like aspartic peptidase whose native biological function was unknown.

Comparative bioinformatic and structural analyses of pepsin and renin

Pepsin, the archetypal pepsin-like aspartic protease, is irreversibly denatured when exposed to neutral pH conditions whereas renin, a structural homologue of pepsin, is fully stable and optimally active in the same conditions despite sharing highly similar enzyme architecture. To gain insight into the structural determinants of differential aspartic protease pH stability, the present study used comparative bioinformatic and structural analyses. In pepsin, an abundance of polar and aspartic acid residues were identified, a common trait with other acid-stable enzymes. Conversely, renin was shown to have increased levels of basic amino acids. In both pepsin and renin, the solvent exposure of these charged groups was high. Having similar overall acidic residue content, the solvent-exposed basic residues may allow for extensive salt bridge formation in renin, whereas in pepsin, these residues are protonated and serve to form stabilizing hydrogen bonds at low pH. Relative differences in structure and sequence in the turn and joint regions of the β-barrel and ψ-loop in both the N- and C-terminal lobes were identified as regions of interest in defining divergent pH stability. Compared to the structural rigidity of renin, pepsin has more instability associated with the N-terminus, specifically the B/C connector. By contrast, renin exhibits greater C-terminal instability in turn and connector regions. Overall, flexibility differences in connector regions, and amino acid composition, particularly in turn and joint regions of the β-barrel and ψ-loops, likely play defining roles in determining pH stability for renin and pepsin.

Extensive expansion of A1 family aspartic proteinases in fungi revealed by evolutionary analyses of 107 complete eukaryotic proteomes

The A1 family of eukaryotic aspartic proteinases (APs) forms one of the 16 AP families. Although one of the best characterized families, the recent increase in genome sequence data has revealed many fungal AP homologs with novel sequence characteristics. This study was performed to explore the fungal AP sequence space and to obtain an in-depth understanding of fungal AP evolution. Using a comprehensive phylogeny of approximately 700 AP sequences from the complete proteomes of 87 fungi and 20 nonfungal eukaryotes, 11 major clades of APs were defined of which clade I largely corresponds to the A1A subfamily of pepsin-archetype APs. Clade II largely corresponds to the A1B subfamily of nepenthesin-archetype APs. Remarkably, the nine other clades contain only fungal APs, thus indicating that fungal APs have undergone a large sequence diversification. The topology of the tree indicates that fungal APs have been subject to both “birth and death” evolution and “functional redundancy and diversification.” This is substantiated by coclustering of certain functional sequence characteristics. A meta-analysis toward the identification of Cluster Determining Positions (CDPs) was performed in order to investigate the structural and biochemical basis for diversification. Seven CDPs contribute to the secondary structure of the enzyme. Three other CDPs are found in the vicinity of the substrate binding cleft. Tree topology, the large sequence variation among fungal APs, and the apparent functional diversification suggest that an amendment to update the current A1 AP classification based on a comprehensive phylogenetic clustering might contribute to refinement of the classification in the MEROPS peptidase database.

RC1339/APRc from Rickettsia conorii is a novel aspartic protease with properties of retropepsin-like enzymes

Members of the species Rickettsia are obligate intracellular, gram-negative, arthropod-borne pathogens of humans and other mammals. The life-threatening character of diseases caused by many Rickettsia species and the lack of reliable protective vaccine against rickettsioses strengthens the importance of identifying new protein factors for the potential development of innovative therapeutic tools. Herein, we report the identification and characterization of a novel membrane-embedded retropepsin-like homologue, highly conserved in 55 Rickettsia genomes. Using R. conorii gene homologue RC1339 as our working model, we demonstrate that, despite the low overall sequence similarity to retropepsins, the gene product of rc1339 APRc (for Aspartic Protease from Rickettsia conorii) is an active enzyme with features highly reminiscent of this family of aspartic proteases, such as autolytic activity impaired by mutation of the catalytic aspartate, accumulation in the dimeric form, optimal activity at pH 6, and inhibition by specific HIV-1 protease inhibitors. Moreover, specificity preferences determined by a high-throughput profiling approach confirmed common preferences between this novel rickettsial enzyme and other aspartic proteases, both retropepsins and pepsin-like. This is the first report on a retropepsin-like protease in gram-negative intracellular bacteria such as Rickettsia, contributing to the analysis of the evolutionary relationships between the two types of aspartic proteases. Additionally, we have also shown that APRc is transcribed and translated in R. conorii and R. rickettsii and is integrated into the outer membrane of both species. Finally, we demonstrated that APRc is sufficient to catalyze the in vitro processing of two conserved high molecular weight autotransporter adhesin/invasion proteins, Sca5/OmpB and Sca0/OmpA, thereby suggesting the participation of this enzyme in a relevant proteolytic pathway in rickettsial life-cycle. As a novel bona fide member of the retropepsin family of aspartic proteases, APRc emerges as an intriguing target for therapeutic intervention against fatal rickettsioses.

Several rickettsiae are pathogenic to humans by causing severe infections, including epidemic typhus (Rickettsia prowazekii), Rocky Mountain spotted fever (Rickettsia rickettsii), and Mediterranean spotted fever (Rickettsia conorii). Progress in correlating rickettsial genes and gene functions has been greatly hampered by the intrinsic difficulty in working with these obligate intracellular bacteria, despite the increasing insights into the mechanisms of pathogenesis of and the immune response to rickettsioses. Therefore, comparison of the multiple available genomes of Rickettsia is proving to be the most practical method to identify new factors that may play a role in pathogenicity. Here, we identified and characterized a novel retropepsin-like enzyme, APRc, that is expressed by at least two pathogenic rickettsial species, R. conorii and R. rickettsii. We have also established that APRc acts to process two major surface antigen/virulence determinants (OmpB/Sca5, OmpA/Sca0) in vitro and we suggest that this processing event is important for protein function. We demonstrate that APRc is specifically inhibited by drugs clinically used to treat HIV infections, providing the exciting possibility of targeting this enzyme for therapeutic intervention. With this work, we demonstrate that retropepsin-type aspartic proteases are indeed present in prokaryotes, suggesting that these enzymes may represent an ancestral form of these proteases.

Biochemical study of the extracellular aspartyl protease Eap1 from the phytopathogen fungus Sporisorium reilianum

In this work, the extracellular protease Eap1 from Sporisorium reilianum was characterized in solid and liquid cultures using different culture media. The results showed that Eap1 was produced in all media and under all culture conditions, with the most activity in solid culture at an acidic pH of 3-5. Following purification, the 41 kDa protease demonstrated aspartyl protease activity. The enzyme was stable at a wide range of temperatures and pH values, but 45°C and pH 3 were optimal. The K(m) and V(max( values obtained were 0.69 mg/mL and 0.66 μmol/min, respectively, with albumin as the substrate. Eap1 degraded hemoglobin as well as proteins obtained from corn germ, roots, stems and slides at pH 3 and also had milk-clotting activity. Sequencing analysis showed that this protein has 100% similarity to the peptide sequence theoretically obtained from the sr11394 gene, which encodes an aspartyl protease secreted by S. reilianum.

Pepsin homologues in bacteria

Background

Peptidase family A1, to which pepsin belongs, had been assumed to be restricted to eukaryotes. The tertiary structure of pepsin shows two lobes with similar folds and it has been suggested that the gene has arisen from an ancient duplication and fusion event. The only sequence similarity between the lobes is restricted to the motif around the active site aspartate and a hydrophobic-hydrophobic-Gly motif. Together, these contribute to an essential structural feature known as a psi-loop. There is one such psi-loop in each lobe, and so each lobe presents an active Asp. The human immunodeficiency virus peptidase, retropepsin, from peptidase family A2 also has a similar fold but consists of one lobe only and has to dimerize to be active. All known members of family A1 show the bilobed structure, but it is unclear if the ancestor of family A1 was similar to an A2 peptidase, or if the ancestral retropepsin was derived from a half-pepsin gene. The presence of a pepsin homologue in a prokaryote might give insights into the evolution of the pepsin family.

Results

Homologues of the aspartic peptidase pepsin have been found in the completed genomic sequences from seven species of bacteria. The bacterial homologues, unlike those from eukaryotes, do not possess signal peptides, and would therefore be intracellular acting at neutral pH. The bacterial homologues have Thr218 replaced by Asp, a change which in renin has been shown to confer activity at neutral pH. No pepsin homologues could be detected in any archaean genome.

Conclusion

The peptidase family A1 is found in some species of bacteria as well as eukaryotes. The bacterial homologues fall into two groups, one from oceanic bacteria and one from plant symbionts. The bacterial homologues are all predicted to be intracellular proteins, unlike the eukaryotic enzymes. The bacterial homologues are bilobed like pepsin, implying that if no horizontal gene transfer has occurred the duplication and fusion event might be very ancient indeed, preceding the divergence of bacteria and eukaryotes. It is unclear whether all the bacterial homologues are derived from horizontal gene transfer, but those from the plant symbionts probably are. The homologues from oceanic bacteria are most closely related to memapsins (or BACE-1 and BACE-2), but are so divergent that they are close to the root of the phylogenetic tree and to the division of the A1 family into two subfamilies.

An aspartic proteinase gene family in the filamentous fungus Botrytis cinerea contains members with novel features

Botrytis cinerea, an important fungal plant pathogen, secretes aspartic proteinase (AP) activity in axenic cultures. No cysteine, serine or metalloproteinase activity could be detected. Proteinase activity was higher in culture medium containing BSA or wheat germ extract, as compared to minimal medium. A proportion of the enzyme activity remained in the extracellular glucan sheath. AP was also the only type of proteinase activity in fluid obtained from B. cinerea-infected tissue of apple, pepper, tomato and zucchini. Five B. cinerea genes encoding an AP were cloned and denoted Bcap1-5. Features of the encoded proteins are discussed. BcAP1, especially, has novel characteristics. A phylogenetic analysis was performed comprising sequences originating from different kingdoms. BcAP1 and BcAP5 did not cluster in a bootstrap-supported clade. BcAP2 clusters with vacuolar APs. BcAP3 and BcAP4 cluster with secreted APs in a clade that also contains glycosylphosphatidylinositol-anchored proteinases from Saccharomyces cerevisiae and Candida albicans. All five Bcap genes are expressed in liquid cultures. Transcript levels of Bcap1, Bcap2, Bcap3 and Bcap4 are subject to glucose and peptone repression. Transcripts from all five Bcap genes were detected in infected plant tissue, indicating that at least part of the AP activity in planta originates from the pathogen.

Immunohistochemical analysis of renin activity in chronic cyclosporine nephropathy in childhood nephrotic syndrome

Although the pivotal role of activation of the intrarenal renin-angiotensin system (RAS) has been demonstrated in the rat model of chronic cyclosporine (CyA) nephropathy, it is still unclear whether intrarenal RAS activation is responsible for chronic CyA nephropathy in humans. Therefore, the distribution of renin in formalin-fixed, paraffin-embedded renal biopsy specimens obtained from 26 children who had idiopathic nephrotic syndrome (NS) and who were treated with long-term moderate-dose CyA was examined with the use of immunohistochemistry with rabbit polyclonal anti-human renin antibody. Nineteen patients had steroid-dependent NS, and 7 had steroid-resistant NS. However, CyA treatment led all of the latter patients into complete remission. All of the patients underwent renal biopsies at the start and the end of CyA treatment. In the pre-CyA specimens, immunoreactivity to renin was detected mainly in those parts of arterioles within the anatomically well-defined juxtaglomerular apparatus. In the post-CyA specimens, it was also detected in those parts of the vessels upstream from the juxtaglomerular apparatus. Moreover, the ratio of the number of renin-positive cells to the number of glomeruli (histologic renin index) increased significantly with long-term CyA treatment (from 1.26+/-0.24 to 4.30+/-0.40, P<0.0001). Eleven of the post-CyA specimens showed mild or moderate CyA-associated arteriolopathy (CAA), whereas 15 showed no CAA. The histologic renin index was significantly higher in specimens with CAA than in those without CAA (5.16+/-0.59 versus 3.67+/-0.48, P = 0.031). Seven CAA-positive patients underwent repeat biopsies 6 to 12 mo after discontinuing the CyA. Their specimens showed an improvement in the CAA and significantly lower histologic renin index compared with the post-CyA (from 4.18+/-0.69 to 2.10+/-0.25, P = 0.018). Eleven of the post-CyA specimens showed mild to moderate interstitial fibrosis, and 15 showed no fibrosis. There was no significant difference in immunoreactivity to renin between the specimens with interstitial fibrosis and those without. However, patients with interstitial fibrosis had significantly longer periods of heavy proteinuria during CyA therapy, because of either steroid-resistant NS or frequent relapses of NS (83+/-18 versus 35+/-12 d, P = 0.030). These findings indicate that long-term CyA treatment for idiopathic NS in children may stimulate renin production in arterioles. They also suggest that CyA-stimulated intrarenal RAS activation is responsible for the development of CAA and that CyA-induced interstitial fibrosis is potentiated by long-term heavy proteinuria and is at least partly independent of CyA-stimulated intrarenal RAS activation.

Crystal structure of the aspartic proteinase from Rhizomucor miehei at 2.15 A resolution

The crystal structure of the aspartic proteinase from Rhizomucor miehei (RMP, EC 3. 4. 23. 23) has been refined to 2.15 A resolution to a crystallographic R-value of 0.215 and an Rfree of 0.281. The root-mean-square (r.m.s.) error for the atomic coordinates estimated from a Luzzati plot is 0.2 A. The r.m.s. deviations for the bond distances and bond angles from ideality are 0.01 A and 1.7 degrees, respectively. RMP contains two domains that consist predominantly of beta-sheets. A large substrate-binding cleft is clearly visible between the two domains, and the two catalytic residues Asp38 and Asp237 are located in the middle of the cleft with a water molecule bridging the carboxyl groups of Asp38 and Asp237. Due to crystal packing, the C-terminal domain is more mobile than the N-terminal domain. Most of the aspartic proteinases (except renin) reach their maximum activity at acidic pH. We propose that the optimum pH of each aspartic proteinase is determined by the electrostatic potential at the active site, which, in turn, is determined by the positions and orientations of all the residues near the active site. RMP is the most glycosylated among the aspartic proteinases. The carbohydrate moieties are linked to Asn79 and Asn188. Asn79 is in the middle of a beta-strand and Asn188 is on a surface loop in contrast to the previous hypothesis proposed by Brown and Yada that they are both on surface beta-turns. RMP has a very high thermal stability. The high thermal stability is probably due to the high level of glycosylation. We propose that the highly flexible carbohydrates act as heat reservoirs to stabilize the conformation of RMP and therefore give the enzyme a high level of thermal stability. Three-dimensional structural and sequence alignments of RMP with other aspartic proteinases show that RMP is most structurally homologous to that of Mucor pusillus (MPP), and differs from other fungal enzymes as much as it does from the mammalian enzymes. This suggests that RMP and MPP diverged from the main stream of aspartic proteinases at an early stage of evolution. The present study adds a second member to this subfamily of aspartic proteinases.

Expression and characterisation of chymosin pH optima mutants produced in Trichoderma reesei

The production of chymosin mutants designed to have altered pH optima using the cellulolytic filamentous fungus Trichoderma reesei is described. The strong promoter of the gene encoding the major cellulase, cellobiohydrolase I (CBHI) has been used for the expression and secretion of active calf chymosin. Structural analysis of the hydrogen bonding network around the two active site aspartates 32 and 215 in chymosin have suggested that residues Thr 218 and Asp 303 may influence the rate and pH optima for catalysis. The chymosin mutants Thr218Ala and the double mutant Thr218Ala/Asp303Ala have been made by site-directed mutagenesis and expressed in T. reesei. Enzyme kinetics of the active enzyme T218A indicate a pH optimum of 4.2 compared to 3.8 for native chymosin B using a synthetic octa-peptide substrate, confirming the previous analysis undertaken in E. coli. The double mutant T218A/D303A exhibits a similar optimum of 4.4 to that reported for the D303A, indicating that the combination of these changes is not additive. The application of protein engineering in the rational design of specific modifications to tailor the properties of enzymes offers a new approach to the development of industrial processes.

Renin assay using a fluorogenic substrate and high performance liquid chromatography

Measurement of renin activity in human fluids using a fluorogenic substrate and high performance liquid chromatography (HPLC) is described. A nine amino acid peptide containing the fluorogenic residue, N-(2-pyridyl) glycine (Pg) is used as a substrate. The peptide sequence is homologous with the cleavage site of human angiotensinogen. This substrate is hydrolyzed by renin to generate fluorogenic and non-fluorogenic products. The amount of fluorogenic product is directly measured by reversed phase HPLC. Optimization of assay conditions and measurement of human serum renin levels are described. Assay results correlated well with those from radioimmunoassay. The method is simple, convenient, highly sensitive and can be used for routine clinical renin assays.

Expression of rat renin in mammalian cells and its purification

Rat renin cDNA was transfected into COS-7 and Chinese hamster ovary (CHO) cells and expressed under the control of the Simian Virus 40 early promoter. Conditioned media of the transfected cells showed renin activity only after trypsin treatment, suggesting prorenin was secreted into the medium. From the trypsinized serum-free culture of the transfected CHO cells active renin was purified to homogeneity by a simple three-step procedure. The active renin had similar specific activity, molecular weight, Km, pH optimum, and isoelectric point compared to native renin. The amino-terminal sequence was the same as that deduced from the renin cDNA. This suggests that the recombinant rat renin is similar to kidney renin in many respects, and is easily obtained by the present procedures.

Sequence requirements for proteolytic cleavage of precursors with paired basic amino acids

When expressed in COS cells, human prorenin was secreted into the medium without being processed to an active renin. Co-expression of furin, a mammalian homologue of the yeast KEX2 gene product, did not affect proteolytic processing of prorenin. A mutant proreninR-4 constructed by site-directed mutagenesis of Pro (-4) to Arg was not cleaved by an endoprotease in the COS cell. However, proreninR-4 was detectably cleaved to yield the active renin upon co-transfection with furin DNA, indicating that Arg at position -4 is important for recognition and processing by furin in addition to the absolute requirement for paired basic amino acids. Another mutant precursor in which Leu (+1) of proreninR-4 was replaced with Ser was found to be much more efficiently processed than proreninR-4, regardless of co-expression of furin. The results suggest that not only a basic amino acid at position -4 but also Leu at position +1 significantly affect the processing of prorenin catalyzed by the COS cell endoprotease or furin.

Human prorenin

Human prorenin is the enzymatically inactive biosynthetic precursor of renin. Recent interest has focused on the posttranslational sorting and processing of prorenin to renin since markedly increased levels of circulating prorenin have been associated with both physiological and pathological changes. These observations raise the question of whether prorenin processing may be a regulatory event in renin production in the kidney. In the juxtaglomerular cells of the kidney, prorenin can be sorted to either of two pathways: 1) the regulated pathway, which is mediated by secretory granules, where a thiol protease resembling cathepsin B processes prorenin to renin by cleavage of the amino terminal 43-amino acid prosegment, which allows exposure of the active site of renin, or 2) the constitutive pathway, which is not regulated and does not involve conversion of prorenin to renin. Studies in which segments of prorenin are modified by site-directed mutagenesis suggest that the prosegment and glycosylation are not required for sorting, although they may influence or participate in sorting, or both. Certain areas in the prosegment are important determinants of enzyme activity and ability to cleave the prosegment. Further structural analysis of prorenin will be useful to assess details of its sorting and processing. In addition, a number of extrarenal tissues such as uterine lining, ovarian theca, corpus luteum, pituitary, and adrenal, express the renin gene. These tissues have different capabilities to sort and process prorenin compared with kidney, and some tissues secrete only prorenin. Whether prorenin-to-renin conversion is necessary to activate these local renin-angiotensin systems is a key issue.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative modeling of proteins in the design of novel renin inhibitors

Renin, the first enzyme in the renin-angiotensin system, is critically important for the maintenance of blood pressure, and, therefore, as a target for antihypertensive therapy. The three-dimensional structure of renin would be an invaluable aid in understanding the functional properties of renin as well as in the design of novel, potent inhibitors. Three-dimensional models of renin have been developed by a number of different groups based on comparative homology modeling from the other known aspartic proteinase structures. These models have been used widely in the drug design process to suggest targets for synthesis and to rationalize the structure-activity relationships of compounds. This review describes the different published renin models and compares them to the extent possible. Applications of these model renin and renin-inhibitor complex structures to biological function and inhibitor design are summarized.

X-ray analyses of aspartic proteinases. II. Three-dimensional structure of the hexagonal crystal form of porcine pepsin at 2.3 A resolution

The molecular structure of the hexagonal crystal form of porcine pepsin (EC 3.4.23.1), an aspartic proteinase from the gastric mucosa, has been determined by molecular replacement using the fungal enzyme, penicillopepsin (EC 3.4.23.6), as the search model. This defined the space group as P6522 and refinement led to an R-factor of 0.190 at 2.3 A resolution. The positions of 2425 non-hydrogen protein atoms in 326 residues have been determined and the model contains 371 water molecules. The structure is bilobal, consisting of two predominantly beta-sheet lobes which, as in other aspartic proteinases, are related by a pseudo 2-fold axis. The strands of the mixed beta-sheets (1N and 1C) of each lobe are related by an intra-lobe topological 2-fold symmetry. Two further beta-sheets, 2N and 2C, are each composed of two topologically related beta-hairpins folded below the 1N and 1C sheets. A further six-stranded sheet (3) spans the two lobes and forms a structure resembling an arch upon which the four other sheets reside. The interface between sheets 1N and 1C forms the catalytic centre consisting of absolutely conserved aspartate residues 32 and 215, which are shielded from solvent by a beta-hairpin loop (75 to 78). The crystal structure of a mammalian aspartic proteinase indicates that interactions with substrate may be more extensive on the prime side of the active site cleft than in the fungal enzymes and involve Tyr189 and the loop 290 to 295, perhaps contributing to the transpeptidase activity of pepsin and the specificity of the renins. Comparison with the high-resolution structure of pepsinogen gives a root-mean-square deviation of 0.9 A and reveals that, in addition to local rearrangement at the active site, there appears to be a rigid group movement of part of the C-terminal lobe of pepsin towards the cleft on activation. A large proportion of the absolutely conserved residues in aspartic proteinases are polar and buried. An examination of the pepsin structure reveals that these side-chains are involved in hydrogen-bond interactions with either the main chain of the protein or other conserved side-chains of the enzyme or propart.

Renin inhibitors

Since the early 1980s, an intensive effort has been focused on the development of orally effective and long-acting inhibitors of renin. During this time, in vitro potency has increased greatly, with several transition-state inhibitor designs yielding inhibitors with subnanomolar IC50 values. In the meantime, both the molecular weight and peptide character of the inhibitors has decreased as important binding elements have been focused into smaller and more stable structures. The resulting inhibitors have shown promising activities in several in vivo models and (in two cases) in man. Nevertheless, renin inhibitors reported to date have limited oral bioavailability and short duration of action, and improvements in both will be necessary for them to compete effectively with ACE inhibitors. Renin inhibitors which have entered clinical studies have at least one naturally occurring amino acid and three or more amide bonds. It is reasonable to expect that continued development will produce wholly nonpeptide inhibitors with still lower MW, and it may be these "second-generation" inhibitors which will succeed as therapeutic agents. Development of orally effective and long-acting inhibitors of renin will enable their long-term antihypertensive efficacy and possible advantages over ACE inhibitor to be investigated.

Substrate specificity of recombinant human renal renin: effect of histidine in the P2 subsite on pH dependence

Steady-state kinetic analysis of human renin demonstrates the histidine proximal to the substrate scissile peptide bond contributes to the unique specificity and pH dependence of this aspartyl protease. Recombinant human renal renin purified from mammalian cell culture appears to be indistinguishable from renin isolated from human kidney with respect to specific activity (1000 Goldblatt units/mg). Recombinant renin contains carbohydrate covalently attached to asparagines at positions 5 and 75 (renin numbering) and disulfide linkages at Cys-51/Cys-58, Cys-217/Cys-221, and Cys-259/Cys-296. Renin pH dependence was evaluated between pH 4.0 and 8.0 by using a synthetic substrate identical with the amino terminus of porcine angiotensinogen (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu*Leu-Val-Tyr-Ser, where the asterisk indicates the scissile peptide bond and the proximal histidine is in italics) and an analogous tetradecapeptide where the proximal histidine was substituted with glutamine. Comparison of the pH profiles shows the catalytic efficiency (V/Km) and maximal velocity (V) of renin are greater above pH 6.5 with the substrate containing histidine proximal to the scissile peptide bond, but below pH 5.0 these parameters are greater with the glutamine substrate analogue. Solvent isotope effects show that proton transfer contributes to the rate-limiting step in catalysis with both substrates and that the proximal histidine does not serve as a base in the catalytic mechanism. Molecular modeling indicates the substrate histidine could hydrogen bond to Asp-226 of the enzyme (renin numbering), thus perturbing the ionization of the catalytic aspartyl groups (Asp-38 and Asp-226).(ABSTRACT TRUNCATED AT 250 WORDS)

Structure of recombinant human renin, a target for cardiovascular-active drugs, at 2.5 A resolution

The x-ray crystal structure of recombinant human renin has been determined. Molecular dynamics techniques that included crystallographic data as a restraint were used to improve an initial model based on porcine pepsinogen. The present agreement factor for data from 8.0 to 2.5 angstroms (A) is 0.236. Some of the surface loops are poorly determined, and these disordered regions border a 30 A wide solvent channel. Comparison of renin with other aspartyl proteinases shows that, although the structural cores and active sites are highly conserved, surface residues, some of which are critical for specificity, vary greatly (up to 10A). Knowledge of the actual structure, as opposed to the use of models based on related enzymes, should facilitate the design of renin inhibitors.