A. Ian Smith

β-Amino Acids: Versatile Peptidomimetics

The use of peptidomimetics has emerged as a powerful means for overcoming the limitations inherent in the physical characteristics of peptides thus improving their therapeutic potential. A peptidomimetic approach that has emerged in recent years with significant potential, is the use of β-amino acids. β-Amino acids are similar to α-amino acids in that they contain an amino terminus and a carboxyl terminus. However, in β-amino acids two carbon atoms separate these functional termini. β-amino acids, with a specific side chain, can exist as the R or S isomers at either the α (C2) carbon or the β (C3) carbon. This results in a total of 4 possible diastereoisomers for any given side chain. The flexibility to generate a vast range of stereo- and regioisomers, together with the possibility of disubstitution, significantly expands the structural diversity of β-amino acids thereby providing enormous scope for molecular design. The incorporation of β-amino acids has been successful in creating peptidomimetics that not only have potent biological activity, but are also resistant to proteolysis. This article reviews the rapidly expanding applications of β-amino acids in the design of bioactive peptide analogues ranging from receptor agonists and antagonists, MHC-binding peptides, antimicrobial peptides and peptidase inhibitors. Given their structural diversity taken together with the ease of synthesis and incorporation into peptide sequences using standard solid-phase peptide synthesis techniques, β-amino acids have the potential to form a new platform technology for peptidomimetic design and synthesis.

Tumor Necrosis Factor-α Convertase (ADAM17) Mediates Regulated Ectodomain Shedding of the Severe-acute Respiratory Syndrome-Coronavirus (SARS-CoV) Receptor, Angiotensin-converting Enzyme-2 (ACE2)

Angiotensin-converting enzyme-2 (ACE2) is a critical regulator of heart function and a cellular receptor for the causative agent of severe-acute respiratory syndrome (SARS), SARS-CoV (coronavirus). ACE2 is a type I transmembrane protein, with an extracellular N-terminal domain containing the active site and a short intracellular C-terminal tail. A soluble form of ACE2, lacking its cytosolic and transmembrane domains, has been shown to block binding of the SARS-CoV spike protein to its receptor. In this study, we examined the ability of ACE2 to undergo proteolytic shedding and investigated the mechanisms responsible for this shedding event. We demonstrated that ACE2, heterologously expressed in HEK293 cells and endogenously expressed in Huh7 cells, undergoes metalloproteinase-mediated, phorbol ester-inducible ectodomain shedding. By using inhibitors with differing potency toward different members of the ADAM (a disintegrin and metalloproteinase) family of proteases, we identified ADAM17 as a candidate mediator of stimulated ACE2 shedding. Furthermore, ablation of ADAM17 expression using specific small interfering RNA duplexes reduced regulated ACE2 shedding, whereas overexpression of ADAM17 significantly increased shedding. Taken together, these data provided direct evidence for the involvement of ADAM17 in the regulated ectodomain shedding of ACE2. The identification of ADAM17 as the protease responsible for ACE2 shedding may provide new insight into the physiological roles of ACE2.

The globin fragment LVV-hemorphin-7 is an endogenous ligand for the AT4 receptor in the brain.

Abstract: Angiotensin IV (Val‐Tyr‐Ile‐His‐Pro‐Phe) has been reported to interact with specific high‐affinity receptors to increase memory retrieval, enhance dopamine‐induced stereotypy behavior, and induce c‐fos expression in several brain nuclei. We have isolated a decapeptide (Leu‐Val‐Val‐Tyr‐Pro‐Trp‐Thr‐Gln‐Arg‐Phe) from sheep brain that binds with high affinity to the angiotensin IV receptor. The peptide was isolated using 125I‐angiotensin IV binding to bovine adrenal membranes to assay receptor binding activity. This peptide is identical to the amino acid sequence 30–39 of sheep βA‐ and βB‐globins and has previously been named LVV‐hemorphin‐7. Pharmacological studies demonstrated that LVV‐hemorphin‐7 and angiotensin IV were equipotent in competing for 125I‐angiotensin IV binding to sheep cerebellar membranes and displayed full cross‐displacement. Using in vitro receptor autoradiography, 125I‐LVV‐hemorphin‐7 binding to sheep brain sections was identical to 125I‐angiotensin IV binding in its pattern of distribution and binding specificity. This study reveals the presence of a globin fragment in the sheep brain that exhibits a high affinity for, and displays an identical receptor distribution with, the angiotensin IV receptor. This globin fragment, LVV‐hemorphin‐7, may therefore represent an endogenous ligand for the angiotensin IV receptor in the CNS.

Upregulation of hepatic angiotensin-converting enzyme 2 (ACE2) and angiotensin-(1–7) levels in experimental biliary fibrosis

Background/Aims Angiotensin-converting enzyme 2 (ACE2), its product, angiotensin-(1–7) and its receptor, Mas, may moderate the adverse effects of angiotensin II in liver disease. We examined the expression of these novel components of the renin angiotensin system (RAS) and the production and vasoactive effects of angiotensin-(1–7) in the bile duct ligated (BDL) rat. Methods BDL or sham-operated rats were sacrificed at 1, 2, 3 and 4 weeks. Tissue and blood were collected for gene expression, enzyme activity and peptide measurements. In situ perfused livers were used to assess angiotensin peptide production and their effects on portal resistance. Results Hepatic ACE2 gene and activity (P <0.0005), plasma angiotensin-(1–7) (P <0.0005) and Mas receptor expression (P <0.01) were increased following BDL compared to shams. Perfusion experiments confirmed that BDL livers produced increased angiotensin-(1–7) (P <0.05) from angiotensin II and this was augmented (P <0.01) by ACE inhibition. Whilst angiotensin II increased vasoconstriction in cirrhotic livers, angiotensin-(1–7) had no effect on portal resistance. Conclusions RAS activation in chronic liver injury is associated with upregulation of ACE2, Mas and hepatic conversion of angiotensin II to angiotensin-(1–7) leading to increased circulating angiotensin-(1–7). These results support the presence of an ACE2-angiotensin-(1–7)-Mas axis in liver injury which may counteract the effects of angiotensin II.

Angiotensin-converting Enzyme 2 (ACE2), But Not ACE, Is Preferentially Localized to the Apical Surface of Polarized Kidney Cells

Angiotensin-converting enzyme-2 (ACE2) is a homologue of angiotensin-I converting enzyme (ACE), the central enzyme of the renin-angiotensin system (RAS). ACE2 is abundant in human kidney and heart and has been implicated in renal and cardiac function through its ability to hydrolyze Angiotensin II. Although ACE2 and ACE are both type I integral membrane proteins and share 61% protein sequence similarity, they display distinct modes of enzyme action and tissue distribution. This study characterized ACE2 at the plasma membrane of non-polarized Chinese hamster ovary (CHO) cells and polarized Madin-Darby canine kidney (MDCKII) epithelial cells and compared its cellular localization to its related enzyme, ACE, using indirect immunofluorescence, cell-surface biotinylation, Western analysis, and enzyme activity assays. This study shows ACE2 and ACE are both cell-surface proteins distributed evenly to detergent-soluble regions of the plasma membrane in CHO cells. However, in polarized MDCKII cells under steady-state conditions the two enzymes are differentially expressed. ACE2 is localized predominantly to the apical surface (∼92%) where it is proteolytically cleaved within its ectodomain to release a soluble form. Comparatively, ACE is present on both the apical (∼55%) and basolateral membranes (∼45%) where it is also secreted but differentially; the ectodomain cleavage of ACE is 2.5-fold greater from the apical surface than the basolateral surface. These studies suggest that both ACE2 and ACE are ectoenzymes that have distinct localization and secretion patterns that determine their role on the cell surface in kidney epithelium and in urine.

The X-ray Crystal Structure of Full-Length Human Plasminogen

Plasminogen is the proenzyme precursor of the primary fibrinolytic protease plasmin. Circulating plasminogen, which comprises a Pan-apple (PAp) domain, five kringle domains (KR1-5), and a serine protease (SP) domain, adopts a closed, activation-resistant conformation. The kringle domains mediate interactions with fibrin clots and cell-surface receptors. These interactions trigger plasminogen to adopt an open form that can be cleaved and converted to plasmin by tissue-type and urokinase-type plasminogen activators. Here, the structure of closed plasminogen reveals that the PAp and SP domains, together with chloride ions, maintain the closed conformation through interactions with the kringle array. Differences in glycosylation alter the position of KR3, although in all structures the loop cleaved by plasminogen activators is inaccessible. The ligand-binding site of KR1 is exposed and likely governs proenzyme recruitment to targets. Furthermore, analysis of our structure suggests that KR5 peeling away from the PAp domain may initiate plasminogen conformational change.

The Subtilisin-Like Protease AprV2 Is Required for Virulence and Uses a Novel Disulphide-Tethered Exosite to Bind Substrates

Many bacterial pathogens produce extracellular proteases that degrade the extracellular matrix of the host and therefore are involved in disease pathogenesis. Dichelobacter nodosus is the causative agent of ovine footrot, a highly contagious disease that is characterized by the separation of the hoof from the underlying tissue. D. nodosus secretes three subtilisin-like proteases whose analysis forms the basis of diagnostic tests that differentiate between virulent and benign strains and have been postulated to play a role in virulence. We have constructed protease mutants of D. nodosus; their analysis in a sheep virulence model revealed that one of these enzymes, AprV2, was required for virulence. These studies challenge the previous hypothesis that the elastase activity of AprV2 is important for disease progression, since aprV2 mutants were virulent when complemented with aprB2, which encodes a variant that has impaired elastase activity. We have determined the crystal structures of both AprV2 and AprB2 and characterized the biological activity of these enzymes. These data reveal that an unusual extended disulphide-tethered loop functions as an exosite, mediating effective enzyme-substrate interactions. The disulphide bond and Tyr92, which was located at the exposed end of the loop, were functionally important. Bioinformatic analyses suggested that other pathogenic bacteria may have proteases that utilize a similar mechanism. In conclusion, we have used an integrated multidisciplinary combination of bacterial genetics, whole animal virulence trials in the original host, biochemical studies, and comprehensive analysis of crystal structures to provide the first definitive evidence that the extracellular secreted proteases produced by D. nodosus are required for virulence and to elucidate the molecular mechanism by which these proteases bind to their natural substrates. We postulate that this exosite mechanism may be used by proteases produced by other bacterial pathogens of both humans and animals.

β-Amino acid-containing hybrid peptides—new opportunities in peptidomimetics

Hybrid peptides consisting of alpha-amino acids with judiciously placed beta-amino acids show great promise as peptidomimetics in an increasing range of therapeutic applications. This reflects a combination of increased stability, high specificity and relative ease of synthesis.

Thiol Activation of Endopeptidase EC 3.4.24.15 A NOVEL MECHANISM FOR THE REGULATION OF CATALYTIC ACTIVITY

Endopeptidase EC 3.4.24.15 (EP24.15) is a thermolysin-like metalloendopeptidase involved in the regulated metabolism of a number of neuropeptides. Unlike other thermolysin-like peptidases EP24.15 displays a unique thiol activation, a mechanism that is not clearly understood. In this study we show that both recombinant and tissue-derived EP24.15 are activated up to 8-fold by low concentrations (0.1 mm) of dithiothreitol. Additionally, under non-reducing conditions, recombinant and native EP24.15 forms multimers that can be returned to the monomeric form by reduction. We have also shown that competitive inhibitor binding occurs only to the monomeric form, which indicates that catalytic site access is restricted in the multimeric forms. Through systematic site-directed mutagenesis we have identified that cysteine residues 246, 253, and possibly 248 are involved in the formation of these multimers. Furthermore, both a double mutant (C246S/C253S) and a triple mutant (C246S/C248S/C253S) are fully active in the absence of reducing agents, as measured by both inhibitor binding and hydrolysis. The formation and disruption of disulfide bonds involving these cysteine residues may be a mechanism by which EP24.15 activity is regulated through changes in intra- and extracellular redox potential.

Angiotensin‐converting enzyme 2 catalytic activity in human plasma is masked by an endogenous inhibitor

Angiotensin‐converting enzyme 2 (ACE2) is thought to act in an opposing manner to its homologue, angiotensin‐converting enzyme (ACE), by inactivating the vasoconstrictor peptide angiotensin II and generating the vasodilatory fragment, angiotensin(1–7). Both ACE and ACE2 are membrane‐bound ectoenzymes and may circulate in plasma as a consequence of a proteolytic shedding event. In this study, we show that ACE2 circulates in human plasma, but its activity is suppressed by the presence of an endogenous inhibitor. Partial purification of this inhibitor indicated that the inhibitor is small, hydrophilic and cationic, but not a divalent metal cation. These observations led us to develop a method for removal of the inhibitor, thus allowing detection of plasma ACE2 levels using a sensitive quenched fluorescent substrate‐based assay. Using this technique, ACE2 activity measured in plasma from healthy volunteers (n= 18) ranged from 1.31 to 8.69 pmol substrate cleaved min−1 ml−1 (mean ±s.e.m., 4.44 ± 0.56 pmol min−1 ml−1). Future studies of patients with cardiovascular, renal and liver disease will determine whether plasma ACE2 is elevated in parallel with increased tissue levels observed in these conditions.