Designed peptide analogues of the potassium channel blocker ShK toxin.

ShK toxin, a potassium channel blocker from the sea anemone Stichodactyla helianthus, is a 35-residue polypeptide cross-linked by 3 disulfide bridges. In an effort to generate truncated peptidic analogues of this potent channel blocker, we have evaluated three analogues, one in which the native sequence was truncated and then stabilized by the introduction of additional covalent links (a non-native disulfide and two lactam bridges), and two in which non-native structural scaffolds stabilized by disulfide and/or lactam bridges were modified to include key amino acid residues from the native toxin. The effect of introducing a lactam bridge in the first helix of ShK toxin (to create cyclo14/18[Lys14,Asp18]ShK) was also examined to confirm that this modification was compatible with activity. All four analogues were tested in vitro for their ability to block Kv1.3 potassium channels in Xenopus oocytes, and their solution structures were determined using 1H NMR spectroscopy. The lactam bridge in full-length ShK is well tolerated, with only a 5-fold reduction in binding to Kv1.3. The truncated and stabilized analogue was inactive, apparently due to a combination of slight deviations from the native structure and alterations to side chains required for binding. One of the peptide scaffolds was also inactive because it failed to adopt the required structure, but the other had a K(d) of 92 microM. This active peptide incorporated mimics of Lys22 and Tyr23, which are essential for activity in ShK, and an Arg residue that could mimic Arg11 or Arg24 in the native toxin. Modification of this peptide should produce a more potent, low molecular weight peptidic analogue which will be useful not only for further in vitro and in vivo studies of the effect of blocking Kv1.3, but also for mapping the interactions with the pore and vestibule of this K(+) channel that are required for potent blockade.     

Synthesis and biological activity of the first cyclic biphalin analogues

Biphalin is a linear octapeptide with strong opioid activity. Its structure is based on two identical sequences derived from enkephalins joined C-terminal to C-terminal by an hydrazide bridge (Tyr-D-Ala-Gly-Phe-NH-NH<--Phe<--Gly<--D-Ala<--Tyr). In this study we present the design, synthesis, and biological evaluation of the first cyclic biphalin analogues. d-Alanine residues in positions 2, 2' of the parent peptide were replaced by d- and l-cysteine and an intramolecular disulfide bond between the cysteine thiol groups was introduced. We obtained two cyclic analogues with quite different biological profiles.     

Cyclic dermorphin tetrapeptide analogues obtained via ring-closing metathesis

The dermorphin-derived cyclic tetrapeptide analogues H-Tyr-c[D-Cys-Phe-Cys]NH(2) and H-Tyr-c[D-Cys-Phe-D-Cys]NH(2) are opioid agonists at the mu and delta receptor. To enhance the metabolic stability of these peptides, we replaced the disulfide bridge with a bis-methylene moiety. This was achieved by solid-phase synthesis of the linear precursor peptide containing allylglycine residues in place of the Cys residues, followed by ring-closing metathesis. In the case of the peptide with L-configuration in the 4-position both the cis and the trans isomer of the resulting olefinic peptides were formed, whereas the cis isomer only was obtained with the peptide having the D-configuration in position 4. Catalytic hydrogenation yielded the saturated -CH(2)-CH(2)- bridged peptides. In comparison with the cystine-containing parent peptides, all olefinic peptides showed significantly reduced mu and delta agonist potencies in the guinea pig ileum and mouse vas deferens assays. The -CH(2)-CH(2)-bridged peptide with l-configuration in the 4-position was equipotent with its cystine-containing parent in both assays, whereas the bis-methylene analogue with D-configuration in position 4 was 10-27-fold less potent compared to its parent. The effect of the disulfide replacements with the -CH=CH- and -CH(2)-CH(2)- moieties on the conformational behavior of these peptides was examined by theoretical conformational analysis which provided plausible explanations in terms of structural parameters for the observed changes in opioid activity.     

Primary and secondary structural determinants in the receptor binding sequence beta-(38-57) from human luteinizing hormone

The intercysteine "loop" sequence 38-57 in the beta subunit has been shown to be a determinant for expression of biological activity in human lutropin (hLH) and choriogonadotropin (hCG) [Keutmann, H. T., Charlesworth, M. C., Mason, K. A., Ostrea, T., Johnson, L., & Ryan, R. J. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 2038]. Together with other sequences, the 38-57 region may contribute to a multicomponent receptor binding domain in hLH/hCG. Because the structural features influencing activity in this important region are not easy to evaluate in the full-length subunit, we have used analogues of hLH beta-(38-57) prepared by solid-phase synthesis. The peptides were tested for inhibition of 125I-labeled hCG binding to rat ovarian membrane receptors. Secondary structure was analyzed by circular dichroism (CD) and by reactivity with antibodies to the native 38-57 peptide. An analogue lacking the 38-57 disulfide linkage retained 20% receptor binding and full immunoreactivity. "Far"-ultraviolet CD profiles were essentially identical with those of the disulfide-intact peptide; a transition from 10% to 30% alpha-helix in 90% trifluoroethanol was characteristic of both. The peptide thus appears not to require the disulfide bridge to retain a looped conformation with amphipathic secondary structure. An essential positive charge at position 43 was shown by complete loss of activity upon substitution of Asp or Ala for the Arg found in all known species of LH. Other analogues showed a requirement for a neutral residue at position 47, also highly conserved.(ABSTRACT TRUNCATED AT 250 WORDS)     

Design,synthesis, conformational analysis,and biological studies of urotensin-II lactam analogues

Human urotensin II (hU-II; H-Glu-Thr-Pro-Asp-cyclo[Cys-Phe-Trp-Lys-Tyr-Cys]-Val-OH) is a disulfide bridged undecapeptide recently identified as the ligand of an orphan G protein-coupled receptor. hU-II has been described as the most potent vasoconstrictor compound identified to date. With the aim of replacing the disulfide bridge by a chemically more stable moiety, we have synthesized and tested a series of lactam analogues of hU-II minimum active fragment, that is hU-II(4-11). The contractile activity of the synthetic analogues on the rat isolated thoracic aorta was found to be dependent upon the dimension of the lactam bridge. The most active peptide, H-Asp-cyclo[Orn-Phe-Trp-Lys-Tyr-Asp]-Val-OH (3), is approximately 2 logs less potent than hU-II (pD(2)=6.3 vs 8.4). A conformational analysis in solution of the active peptide 3, one of the inactive analogues, and hU-II was performed, using NMR and molecular modelling techniques. A superposition of the calculated structures of hU-II and 3 clearly shows that three out of four key residues (i.e., Phe(6), Lys(8) and Tyr(9)) maintain the same side- chain orientation, while the fourth one, Trp(7), cannot be superimposed. This observation could explain the reduced biological activity of the synthetic analogue.     

Disulfide bond mutagenesis and the structure and function of the head-to-tail macrocyclic trypsin inhibitor SFTI-1

SFTI-1 is a novel 14 amino acid peptide comprised of a circular backbone constrained by three proline residues, a hydrogen-bond network, and a single disulfide bond. It is the smallest and most potent known Bowman-Birk trypsin inhibitor and the only one with a cyclic peptidic backbone. The solution structure of [ABA(3,11)]SFTI-1, a disulfide-deficient analogue of SFTI-1, has been determined by (1)H NMR spectroscopy. The lowest energy structures of native SFTI-1 and [ABA(3,11)]SFTI-1 are similar and superimpose with a root-mean-square deviation over the backbone and heavy atoms of 0.26 +/- 0.09 and 1.10 +/- 0.22 A, respectively. The disulfide bridge in SFTI-1 was found to be a minor determinant for the overall structure, but its removal resulted in a slightly weakened hydrogen-bonding network. To further investigate the role of the disulfide bridge, NMR chemical shifts for the backbone H(alpha) protons of two disulfide-deficient linear analogues of SFTI-1, [ABA(3,11)]SFTI-1[6,5] and [ABA(3,11)]SFTI-1[1,14] were measured. These correspond to analogues of the cleavage product of SFTI-1 and a putative biosynthetic precursor, respectively. In contrast with the cyclic peptide, it was found that the disulfide bridge is essential for maintaining the structure of these open-chain analogues. Overall, the hydrogen-bond network appears to be a crucial determinant of the structure of SFTI-1 analogues.     

Chemical synthesis and kinetic study of the smallest naturally occurring trypsin inhibitor SFTI-1 isolated from sunflower seeds and its analogues

The smallest known naturally occurring trypsin inhibitor SFTI-1 (14 amino acid residues head-to-tail cyclic peptide containing one disulfide bridge) and its two analogues with one cycle each were synthesized by the solid phase method. Their trypsin inhibitory activity was determined as association equilibrium constants (K(a)). Additionally, hydrolysis rates with bovine beta-trypsin were measured. Among all three peptides, the wild SFTI-1 and the analogue with the disulfide bridge only had, within the experimental error, the same activity (the K(a) values 1.1 x 10(10) and 9.9 x 10(9) M(-1), respectively). Both peptides displayed unchanged inhibitory activity up to 6 h. The trypsin inhibitory activity of the analogue with the head-to-tail cycle only was 2.4-fold lower. It was also remarkably faster hydrolyzed (k = 1.1 x 10(-4) mol(peptide) x mol(enzyme)(-1) x s(-1)) upon the incubation with the enzyme than the other two peptides. This indicates that the head-to-tail cyclization is significantly less important than the disulfide bridge for maintaining trypsin inhibitory activity.     

Cyclic parathyroid hormone related protein antagonists: lysine 13 to aspartic acid 17 [i to (i + 4)] side chain to side chain lactamization.

Cyclization of parathyroid hormone related protein (7-34)amide [PTHrP(7-34)NH2] via covalent bond formation between the epsilon-amino of Lys13 and the beta-carboxyl of Asp17 yielded a 20-membered ring lactam. This analogue, [Lys13,Asp17]PTHrP(7-34)NH2, was 5-10-fold more potent than the linear parent peptide (Kb = 15 and 18 nM in PTH receptor binding assays, and Ki = 130 and 17 nM in PTH-stimulated adenylate cyclase assays in bovine renal cortical membrane and in human bone derived B10 cells, respectively). In contrast, a linear analogue in which charges in positions 13 and 17 were eliminated and other stereoisomers of the above-mentioned lactam in which either Lys13 and/or Asp17 were replaced by the corresponding D-amino acids were much less potent with regard to antagonist bioactivity than the parent peptide. The rationale for the design of the lactam as well as the conformational implications for the PTHrP sequence in light of reported models suggested for the 1-34 peptide are described. The potential use of conformationally constrained analogues for elucidating the "bioactive conformation" of antagonists and for the design of substantially simplified molecular structures for antagonists is discussed.     

Toward parathyroid hormone minimization: conformational studies of cyclic PTH(1-14) analogues

The N-terminal fragment of PTH(1-34) is critical for PTH1 receptor activation. Various modifications of PTH(1-14) have been shown to result in a considerable increase in signaling potency [Shimizu et al. (2000) J. Biol. Chem. 275, 21836-21843]. Our structural investigations revealed an unusually stable helical structure of the signaling domain (1-14), where residues 6 (Gln) and 10 (Gln or Asn) were located on the same face of the alpha-helix. To test whether a stable N-terminal alpha-helix is required for productive interaction with PTH1 receptor, we designed two conformationally restricted PTH(1-14) analogues, each containing a lactam bridge at positions 6 and 10. Specifically, substitutions Gln(6)-->Glu(6) and Asn(10)-->Lys(10) were introduced into the most potent [Ala(1,3,12),Gln(10),Har(11),Trp(14)]PTH(1-14)NH2 agonist. Both the Glu(6)-Lys(10) and Lys(6)-Glu(10) lactam-bridged analogues were characterized to examine the importance of orientation of the lactam. According to biological studies [Shimizu et al. (2003) Biochemistry 42, 2282-2290], none of the 6/10 substituted analogues (linear or cyclic) remained as active as the parent peptide. However, relative to their corresponding linear peptides, lactam-bridged analogues either maintained potency or showed 6-fold improvement. High-resolution structures as determined by 1H NMR and NOE-restrained molecular dynamics simulations clearly illustrate the structural differences between the linear and cyclic PTH(1-14) fragments, supporting the hypothesis that an alpha-helix is the preferred bioactive conformation of the N-terminal fragment of PTH. In addition, our results demonstrate that the structural order of the very first residues (1-4) of the signaling domain plays a significant role in PTH action.     

Synthesis and biological activities of new side chain and backbone cyclic bradykinin analogues.

A series of conformationally constrained cyclic analogues of the peptide hormone bradykinin (BK, Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) was synthesized to check different turned structures proposed for the bioactive conformation of BK agonists and antagonists. Cycles differing in the size and direction of the lactam bridge were performed at the C- and N-terminal sequences of the molecule. Glutamic acid and lysine were introduced into the native BK sequence at different positions for cyclization through their side chains. Backbone cyclic analogues were synthesized by incorporation of N-carboxy alkylated and N-amino alkylated amino acids into the peptide chain. Although the coupling of Fmoc-glycine to the N-alkylated phenylalanine derivatives was effected with DIC/HOAt in SPPS, the dipeptide building units with more bulky amino acids were pre-built in solution. For backbone cyclization at the C-terminus an alternative building unit with an acylated reduced peptide bond was preformed in solution. Both types of building units were handled in the SPPS in the same manner as amino acids. The agonistic and antagonistic activities of the cyclic BK analogues were determined in rat uterus (RUT) and guinea-pig ileum (GPI) assays. Additionally, the potentiation of the BK-induced effects was examined. Among the series of cyclic BK agonists only compound 3 with backbone cyclization between positions 2 and 5 shows a significant agonistic activity on RUT. To study the influence of intramolecular ring closure we used an antagonistic analogue with weak activity, [D-Phe7]-BK. Side chain as well as backbone cyclization in the N-terminus of [D-Phe7]-BK resulted in analogues with moderate antagonistic activity on RUT. Also, compound 18 in which a lactam bridge between positions 6 and 9 was achieved via an acylated reduced peptide bond has moderate antagonistic activity on RUT. These results support the hypothesis of turn structures in both parts of the molecule as a requirement for BK antagonism. Certain active and inactive agonists and antagonists are able to potentiate the bradykinin-induced contraction of guinea-pig ileum.     

Synthesis and helical structure of lactam bridged BH3 peptides derived from pro-apoptotic Bcl-2 family proteins

Protein-protein interactions within the Bcl-2 family are mediated by the helical BH3 domains of pro-apoptotic family members. To study the mechanism of this BH3 domain-protein interaction, a series of cyclic lactam bridged BH3 peptide analogues were synthesized by a novel combined Fmoc/tBu/Bzl protections strategy. These peptide analogues were studied by circular dichroism spectroscopy and found to adopt highly helical structure. These helical peptides stabilized by a lactam bridge serve as useful models to analyze the structure-function relationship of the pro-apoptotic BH3 domains. Furthermore, the synthetic method for lactam bridge incorporation reported here may find application in studies of other helical structures and development of helix mimics.     

A tetradecapeptide somatostatin dicarba-analog: Synthesis,structural impact and biological activity

We described here the first tetradecapeptide somatostatin-analogue where the disulfide bridge has been replaced by a carbon–carbon double bond. This analogue was prepared using microwave assisted ring closing metathesis (RCM) using the 2nd generation Grubbs as catalyst. Under our optimized conditions the cyclization between allylGly 3 and 14 proceeded in moderate yield, excellent cyclic/linear ratio and very high Z-double bond selectivity. NMR studies also demonstrated that the conformational flexibility of this peptide is increased in comparison to that of the natural hormone. Remarkably, this alkene-bridged somatostatin analog is highly selective against somatostatin receptors 1 and 5, suggesting that conformational rigidity is not required for the efficient interaction of somatostatin analogues with these two receptors.     

Epitopes associated with a synthetic hepatitis B surface antigen peptide

A synthetic peptide (SP1), corresponding to the amino acid residues 122 through 137 of the major polypeptide derived from hepatitis B surface antigen (HBsAg), subtype ayw, was analyzed for the presence of the major epitopes of HBsAg. Both a cyclic form, produced by introduction of an intrachain disulfide bond, and a linear form of the peptide were characterized. A panel of monoclonal antibodies with defined specificity for the cross-reactive group a antigenic determinant(s) and for the y and w subtype specificities was used for this analysis. The cyclic, but not the linear, form of SP1 reacted with five of 14 anti-a monoclonal antibodies, demonstrating that the cyclic peptide contains a conformation-dependent a epitope. Only one anti-a antibody was found to react with both cyclic and linear forms of SP1. Because SP1 failed to react with the remaining 8 anti-a monoclonal antibodies, it was concluded that the a antigenic reactivity associated with HBsAg contains an additional epitope(s) unrelated to that expressed on SP1. Both cyclic and linear SP1 reacted with three of three anti-y monoclonal antibodies, indicating that a sequential y epitope is also present on SP1; no w reactivity was detected. Analysis of the idiotypes associated with the monoclonal antibodies showed those that combined with cyclic SP1 also inhibited the binding of a common human anti-HBs (CHBs) idiotype with its rabbit anti-idiotype serum, whereas a monoclonal antibody that did not react with the cyclic SP1 epitope failed to inhibit the CHBs idiotype-anti-idiotype reaction. Thus, the conformational a epitope present on cyclic SP1 appears to contain the predominant epitope recognized by humans in response to a natural HBV infection.     

Structure-activity relationship studies of gomesin: importance of the disulfide bridges for conformation, bioactivities, and serum stability

Gomesin is an antimicrobial peptide isolated from hemocytes of the Brazilian spider Acanthoscurria gomesiana that contains two disulfide bridges Cys(2-15)/Cys(6-11) and presents a beta-hairpin structure. To investigate the role of the disulfide bridges on gomesin conformation, bioactivities, and serum stability, structure-activity relationship (SAR) studies were conducted. Initially, gomesin and variants lacking one or both disulfide bridges were synthesized. CD studies showed that the gomesin structure is very rigid independently of the solvent environment. On the other hand, the linearized analogues adopted secondary structures according to the environment, while the monocyclic disulfide-bridged peptides had a tendency to adopt a turn structure. The absence of one or both bridges resulted in a decrease in the antimicrobial and hemolytic activities. In addition, serum stability studies revealed that, contrasting to gomesin that was stable even after 48 h of incubation, the linearized analogues were rapidly degraded. The replacement of the disulfide bounds by lactam bridges led to monocyclic and bicyclic compounds. SAR studies indicated that the monocyclic lactam-bridged analogues tend to assume a alpha-helical structure being less potent, hemolytic, and serum stable than the wild-type gomesin. On the other hand, the bicyclic lactam/disulfide-bridged analogues displayed a similar conformation and degradation kinetics identical to gomesin. However, the antimicrobial activity appeared to be dependent on the lactam bridge position and size. These findings indicated that (i) the secondary structure plays a pivotal role for the full activity of gomesin; (ii) the antimicrobial and hemolytic activities of gomesin are correlated events; (iii) while at least one of the disulfide bridges is needed for the maintenance of a significant antimicrobial activity of gomesin, both bridges are required for high serum stability and optimal conformation; and finally (iv) the best analogue obtained was the bicyclo (2-15,6-11)[Glu2, Cys(6,11), Lys15]-Gm since it is as stable and potent as gomesin.     

Functional fine-mapping and molecular modeling of a conserved loop epitope of the measles virus hemagglutinin protein.

Neutralizing and protective monoclonal antibodies (mAbs) were used to fine-map the highly conserved hemagglutinin noose epitope (H379-410, HNE) of the measles virus. Short peptides mimicking this epitope were previously shown to induce virus-neutralizing antibodies [El Kasmi et al. (2000) J. Gen. Virol.81, 729-735]. The epitope contains three cysteine residues, two of which (Cys386 and Cys394) form a disulfide bridge critical for antibody binding. Substitution and truncation analogues revealed four residues critical for binding (Lys387, Gly388, Gln391 and Glu395) and suggested the binding motif X7C[KR]GX[AINQ]QX2CEX5 for three distinct protective mAbs. This motif was found in more than 90% of the wild-type viruses. An independent molecular model of the core epitope predicted an amphiphilic loop displaying a remarkably stable and rigid loop conformation. The three hydrophilic contact residues Lys387, Gln391 and Glu395 pointed on the virus towards the solvent-exposed side of the planar loop and the permissive hydrophobic residues Ile390, Ala392 and Leu393 towards the solvent-hidden side of the loop, precluding antibody binding. The high affinity (Kd = 7.60 nm) of the mAb BH216 for the peptide suggests a high structural resemblance of the peptide with the natural epitope and indicates that most interactions with the protein are also contributed by the peptide. Improved peptides designed on the basis of these findings induced sera that crossreacted with the native measles virus hemagglutinin protein, providing important information about a lead structure for the design of more stable antigens of a synthetic or recombinant subunit vaccine.     

Mimetics of the disulfide bridge between the N- and C-terminal cysteines of the KLK3-stimulating peptide B-2

Human prostate produces kallikrein-related peptidase 3 (KLK3, also known as prostate specific antigen), which is widely used as a prostate cancer marker. Proteolytically active KLK3 has been shown to inhibit angiogenesis and its expression decreases in poorly differentiated tumors. Thus, it may be possible to control prostate cancer growth with agents that stimulate the proteolytic activity of KLK3. We have earlier developed synthetic peptides, which bind specifically to KLK3 and promote its proteolytic activity. These peptides are cyclic, all containing a disulfide bridge between the N- and C-terminal cysteines. To increase the in vivo stability of the KLK3-stimulating peptide B-2, we made differently cyclized analogues by replacing both terminal cysteines and the disulfide bridge between them. A replacement consisting of γ-amino butyric acid and aspartic acid, where the amino group from the former was linked to the main chain carboxyl group of the latter, was found to be, at high concentrations, more active than the B-2 peptide. Furthermore, as compared to the parent peptide, this analog had an improved stability in plasma and against the enzymatic degradation by KLK3. In addition, the series of analogues also provided valuable information of the structure–activity relationships of the B-2 peptide.     

Implication of the disulfide bridge in trypsin inhibitor SFTI-1 in its interaction with serine proteinases

Fourteen monocyclic analogues of trypsin inhibitor SFTI-1 isolated from sunflower seeds were synthesized by the solid-phase method. The purpose of this work was to establish the role of a disulfide bridge present in inhibitor’s side chains of Cys3 and Cys11 in association with serine proteinases. This cyclic fragment was replaced by the disulfide bridges formed by l-pencillamine (Pen), homo-l-cysteine (Hcy), N-sulfanylethylglycine (Nhcy) or combination of the three with Cys. As in the substrate specificity the P₁ position of the synthesized analogues Lys, Nlys [N-(4-aminobutyl)glycine], Phe or Nphe (N-benzylglycine) were present, and they were checked for trypsin and chymotrypsin inhibitory activity. The results clearly indicated that Pen and Nhcy were not acceptable at the position 3, yielding inactive analogues, whereas another residue (Cys11) could be substituted without any significant impact on the affinity towards proteinase. On the other hand, elongation of the Cys3 side chain by introduction of Hcy did not affect inhibitory activity, and an analogue with the Hcy–Hcy disulfide bridge was more than twice as effective as the reference compound ([Phe⁵] SFTI-1) in inhibition of bovine α-chymotrypsin.     

Synthesis and evaluation of disulfide bond mimetics of amylin-(1-8) as agents to treat osteoporosis

Osteoporotic fracture is a significant public health problem, resulting in fractures in >50% of women and in almost one third of men age 65 and older. Most of the existing therapies act by slowing bone loss, through inhibiting the action of bone resorbing cells. However, more substantial reductions of fracture numbers will only result from treatments that can rebuild bone. Our own animal studies demonstrated the anabolic potential of the small but unstable octapeptide fragment of amylin-(1-37), namely amylin-(1-8) containing one disulfide bridge (Cys/2 and Cys/7) [Am. J. Physiol. Endocrinol. Metab.2000, 279, E730]. Herein, we describe the synthesis of amylin-(1-8) octapeptide and seven analogues thereof wherein the disulfide bridge is modified either via insertion of different linkers or bridges of a different nature in order to improve the stability and/or bone anabolic activity of the parent peptide. The peptide analogues were screened for proliferative activity in primary foetal rat bone-forming cells or osteoblasts at physiological concentrations. One such analogue showed promising biological activity.     

Structural and functional studies with antibodies to the integrin beta 2 subunit. A model for the I-like domain

To establish a structure and function map of the beta2 integrin subunit, we mapped the epitopes of a panel of beta2 monoclonal antibodies including function-blocking, nonblocking, and activating antibodies using human/mouse beta2 subunit chimeras. Activating antibodies recognize the C-terminal half of the cysteine-rich region, residues 522-612. Antibodies that do not affect ligand binding map to residues 1-98 and residues 344-521. Monoclonal antibodies to epitopes within a predicted I-like domain (residues 104-341) strongly inhibit LFA-1-dependent adhesion. These function-blocking monoclonal antibodies were mapped to specific residues with human --> mouse knock-out or mouse --> human knock-in mutations. Combinatorial epitopes involving residues distant in the sequence provide support for a specific alignment between the beta-subunit and I domains that was used to construct a three-dimensional model. Antigenic residues 133, 332, and 339 are on the first and last predicted alpha-helices of the I-like domain, which are adjacent on its "front." Other antigenic residues in beta2 and in other integrin beta subunits are present on the front. No antigenic residues are present on the "back" of the domain, which is predicted to be in an interface with other domains, such as the alpha subunit beta-propeller domain. Most mutations in the beta2 subunit in leukocyte adhesion deficiency are predicted to be buried in the beta2 subunit I-like domain. Two long insertions are present relative to alpha-subunit I-domains. One is tied down to the back of the I-like domain by a disulfide bond. The other corresponds to the "specificity-determining loop" defined in beta1 and beta3 integrins and contains the antigenic residue Glu(175) in a disulfide-bonded loop located near the "top" of the domain.     

Biological and conformational evaluation of angiotensin II lactam bridge containing analogues

Angiotensin II (AII) is the active octapeptide product of the renin enzymatic cascade, which is responsible for sustaining blood pressure. In an attempt to establish the AII-receptor-bound conformation of this octapeptide, we designed conformationally constrained analogues by scanning the entire AII sequence with an i-(i+2) and i-(i+3) lactam bridge consisting of an Asp-(Xaa)(n)-Lys scaffold. Most analogues presented low agonistic activity when compared to AII in the different bioassays tested. The exceptions are cyclo(0-1a) [Asp(0), endo-(Lys(1a))]-AII (1) and [Asp(0), endo-(Lys(1a))]-AII (2), both of which showed activity similar to AII. Based on peptide 1 and the analogue cyclo(3-5)[Sar(1), Asp(3), Lys(5)]-AII characterized by Matsoukas et al., we analyzed the agonistic and antagonistic activities, respectively, through a new monocyclic peptide series synthesized by using the following combinations of residues as bridgehead elements for the lactam bond formation: D- or L-Asp combined with D- or L-Lys or L-Glu combined with L-Orn. Six analogues showed an approximately 20% increase in biological activity when compared with peptide (1) and were equipotent to AII. In contrast, six analogues presented antagonistic activity. These results suggest that the position of the lactam bridge is more important than the bridge length or chirality for recognition of and binding to the angiotensin II AT1-receptor.