Investigation of cis/trans proline isomerism in a multiply occurring peptide fragment from human salivary proline-rich glycoprotein

The solution-state conformations of eight proline-containing peptide fragments found in human salivary proline-rich glycoprotein (PRG) were investigated in 2 x distilled water (treated with metal ion chelating resin) using 13C-nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. The peptide sequences and acronyms were as follows: PRG9-2 = NH2-G(1)-P(2)-CONH2, PRG9-3 = NH2-G(1)P(2)-P(3)-CONH2, PRG9-4 = NH2-G(1)-P(2)-P(3)-P(4)-CONH2, PRG9-5 = NH2-G(1)-P(2)-P(3)-P(4)-H(5)-CONH2, PRG9-6 = NH2-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-CONH2, PRG9-7 = NH2-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-G(7)-CONH2, PRG9-8 = NH2-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-G(7)-K(8)-CONH2 and PRG9-9 = NH2-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-G(7)-K(8)-P(9)-CONH2. Sequence-specific resonance assignments from the 13C-NMR spectra indicated that the trans proline isomer dominated the conformations of the peptides. CD results clearly showed the presence of the poly-L-proline II helix as the major conformation in PRG9-3----PRG9-5, supplemented by beta- and/or gamma-turns in PRG9-6----PRG9-9. These data suggest that in "metal free" water, native PRG could contain several small poly-L-proline II helices along with beta- and/or gamma-turns. Since proline is the major amino acid present in native PRG, these localized conformations may contribute to PRG's global conformation and act as a primary force in determining its biological activities.

Folding of chymotrypsin inhibitor 2. 2. Influence of proline isomerization on the folding kinetics and thermodynamic characterization of the transition state of folding

The refolding of chymotrypsin inhibitor 2 (CI2) is, at least, a triphasic process. The rate constants are 53 s-1 for the major phase (77% of the total amplitude) and 0.43 and 0.024 s-1 for the slower phases (23% of the total amplitude) at 25 degrees C and pH 6.3. The multiphase nature of the refolding reaction results from heterogeneity in the denatured state because of proline isomerization. The fast phase corresponds to the refolding of the fraction of protein that has all its prolines in a native trans conformation in the denatured state. It is not catalyzed by peptidyl-prolyl isomerase. The rate-limiting step of folding for the slower phases, however, is proline isomerization, and they are both catalyzed by peptidyl-prolyl isomerase. The slowest phase has properties consistent with a process involving proline isomerization in a denatured state. In particular, the activation enthalpy is large, 16 kcal mol-1 K-1, and the rate is independent of guanidinium chloride concentration ([GdnHCl]). In comparison, the intermediate phase shows properties consistent with a process involving proline isomerization in a partially structured state. The activation enthalpy is small, 8 kcal mol-1 K-1, and the rate has a strong dependence on [GdnHCl]. Temperature dependences of the rate constants for unfolding and for the fast refolding phase, both in the absence and in the presence of GdnHCl, were used to characterize the thermodynamic nature of the transition state and its relative exposure to solvent. The Eyring plot for unfolding is linear, indicating that there is relatively little change in heat capacity between native state and transition state.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the carboxy-terminal lysine of urokinase on the catalysis of plasminogen activation

When single-chain pro-UK is activated by plasmin or kallikrein, the Lys158-Ile159 bond is cleaved, leaving a C-terminal lysine on the A-chain (Lys-UK). Two-chain, high molecular weight urokinase (UK) purified from urine, however, has been shown to contain a phenylalanine residue as the C-terminal of the A-chain (Phe-UK). Since C-terminal lysine residues have a strong binding affinity for plasminogen that may promote its activation, we undertook kinetic studies comparing plasminogen activation by Lys- and Phe-UK. A two-stage method was employed in order to minimize factors known to interfere with plasminogen activation and plasmin determination. The Lys-UK was prepared by plasmin activation of pro-UK purified from human fetal kidney cell culture medium. The Phe-UK was prepared by carboxypeptidase B (CpB) treatment of Lys-UK. Removal of the C-terminal lysine of Lys-UK by CpB produced small but significant increases in the Michaelis constants for the activation of both Glu- and Lys-plasminogen. The apparent Michaelis constants for Glu-plasminogen activation by Lys- and Phe-UK were 3.7 microM +/- .36 microM and 5.9 microM +/- .70 microM, respectively and the Michaelis constants for Lys-plasminogen activation by Lys- and Phe-UK were 5.4 microM +/- .72 microM and 15.2 microM +/- 1.4 microM, respectively. The catalytic efficiency (kcat/Km) of Lys-UK was approximately 2-fold greater than that of Phe-UK for the activation of either Glu- or Lys-plasminogen. When the fibrinolytic activities of Lys- and Phe-UK were compared in a plasma milieu no significant differences were detected. In conclusion, the findings indicate that the C terminal lysine on the A-chain of UK significantly promotes the catalysis of plasminogen in a purified system. However, the higher catalytic efficiency of Lys-UK was not found to induce significant acceleration of clot lysis at pharmacological concentrations in plasma.

NMR studies of an oligoproline-containing peptide analogue that binds specifically to the H-2Kd histocompatibility molecule

T lymphocytes expressing variable cell surface antigen receptors recognize "processed" forms of antigen, presented on the surface of other cells by molecules of the major histocompatibility complex (MHC). Naturally processed antigenic peptides can be replaced by synthetic ones. The synthetic peptide AYPPPPPTLA (P5) is an active competitor to the antigenic peptide HLA A24 170-182 (sequence RYLENGKETLQRA) that is recognized by A24 specific T cells in association with the H-2Kd class I MHC molecule. In P5 the five prolines were designed to play the role of a rigid spacer between the residue Y and the T-L unit, so as to mimic the role of Y171, T178, and L179 in the HLA A24 antigenic peptide, since these residues have proven to be the most important with respect to the binding of the HLA A24 peptide with the H-2Kd MHC molecule. Nuclear magnetic resonance studies allow us to demonstrate that in aqueous solution P5 adopts at least three long-lived conformations that can be classified with respect to the Y2-P3-P4 amide bonds as trans-trans, cis-trans, and cis-cis. Among these, the trans-trans form is present in 67% of the molecules while the two others share the remaining 33%.

Secondary structure and side-chain 1H and 13C resonance assignments of calmodulin in solution by heteronuclear multidimensional NMR spectroscopy

Heteronuclear 2D and 3D NMR experiments were carried out on recombinant Drosophila calmodulin (CaM), a protein of 148 residues and with molecular mass of 16.7 kDa, that is uniformly labeled with 15N and 13C to a level of greater than 95%. Nearly complete 1H and 13C side-chain assignments for all amino acid residues are obtained by using the 3D HCCH-COSY and HCCH-TOCSY experiments that rely on large heteronuclear one-bond scalar couplings to transfer magnetization and establish through-bond connectivities. The secondary structure of this protein in solution has been elucidated by a qualitative interpretation of nuclear Overhauser effects, hydrogen exchange data, and 3JHNH alpha coupling constants. A clear correlation between the 13C alpha chemical shift and secondary structure is found. The secondary structure in the two globular domains of Drosophila CaM in solution is essentially identical with that of the X-ray crystal structure of mammalian CaM [Babu, Y., Bugg, C. E., & Cook, W.J. (1988) J. Mol. Biol. 204, 191-204], which consists of two pairs of a "helix-loop-helix" motif in each globular domain. The existence of a short antiparallel beta-sheet between the two loops in each domain has been confirmed. The eight alpha-helix segments identified from the NMR data are located at Glu-6 to Phe-19, Thr-29 to Ser-38, Glu-45 to Glu-54, Phe-65 to Lys-77, Glu-82 to Asp-93, Ala-102 to Asn-111, Asp-118 to Glu-127, and Tyr-138 to Thr-146. Although the crystal structure has a long "central helix" from Phe-65 to Phe-92 that connects the two globular domains, NMR data indicate that residues Asp-78 to Ser-81 of this central helix adopt a nonhelical conformation with considerable flexibility.

The Pro117 to glycine mutation of staphylococcal nuclease simplifies the unfolding-folding kinetics

Kinetics of unfolding and refolding of a staphylococcal nuclease mutant, in which Pro117 is replaced by glycine, have been investigated by stopped-flow circular dichroism, and the results are compared with those for the wild-type protein. In contrast to the biphasic unfolding of the wild-type nuclease, the unfolding of the mutant is represented by a single-phase reaction, indicating that the biphasic unfolding for the wild-type protein is caused by cis-trans isomerization about the prolyl peptide bond in the native state. The proline mutation also simplifies the kinetic refolding. Importance of the results in elucidating the folding mechanism is discussed.

Vibrational circular dichroism spectra of unblocked proline oligomers

Vibrational Circular Dichroism (VCD) spectra of unblocked L-proline oligopeptides, (Pro)n n = 3 to 7, dissolved in D2O are reported. For these oligomers, the VCD spectra can be attributed to a conformational dominance of the trans amide conformation with subunits interrelated by a left-handed twist, particularly for the longer oligomers. As a function of oligomer length, formation of this conformation starts at n = 3; and by n = 5 a spectrum closely resembling that of the poly-L-proline II helix in shape and magnitude is seen. The VCD data are compared with previous (Pro)n results using IR, CD, Raman and NMR spectroscopies, and reasons for the variations in interpretation are discussed.

Missense mutation serine106----proline causes 17 alpha-hydroxylase deficiency

Steroid 17 alpha-hydroxylase deficiency is caused by defects in cytochrome P450c17, the single enzyme that has 17-alpha hydroxylase and 17,20-lyase activities. We describe a rapid and efficient polymerase chain reaction tactic for identifying these genetic lesions and identify Ser106----Pro as the cause of 17 alpha-hydroxylase deficiency in two unrelated homozygous patients from Guam. We used site-directed mutagenesis of the normal P450c17 cDNA to construct the Pro106 mutant, and expressed both the normal and mutant sequences in monkey COS-1 cells and in yeast. Expression of the normal sequence permitted the cells to convert pregnenolone to 17-OH pregnenolone, progesterone to 17-OH progesterone, and 17-OH pregnenolone to dehydroepiandrosterone, showing the normal sequence conferred both 17 alpha-hydroxylase and 17,20-lyase activities. Expression of the mutant sequence generated P450c17 mRNA, but conferred none of these activities, proving that the Ser106----Pro mutation abolished the 17 alpha-hydroxylase and 17,20-lyase activities. An HhaI restriction site created by the mutation should permit screening of large populations.

Cell adhesion promoting peptide GVKGDKGNPGWPGAP from the collagen type IV triple helix: cis/trans proline-induced multiple 1H NMR conformations and evidence for a KG/PG multiple turn repeat motif in the all-trans proline state

Peptide GVKGDKGNPGWPGAPY (called peptide IV-H1), derived from the protein sequence of human collagen type IV, triple-helix domain residues 1263-1277, represents an RGD-independent, cell-specific, adhesion, spreading, and motility promoting domain in type IV collagen. In this study, peptide IV-H1 has been investigated by 1H NMR (500 MHz) spectroscopy. Cis-trans proline isomerization at each of the three proline residues gives rise to a number of slowly exchanging (500-MHz NMR time scale) conformation states. At least five such states are observed, for example, for the well-resolved A14 beta H3 group, and K3, which is six residues sequentially removed from the nearest proline, i.e., P9, shows two sets. The presence of more than two sets of resonances for residues sequentially proximal to a proline, e.g., A14-cis-P15 and A14-trans-P15, and more than one set for a residue sequentially well-removed from a proline, e.g., K3, indicates long range conformation interactions and the presence of preferred structure in this short linear peptide. Many resonances belonging to these multiple species have been assigned by using mono-proline-substituted analogues. Conformational (isomer) state-specific 2D 1H NMR assignments for the combination of cis and trans proline states have been made via analysis of COSY-type, HOHAHA, and NOESY spectra. Peptide IV-H1 in the all-trans proline state ttt exists in relatively well-defined conformation populations showing numerous short- and long-range NOEs and long-lived backbone amide protons and reduced backbone NH temperature coefficients, suggesting hydrogen-bonding, and structurally informative 3J alpha N coupling constants. The NMR data indicate significant beta-turn populations centered at K3-G4, K5-G6, P9-G10, and P12-G13, and a C-terminal gamma-turn within the A14-P15-Y16 sequence. These NMR data are supported by circular dichroic studies which indicate the presence of 52% beta-turn, 10% helix, and 38% random coil structural populations. Since equally spaced KG and PG residues are found on both sides of peptide IV-H1 in the native collagen type IV sequence, this multiple turn repeat motif may continue through a longer segment of the protein. Synthetic peptide IV-H1 overlapping sequence "walk throughs" indicate that the primary biological activity is localized in the GNPGWPGAP double beta-turn domain, which contains the backbone constraining proline residues. This proline-domain conformation may suggest a collagen type IV receptor-specific, metastatic cell adhesion promoting binding domain.

The conserved, buried aspartic acid in oxidized Escherichia coli thioredoxin has a pKa of 7.5. Its titration produces a related shift in global stability

Aspartic acid 26 in Escherichia coli thioredoxin is located at the bottom of a hydrophobic cavity, near the redox-active disulfide of the active site. Asp 26 is embedded in the protein except for part of the surface of one carboxyl oxygen. The high degree of evolutionary conversion of Asp 26 suggests that it plays a critical role in thioredoxin function. We have determined the pKa of Asp 26 by a novel electrophoretic method based on the relative electrophoretic mobilities of wild-type thioredoxin and of D26A thioredoxin (with Asp 26 replaced by alanine). The pKa of Asp 26 determined by this technique is 7.5, more than 3 units above the pKa of a solvated carboxyl side chain. The titration of Asp 26 is thermodynamically linked to the stability of thioredoxin. As expected if thioredoxin stability depends on the ionization state of Asp 26, delta Go WT, the free energy of the cooperative denaturation reaction of wild-type thioredoxin by guanidine hydrochloride, varies with pH in a sigmoidal fashion in the vicinity of pH 7.5. Over the same pH range, the free energy for D26A folding, delta Go D26A, is pH independent and D26A is highly stabilized compared to wild type. From the thermodynamic cycle describing the linkage of Asp 26 titration to thioredoxin stability, the difference in free energy between wild-type thioredoxin with protonated Asp 26 and wild-type thioredoxin with deprotonated Asp 26, delta delta Go (COOH----COO-), is calculated to be 4.9 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Heteronuclear 2D NMR studies of an engineered insulin monomer: assignment and characterization of the receptor-binding surface by selective 2H and 13C labeling with application to protein design

Insulin provides an important model for the application of genetic engineering to rational protein design and has been well characterized in the crystal state. However, self-association of insulin in solution has precluded complementary 2D NMR study under physiological conditions. We demonstrate here that such limitations may be circumvented by the use of a monomeric analogue that contains three amino acid substitutions on the protein surface (HisB10----Asp, ProB28----Lys, and LysB29----Pro); this analogue (designated DKP-insulin) retains native receptor-binding potency. Comparative 1H NMR studies of native human insulin and a series of three related analogues--(i) the singly substituted analogue [HisB10----Asp], (ii) the doubly substituted analogue [ProB28----Lys; LysB29----Pro], and (iii) DKP-insulin--demonstrate progressive reduction in concentration-dependent line-broadening in accord with the results of analytical ultracentrifugation. Extensive nonlocal interactions are observed in the NOESY spectrum of DKP-insulin, indicating that this analogue adopts a compact and stably folded structure as a monomer in overall accord with crystal models. Site-specific 2H and 13C isotopic labels are introduced by semisynthesis as probes for the structure and dynamics of the receptor-binding surface. These studies confirm and extend under physiological conditions the results of a previous 2D NMR analysis of native insulin in 20% acetic acid [Hua, Q. X., & Weiss, M. A. (1991) Biochemistry 30, 5505-5515]. Implications for the role of protein flexibility in receptor recognition are discussed with application to the design of novel insulin analogues.

Folding studies of the cysteine proteinase inhibitor--human stefin A

Reversible GuHCl denaturation of human stefin A (25 degrees C, pH 8) was monitored by the tyrosine fluorescence, by circular dichroism in the near UV and by circular dichroism in the far UV. In each case a midpoint of 2.8 +/- 0.1 M GuHCl was obtained, demonstrating the cooperativity of the denaturation. Kinetics of the slow folding on diluting the protein from the GuHCl denatured state, was also measured by the three spectroscopic probes (10 degrees C, pH 8). Results conform to a sequential mechanism. Denaturant concentration and temperature dependence of the slow folding were measured by fluorescence. From a linear Arrhenius plot the Ea of 100 +/- 5 kJ/mol was read. 'Double mixing' experiments revealed a slow reaction going on in the unfolded state which influenced the amplitude of the fluorescence changes. 'Double mixing' experiments performed by FPLC have shown that the folding itself, i.e., the formation of a compact state, was not dependent on the time spent under unfolding conditions.

The effect of hydration stress solutes on the phase behavior of hydrated dipalmitoylphosphatidylcholine

We have investigated the interaction of solutes found to accumulate in biological systems during chilling, dehydration, and salt stress with fully hydrated multilamellar and unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC). We have focused on a series of mono-, di-, and tri-substituted amines (glycine, 4-hydroxyproline, proline, and betaine) and contrasted the action of these solutes to trehalose, a protective disaccharide. Differential scanning calorimetry studies show that when DPPC is scanned in the presence of increasing concentrations of these solutes (up to 3 M), there is a moderate increase in the pre-transition temperature (1-6 degrees C) with a smaller increase (1-2 degrees C) in the main transition temperature of hydrated multilamellar vesicles of DPPC. Other calorimetric parameters (delta H, delta T1/2, Cpmax) determined for the pre-transition and main transition were similar independent of the solute. In each case, the main phase transition was broadened with increasing solute while the transition enthalpy was not significantly affected.

Alteration of HIV-1 infectivity and neutralization by a single amino acid replacement in the V3 loop domain

The V3 loop (residues 303-338) of the human immunodeficiency virus type 1 (HIV-1) gp120 envelope protein represents a principal neutralizing determinant for the virus. An HIV-1 proviral clone containing a mutation in the V3 loop was constructed in which the proline residue at position 313 was changed to an alanine (P313-A). This mutation alters the conserved GPGR sequence that is found in the V3 loop sequences of different HIV-1 isolates. The P313-A clone produced virus particles, which were infectious for a number of T-cell lines including MOLT-4, CEM, and SupT1, but demonstrated a relatively low infectivity on the AA5 B-cell line when compared with wild-type viruses, HTLV-IIIB, HXB2/10 (a chimeric molecular clone), and another mutant virus (Q290-T). V3 loop-specific neutralizing polyclonal sera and the 9284 monoclonal antibody, which recognizes the amino side of the V3 loop sequence, effectively blocked infectivity and syncytia formation of all viruses tested. In contrast, the 0.5 beta monoclonal antibody, which is biologically more potent than 9284 and recognizes a different V3 loop determinant, failed to neutralize the P313-A virus. These results suggest that the proline residue in the relatively conserved GPGR "turn" region of the V3 loop is crucial for recognition by the 0.5 beta antibody. The observed variation in sensitivity of the B-cell line to the P313-A virus may reflect the presence of cell-specific factors which could be important in establishing an HIV-1 infection.

Synthesis, conformational properties, and antibody recognition of peptides containing beta-turn mimetics based on alpha-alkylproline derivatives

Peptide recognition by monoclonal antibodies may provide a useful model for drug development, in particular to test the effects of conformational restriction on ligand binding. We have tested the influence of novel peptide mimetics upon conformation and binding affinity for the case of monoclonal antibodies raised to a peptide antigen which displays a preference for a beta-turn conformation in aqueous solution. Two monoclonals were isolated that recognized the peptide Ac-Tyr-Pro-Tyr-Asp-Val-Pro-Asp-Tyr-Ala specifically at the beta-turn formed by Tyr-Pro-Tyr-Asp. Peptide analogues were then synthesized containing mimetics designed to stabilize this conformation. One, analogue (3), contained a spirocyclic gamma-lactam bridge between the alpha-position of proline-2 and the N atom of tyrosine-3, while another (2) contained (S)-alpha-methylproline at position 2. NMR spectroscopy and molecular modeling suggest that both analogues adopt reverse-turn conformations stabilized relative to that in the native sequence. For the (S)-alpha-methylproline analogue binding to both monoclonal antibodies was substantially improved, compared with the native antigen, whereas the gamma-lactam analogue (3) was not recognized by either antibody. Quantitative equilibrium ultrafiltration binding assays showed that the affinities of the (S)-alpha-methylproline analogue (2) for the two antibodies were improved over those measured with the native antigen by -2.3 and -0.65 kcal/mol. The origins of these free energy differences cannot be explained wholly on the basis of presumed extra hydrophobic contacts between the new methyl substituent and the antigen binding sites. We propose that the increased conformational stability of the analogue plays a decisive role, implying that the reverse turn detected in the native antigen, possibly a type-I turn, is important for recognition by the two antibodies.

Conformation and formation tendency of the cyclotetrapeptide cyclo (D-Pro-D-Pro-L-Pro-L-Pro): experimental results and molecular modeling studies

The title compound represents the smallest member of cyclic proline peptides corresponding to the general formula c(DDLL-Pro4)n with a strictly D,D,L,L double-alternating sequence of the chiral amino acid residues. The cyclopeptides with n greater than or equal to 2 could be synthesized from both DDLL-Pro4 (1) and DLLD-Pro4 (2). The cyclic monomer (n = 1) resulted only from 2, whereas not even a trace could be found by cyclization of 1. The peptide exists in a strongly strained Ci symmetrical conformation (x-ray analysis) with alternating cis and trans peptide bonds (ctct form I). The cis peptide bonds deviate from planarity (omega = 22 degrees); two of the pyrrolidine rings show a "South" conformation (phi = -94 degrees), whereas the other residues exhibit C alpha-endo puckering (phi = -124 degrees). Two of the psi angles surprisingly occur at +41 degrees (anti-cis'), the others are located in the trans' region. A quantitative ring opening occurs with trifluoroacetic acid at room temperature. In solution the existence of an isomeric ctcc sequence (form Ia) is indicated. Dreiding model studies also suggested a favorable conformation with a tctc sequence (form II). Consequently, we performed molecular mechanics calculations, based on the CHARMM force field and semiempirical quantum mechanical AM1 calculations (MOPAC program). Pronounced differences in the backbone parameters were found using these two methods. However, the theoretical studies evidenced the experimentally obtained differences in the cyclization tendencies of the linear precursors.

Biological and conformational studies of [Val4]morphiceptin and [D-Val4]morphiceptin analogs incorporating cis-2-aminocyclopentane carboxylic acid as a peptidomimetic for proline

We report the synthesis, biological activity, and conformational analysis of tetrapeptide analogs related to [Val4]morphiceptin and [D-Val4]morphiceptin in which the proline at the second position has been replaced with cis-2-aminocyclopentane carboxylic acid (cis-2-Ac5c). Since the cis-2-Ac5c residue contains a normal amide, only the trans form has been observed about the amide bond between the first and second residues. The cis-2-Ac5c is a beta amino acid with two chiral centers resulting in two possible configurational isomers, namely (1S, 2R) and 1R, 2S) forms. The analogs containing the (1S, 2R)-Ac5c residue show activity at the mu-receptor but are inactive at the delta-receptor, resulting in a high selectivity for the mu-receptor. The (1R,2S)-Ac5c containing analogs are completely inactive at both the mu- and delta-receptors. The conformational analysis indicates that the separation of the aromatic rings of the tyrosine and phenylalanine residues, as expressed by the center-to-center distance, is 10.1-12.7 A for the preferred conformations of the bioactive analogs containing the (1S,2R)-Ac5c residue while a range of 4.8-7.0 A is observed for the preferred conformations of the inactive analogs with the (IR,2S)-Ac5c residue. A comparison of the findings from the conformational analysis and biological assays establishes the fact that a relatively large separation of the two aromatic side chains is required for the mu-opioid receptor activity of these molecules. Since the tetrapeptide amides studied in this investigation show similar biological profiles to those of the morphiceptin-related analogs, we have compared the preferred conformations estimated for the cis-2-Ac5c containing analogs with those of morphiceptin. One of the low energy conformations calculated for morphiceptin with the cis form about the tyrosine and proline residues has considerable topological similarity with the bioactive analogs containing the (1S,2R)-Ac5c residue, indicating that the cis from about these two residues is required for the biological activity of the morphiceptin-related analogs containing the proline at the second position.

Proline kinks in transmembrane alpha-helices

Integral membrane proteins often contain proline residues in their presumably alpha-helical transmembrane segments. This is in marked contrast to globular proteins, where proline is rarely found inside alpha-helices. Proline residues cause kinks in helices, and, in addition to leaving the i-4 backbone carbonyl without its normal hydrogen bond donor, also sterically prevent the (i-3)-carbonyl-(i + l)-amide backbone hydrogen bond from forming. Here, some structural aspects of proline kinks in transmembrane helices are discussed on the basis of an analysis of Pro-kinked helices in the photosynthetic reaction center and bacteriorhodopsin, as well as results from an analysis of Pro-containing transmembrane segments identified in the NBRF Protein Sequence Databank.

Thermo-responsive hydrogels based on acryloyl-L-proline methyl ester and their use as long-acting testosterone delivery systems

New thermo-responsive hydrogels were synthesized by copolymerizing acryloyl-L-proline methyl ester (A-ProOMe) with minor amounts of 2-hydroxypropyl methacrylate (HPMA) or polyethylene glycol 600 dimethacrylate (14G), using gamma-rays from a 60Co source. In water, extensive swelling of the hydrogels occurred at 10 degrees C, but there was marked deswelling as the temperature was raised to 37 degrees C. The poly(A-ProOMe-co-HPMA) hydrogel was characterized by an initial rapid shrinkage at the surface in the deswollen state; this shrinkage arose because of the formation of a rigid membrane barrier devoid of micropores. The system is therefore 'surface regulated'. In contrast, no such a barrier formed in the deswollen poly(A-ProOMe-co-14G) hydrogel. The whole matrix shrunk without the disappearance of micropores, and it is therefore a 'matrix pumping' system. Testosterone was incorporated into both these types of hydrogels, and the drug-loaded hydrogels were implanted subcutaneously into the backs of castrated rats. The daily dose of testosterone released in vivo from the poly(A-ProOMe-co-HPMA) hydrogel was constant at approximately 30 micrograms/day throughout an experimental period of 54 weeks. In contrast, drug release from the poly(A-ProOME-co-14G) hydrogel reached a maximum after one week and then decreased linearly with time down to the 7th week, when it was undetectable. These conclusions were supported by the changes in weight of the ventral prostates and right-side seminal vesicles of the rats, which were restored to normal when delivery of the testosterone was sustained.

Effects of proline ring conformation on theoretical pi-pi* absorption and CD spectra of helical poly(L-proline) forms I and II

Absorption and CD spectra of the pi-pi* transition near 200 nm are calculated for helical (Pro)10 forms I and II with a variable proline ring conformation characterized by torsion angle chi 2 in the range -60 degrees to 60 degrees. The spectra for poly(Pro) I are not sufficiently sensitive to chi 2 to suggest a preferred ring conformation. The spectra for poly(Pro) II are more sensitive to chi 2, and suggest preferred ring conformations near either or both of the chi 2 regions -50 +/- 10 degrees and 50 +/- 10 degrees.

Influence of proline residues on protein conformation

To study the influence of proline residues on three-dimensional structure, an analysis has been made of all proline residues and their local conformations extracted from the Brookhaven Protein Data bank. We have considered the conformation of the proline itself, the relative occurrence of cis and trans peptides preceding proline residues, the influence of proline on the conformation of the preceding residue and the conformations of various proline patterns (Pro-Pro, Pro-X-Pro, etc.). The results highlight the unique role of proline in determining local conformation.

Proline in alpha-helix: stability and conformation studied by dynamics simulation

Free-energy simulations have been used to estimate the change in the conformational stability of short polyalanine alpha-helices when one of the alanines is replaced by a proline residue. For substituting proline in the middle of the helix the change in free energy of folding (delta delta G degrees) was calculated as 14 kJ/mol (3.4 kcal/mol), in excellent agreement with the one available experimental value. The helix containing proline was found to be strongly kinked; the free energy for reducing the angle of the kink from 40 degrees to 15 degrees was calculated, and found to be small. A tendency to alternate hydrogen bonding schemes was observed in the proline-containing helix. These observations for the oligopeptide agree well with the observation of a range of kink angles (18-35 degrees) and variety of hydrogen bonding schemes, in the rare instances where proline occurs in helices in globular proteins. For substituting proline at the N-terminus of the helix the change in free energy of folding (delta delta G degrees) was calculated as -4 kJ/mol in the first helical position (N1) and +6 kJ/mol in the second helical position (N2). The observed frequent occurrence of proline in position N1 in alpha-helices in proteins therefore has its origin in stability differences of secondary structure. The conclusion reached here that proline may be a better helix former in position N1 than (even) alanine, and thus be a helix initiator may be testable experimentally by measurements of fraction helical conformation of individual residues in oligopeptides of appropriate sequence. The relevance of these results in regards to the frequent occurrence of proline-containing helices in certain membrane proteins is discussed.

cDNA cloning of carboxyl ester lipase from human pancreas reveals a unique proline-rich repeat unit

We report the isolation and nucleotide sequence of the cDNA for carboxyl ester lipase (CEL) from human pancreas. CEL was purified from human pancreas and microsequence analysis was performed on the amino-terminal and internal peptides. Peptide sequence was used to design oligonucleotide probes for screening a human pancreas cDNA library. Partial length cDNAs for CEL were isolated from the library, and the 5' portion of the cDNA was obtained using the anchored polymerase chain reaction. The deduced amino acid sequence indicates that mature CEL contains 722 amino acids and is synthesized with a 20 amino acid leader peptide. The amino acid sequence is rich in proline (12.2%), with 68% of the proline residues occurring within the final 25% of protein length. This is due to the occurrence of a series of proline-rich tandem repeat units near the carboxyl terminus, and accounts for the previously observed species variation in CEL size and amino acid composition. The primary sequence of CEL shows strong similarity to members of the serine esterase family, including the identical G-E-S-A-G motif at the putative active site. A striking homology also occurs between CEL and acetylcholinesterase and cholinesterase, essential enzymes of the nervous system. Proteins with cholesteryl esterase activity have been detected in extra-pancreatic tissues including liver, intestine, kidney, aorta, macrophage, and in the milk of some species (human, gorilla, cat, dog), but not others (rat, cow). To clarify the structural relationships between these various esterases and CEL, we used the CEL cDNA to study expression in pancreas and liver. CEL mRNA was abundant in pancreas of human and rat, with the human CEL mRNA approximately 300 nucleotides larger than that from rat. CEL mRNA was not detected in human adult or fetal liver, nor in rat liver. These results indicate that CEL is not synthesized in significant amounts in liver, and suggest that the cholesterol esterase activity that has been described in liver may be due to a distinct enzyme, or may be derived from pancreas, as has been proposed for the cholesterol esterase activity in intestine.

Role of conserved proline residues in stabilizing tryptophan synthase alpha subunit: analysis by mutants with alanine or glycine

To study the role of Pro residues in the conformation and conformational stability of a protein, nine mutant alpha subunits of tryptophan synthase from Escherichia coli, in which Ala or Gly was substituted for each of six Pro residues (positions 28, 57, 62, 96, 132, and 207) that are conserved in 10 microorganisms, were constructed by means of site-directed mutagenesis. The far-ultraviolet (UV) CD spectra of five mutant alpha subunits with Ala in place of Pro were identical to the spectrum of the wild-type protein, the exception being the mutant at position 207 (P207A). CD values in the far-UV region were less negative for P207A, indicating that the Pro residue at position 207 plays a role in maintaining the intact structure of the alpha subunit. The negative CD values of the Gly mutants examined (P28G, P96G, and P132G) were also decreased. Calorimetric measurements showed that the two mutants at position 28 (P28G and P28A) gave two peaks in the excess heat capacity curve, whereas the wild type and other Pro mutants had only a single peak. The stability of each mutant protein relative to that of the wild type was about the same for P57A, less for P62A and P132A, and markedly decreased for P96A and P207A, which are substituted at less mobile positions. The changes of denaturation entropy (delta delta dS) at the denaturation temperature of the wild-type protein (54.1 degrees C at pH 9.0) were positive for P57A, P62A, and P132A, but negative for P96A, P207A, and P132G.(ABSTRACT TRUNCATED AT 250 WORDS)