A proton NMR investigation of proline-24 cis-trans isomerism in corticotropin 1-32 and related peptides

Characterization of secondary amide peptide bond isomerization: thermodynamics and kinetics from 2D NMR spectroscopy

Secondary amide cis peptide bonds are of even lower abundance than the cis tertiary amide bonds of prolines, yet they are of biochemical importance. Using 2D NMR exchange spectroscopy (EXSY) we investigated the formation of cis peptide bonds in several oligopeptides: Ac-G-G-G-NH(2) , Ac-I-G-G-NH(2) , Ac-I-G-G-N-NH(2) and its cyclic form: I-G-G-N in dimethylsulfoxide (DMSO). From the NMR studies, using the amide protons as monitors, an occurrence of 0.13-0.23% of cis bonds was obtained at 296 K. The rate constants for the trans to cis conversion determined from 2D EXSY spectroscopy were 4-9 × 10(-3) s(-1) . Multiple minor conformations were detected for most peptide bonds. From their thermodynamic and kinetic properties the cis isomers are distinguished from minor trans isomers that appear because of an adjacent cis peptide bond. Solvent and sequence effects were investigated utilizing N-methylacetamide (NMA) and various peptides, which revealed a unique enthalpy profile in DMSO. The cyclization of a tetrapeptide resulted in greatly lowered cis populations and slower isomerization rates compared to its linear counterpart, further highlighting the impact of structural constraints.

Investigating diproline segments in proteins: occurrences, conformation and classification

The covalent linkage between the side-chain and the backbone nitrogen atom of proline leads to the formation of the five-membered pyrrolidine ring and hence restriction of the backbone torsional angle ϕ to values of -60 °± 30° for the L-proline. Diproline segments constitute a chain fragment with considerably reduced conformational choices. In the current study, the conformational states for the diproline segment (( L) Pro-( L) Pro) found in proteins has been investigated with an emphasis on the cis and trans states for the Pro-Pro peptide bond. The occurrence of diproline segments in turns and other secondary structures has been studied and compared to that of Xaa-Pro-Yaa segments in proteins which gives us a better understanding on the restriction imposed on other residues by the diproline segment and the single proline residue. The study indicates that P(II) -P(II) and P(II) -α are the most favorable conformational states for the diproline segment. The analysis on Xaa-Pro-Yaa sequences reveals that the Xaa-Pro peptide bond exists preferably as the trans conformer rather than the cis conformer. The present study may lead to a better understanding of the behavior of proline occurring in diproline segments which can facilitate various designed diproline-based synthetic templates for biological and structural studies.

NMR studies of adrenocorticotropin hormone peptides in sodium dodecylsulfate and dodecylphosphocholine micelles: proline isomerism and interactions of the peptides with micelles

Three adrenocorticotropin hormone (ACTH) fragments (1-10, 1-24, and 11-24) have been studied in water and in sodium dodecylsulfate (SDS) and dodecylphosphocholine (DPC) micelles by nuclear magnetic resonance spectroscopy. The trans-cis isomerism at all three proline sites (at positions 12, 19, and 24) was found in the 11-24 segment of the peptide. The population of the cis isomers changes with the environment of the peptide. Specifically, the presence of the DPC micelle does not affect the trans-cis equilibrium in the 11-24 segment from that in water. In contrast, the presence of the SDS micelles decreases the population of the cis isomer at Pro(24), but increases its population at Pro(12) and Pro(19). The effect of SDS micelles on the trans-cis equilibrium at these proline sites was discussed. Intermolecular nuclear Overhauser effect (NOE) correlations between the ACTH peptides and the micelles were observed. These correlations occurred only in the 1-10 segment of the peptides, and the hydrophobic side chains contributed most to the intermolecular NOE. The intermolecular NOE pattern corroborates the suggestion that the 1-10 segment of the ACTH peptides bind to these micelles via a surface-binding mode, with most of the interactions coming from the insertion of the hydrophobic side chains.

Studies of the binding and structure of adrenocorticotropin peptides in membrane mimics by NMR spectroscopy and pulsed-field gradient diffusion

The partition and structure of three adrenocorticotropic hormone peptides ACTH(1-10), ACTH(1-24), and ACTH(11-24) in water and in sodium dodecylsulfate (SDS) and dodecylphosphocholine (DPC) micelles were studied by 2D NMR and NMR gradient diffusion measurements. The diffusion rates, the NH chemical shifts, and the nuclear Overhauser effect patterns provided a coherent picture of binding of these peptides. All three peptides are significantly partitioned in the negatively charged SDS micelles and possess definite secondary structure, as opposed to random structures in water. For ACTH (1-24), the hydrophobic 1-10 segment is partitioned in DPC micelles, but the charged 11-24 segment prefers to remain in the aqueous region. ACTH(11-24) does not bind significantly to the DPC micelles. The binding of the ACTH peptides in these two widely used "membrane mimics" are substantially different from that in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers obtained by attenuated total reflection infrared spectroscopy and from our preliminary diffusion studies of the same peptides in POPC vesicles. This study showed that, in a given micellar medium, all corresponding segments of these peptides are located in the same membrane environment in the system, regardless of whether these segments exist by themselves or are attached to other segments. This result may contradict the membrane-compartments concept of Schwyzer, which suggests that ACTH(1-10) and ACTH(1-24) are located in different membrane compartments because they have different address segments, and consequently, bind to different receptors. The present results also suggest that the assumption that micelles are good membrane mimics should be carefully examined.

Different requirements for productive interaction between the active site of HIV-1 proteinase and substrates containing -hydrophobic*hydrophobic- or -aromatic*pro- cleavage sites

The sequence requirements for HIV-1 proteinase catalyzed cleavage of oligopeptides containing two distinct types of junctions (-hydrophobic*hydrophobic- or -aromatic*Pro-) has been investigated. For the first type of junction (-hydrophobic*hydrophobic-) the optimal residues in the P2 and P2' positions were found to be Val and Glu, respectively, in accord with recent statistical analysis of natural cleavage sites [Poorman, R. A., Tomasselli, A. G., Heinrikson, R. L., & Kézdy, F. J. (1991) J. Biol. Chem. 266, 14554-14561]. For the -aromatic*Pro- type of junction, in the specific sequence context studied here, the value of Glu in the P2' position was again observed. An explanation for the inefficient cleavage observed for peptides with the sequence -Val-Tyr*Pro- has been provided from molecular modeling of the putative enzyme-substrate complex. A significant effect upon cleavage rates due to the amino acid in the P5 position has also been documented. While lysine in the P5 position in one sequence of the -hydrophobic*hydrophobic- type produces a peptide cleaved very efficiently (kcat greater than 15 s-1 for Lys-Ala-Arg-Val-Nle*p-nitrophenylalanine-P2'-Ala-Nle-NH2, for P2' = Glu, Gln, Ile, Val, or Ala), for substrates of the -aromatic*Pro- type, the P5 residue can exert either a positive or negative effect on cleavage rates. These results have again been interpreted in light of molecular modeling. We suggest that interaction of the substrate sequence on the periphery of the active site cleft may influence the match of the enzyme-substrate pair and, hence, control the efficiency of catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential 1H NMR assignment and secondary structure determination of salmon calcitonin in solution

Salmon calcitonin (sCT) has been investigated by NMR at 500 MHz in a 90% DMSOd6-10% 1H2O (v/v) mixture at 278 K. All backbone and side-chain resonances of the hormone have been assigned by using high-resolution phase-sensitive two-dimensional techniques. Analysis of the type and magnitude of the observed sequential nuclear Overhauser effects, the NH-alpha CH spin-spin coupling constants, and the 1H/2H exchange kinetics measured in 80% DMSOd6-20% 2H2O (v/v) at 278 K enabled prediction of the secondary structure. Overall, an extended conformation is the dominant feature of the solution, but there are clear indications for a short double-stranded antiparallel beta sheet in the central region comprising residues 12-18, connected by a three-residue hairpin loop formed by residues 14-16. Two tight turns, made by residues 6-9 and 25-28, were also identified, but no evidence was found for the presence of a regular helical segment. The beta sheet favors an amphipathic distribution of the residues, orienting the predominantly hydrophilic Ser13, Glu15, and His17 side chains above the plane of the sheet, and the predominantly hydrophobic Leu12, Gln14, and Leu16 below it. This is interpreted as the "seed" of the amphipathic alpha helix postulated to be responsible for the interaction of sCT with lipids, a situation reminiscent of the folding mechanism of signal peptides in the interaction with membranes. The possible significance of the cis-trans Pro23 isomerism is discussed.

Conformation and mobility of tyrosine side chain in tetrapeptides. Specific effects of cis- and trans-proline in Tyr-Pro- and Pro-Tyr-segments

We examined the properties of tyrosine in four free tetrapeptides: Ala-Ala-Tyr-Ala (AATA), Ala-Pro-Tyr-Ala (APTA), Ala-Tyr-Ala-Ala (ATAA) and Ala-Tyr-Pro-Ala (ATPA) by CD, n.m.r. and energy calculations. Experimental data (the aromatic 1Lb signal, rotamer populations around the C alpha-C beta bond (chi 1), rotations around C beta-C gamma(chi 2), chemical shifts of ortho- and meta-protons in the phenolic ring (in aqueous and Me2SO solutions), NH proton temperature coefficients and vicinal coupling constants 3JNH-C alpha H in the backbone (Me2SO solution) were compared with calculated minimum energy conformations. We find qualitative agreement between the results of the different techniques with respect to global tendencies of conformational behaviour: we present experimental evidence showing that the presence of proline in the sequence has a more pronounced effect on the side chain organization of the residues preceding it than on one succeeding it. This steric influence of proline on its immediate neighbor is even stronger in the cis isomer than in the more common trans isomer. The strong preference for Rotamer II (chi 1 = 180 degrees) over Rotamer I (chi 1 = -60 degrees) in ATPA (cis-form) concomitant with a noticeable deviation of chi 2 is a striking example.

Conformation of corticotropin. An infrared spectrometry study of hydrogen exchange kinetics

1H--2H exchange kinetics of the peptide hydrogens in corticotropin have been examined in 2H2O and CF3C2H2O2H solutions by means of infrared absorption measurements. In aqueous solution, around pH 3, the experimental data suggest a partially ordered structure, since in the two corticotropins 1--24 and 1--32 about 6 slowly exchanging peptide protons are numbered. These might belong to the N-terminal part of the molecule. The C-terminal 25--32 octapeptide segment appears to be unordered and slightly destabilizes the overall hormone conformation. For corticotropin1--24 in CF3C2H2O2H, the qualitative interpretation of infrared spectra and the quantitative analysis of exchange data give evidence of a strong stabilization: a predominantly alpha-helical structure is induced by trifluoroethanol.