Influence of cation-π interactions to the structural stability of phycocyanin proteins: A computational study.

The influences of cation-π interactions in phycocyanin proteins and their environmental preferences were analyzed. The number of interactions formed by arginine showed to be higher than those formed by the lysine in the cationic group, while histidine is comparatively higher than phenylalanine and N-terminal residue in the π group. Arg-Tyr and Arg-Phe interacting pairs are predominant among the various pairs analyzed. Cation-π interactions are distance-dependent and can be realized above a wider area above the π ring. We analyzed the energy contribution resulting from cation-π interactions using ab initio calculations. The energy contribution resulting from the most frequent cation-π interactions was in the lower range of strong hydrogen bonds. The results showed that, while most of their interaction energies lay ranged from - 2 to - 8 kcal/mol, those energies could be up to -12- 12 kcal/mol. Stabilization centers for these proteins showed that all residues found in cation-π interactions are important in locating one or more of such centers. In the cation-π interacting residues, 54% of the amino acid residues involved in these interactions might be conserved in phycocyanins. From this study, we infer that cation-π forming residues play an important role in the stability of the multiply commercially used phycocyanin proteins and could help structural biologists and medicinal chemists to design better and safer drugs.

Computational Analysis of Non-covalent Interactions in Phycocyanin Subunit Interfaces.

Protein-protein interactions are an important phenomenon in biological processes and functions. We used the manually curated non-redundant dataset of 118 phycocyanin interfaces to gain additional insight into this phenomenon using a robust inter-atomic non-covalent interaction analyzing tool PPCheck. Our observations indicate that there is a relatively high composition of hydrophobic residues at the interfaces. Most of the interface residues are clustered at the middle of the range which we call "standard-size" interfaces. Furthermore, the multiple interaction patterns founded in the present study indicate that more than half of the residues involved in these interactions participate in multiple and water-bridged hydrogen bonds. Thus, hydrogen bonds contribute maximally towards the stability of protein-protein complexes. The analysis shows that hydrogen bond energies contribute to about 88 % to the total energy and it also increases with interface size. Van der Waals (vdW) energy contributes to 9.3 %±1.7 % on average in these complexes. Moreover, there is about 1.9 %±1.5 % contribution by electrostatic energy. Nevertheless, the role by vdW and electrostatic energy could not be ignored in interface binding. Results show that the total binding energy is more for large phycocyanin interfaces. The normalized energy per residue was less than -16 kJ mol-1 , while most of them have energy in the range from -6 to -14 kJ mol-1 . The non-covalent interacting residues in these proteins were found to be highly conserved. Obtained results might contribute to the understanding of structural stability of this class of evolutionary essential proteins with increased practical application and future designs of novel protein-bioactive compound interactions.

Anion-π interactions in active centers of superoxide dismutases.

We investigated 1060 possible anion-π interactions in a data set of 41 superoxide dismutase active centers. Our observations indicate that majority of the aromatic residues are capable to form anion-π interactions, mainly by long-range contacts, and that there is preference of Trp over other aromatic residues in these interactions. Furthermore, 68% of total predicted interactions in the dataset are multiple anion-π interactions. Anion-π interactions are distance and orientation dependent. We analyzed the energy contribution resulting from anion-π interactions using ab initio calculations. The results showed that, while most of their interaction energies lay in the range from -0 to -4kcalmol-1, those energies can be up to -9kcalmol-1 and about 34% of interactions were found to be repulsive. Majority of the suggested anion-π interacting residues in ternary complexes are metal-assisted. Stabilization centers for these proteins showed that all the six residues found in predicted anion-π interactions are important in locating one or more of such centers. The anion-π interacting residues in these proteins were found to be highly conserved. We hope that these studies might contribute useful information regarding structural stability and its interaction in future designs of novel metalloproteins.

Anion-π interactions in complexes of proteins and halogen-containing amino acids.

We analyzed the potential influence of anion-π interactions on the stability of complexes of proteins and halogen-containing non-natural amino acids. Anion-π interactions are distance and orientation dependent and our ab initio calculations showed that their energy can be lower than -8 kcal mol(-1), while most of their interaction energies lie in the range from -1 to -4 kcal mol(-1). About 20 % of these interactions were found to be repulsive. We have observed that Tyr has the highest occurrence among the aromatic residues involved in anion-π interactions, while His made the least contribution. Furthermore, our study showed that 67 % of total interactions in the dataset are multiple anion-π interactions. Most of the amino acid residues involved in anion-π interactions tend to be buried in the solvent-excluded environment. The majority of the anion-π interacting residues are located in regions with helical secondary structure. Analysis of stabilization centers for these complexes showed that all of the six residues capable of anion-π interactions are important in locating one or more of such centers. We found that anion-π interacting residues are sometimes involved in simultaneous interactions with halogens as well. With all that in mind, we can conclude that the anion-π interactions can show significant influence on molecular organization and on the structural stability of the complexes of proteins and halogen-containing non-natural amino acids. Their influence should not be neglected in supramolecular chemistry and crystal engineering fields as well.

Contribution of cation-π interactions to the stability of Sm/LSm oligomeric assemblies.

In this work, we have analyzed the influence of cation-π interactions to the stability of Sm/LSm assemblies and their environmental preferences. The number of interactions formed by arginine is higher than lysine in the cationic group, while histidine is comparatively higher than phenylalanine and tyrosine in the π group. Arg-Tyr interactions are predominant among the various pairs analyzed. The furcation level of multiple cation-π interactions is much higher than that of single cation-π interactions in Sm/LSm interfaces. We have found hot spot residues forming cation-π interactions, and hot spot composition is similar for all aromatic residues. The Arg-Phe pair has the strongest interaction energy of -8.81 kcal mol(-1) among all the possible pairs of amino acids. The extent of burial of the residue side-chain correlates with the ΔΔG of binding for residues in the core and also for hot spot residues cation-π bonded across the interface. Secondary structure of the cation-π residues shows that Arg and Lys preferred to be in strand. Among the π residues, His prefers to be in helix, Phe prefers to be in turn, and Tyr prefers to be in strand. Stabilization centers for these proteins showed that all the five residues found in cation-π interactions are important in locating one or more of such centers. More than 50 % of the cation-π interacting residues are highly conserved. It is likely that the cation-π interactions contribute significantly to the overall stability of Sm/LSm proteins.

Contribution of anion-π interactions to the stability of Sm/LSm proteins.

We have analyzed the influence of anion-π interactions to the stability of Sm/LSm assemblies. The side chain of Glu is more likely to be in anion-π interactions than Asp. Phe has the highest occurrence in these interactions than the other two π residues. Among the anion-π residue pairs, Glu-Phe residue pair showed the maximum number of anion-π. We have found hot-spot residues forming anion-π interactions, and Glu-Phe is the most common hot-spot interacting pair. The significant numbers of anion-π interacting residues identified in the dataset were involved in the formation of multiple anion-π interactions. More than half of the residues involved in these interactions are evolutionarily conserved. The anion-π interaction energies are distance and orientation dependent. It was found that anion-π interactions showed energy less than -15 kcal mol(-1), and most of them have energy in the range -2 to -9 kcal mol(-1). Solvent accessibility pattern of Sm/LSm proteins reveals that all of the interacting residues are preferred to be in buried regions. Most of the interacting residues preferred to be in strand. A significant percentage of anion-π interacting residues are located as stabilization centers and thus might provide additional stability to these proteins. The simultaneous interaction of anions and cations on different faces of the same π-system has been observed. On the whole, the results presented in this work will be very useful for understanding the contribution of anion-π interaction to the stability of Sm/LSm proteins.

Halogen bonding in complexes of proteins and non-natural amino acids.

In this work, we have analyzed the influence of halogen bonding to the stability of 44 complexes of proteins and non-natural amino acids. Fluorine- and chlorine-containing non-natural amino acids are more prevalent in the dataset, and an even larger number of contacts made by iodine-containing ligands are found. Only few halogen bonds with the hydroxyl oxygens and carboxylate side chains are found in the dataset. Halogen bonds with the nitrogen-containing side chains have higher occurrence than other acceptors. Backbone carbonyl oxygens and nitrogens are to a substantial extent involved in our dataset. We have observed a small percentage of interactions involving water as hydrogen bond donors. Additionally, most of the interacting residues comprising the interfaces also show a great degree of conservation. There is a clear interaction hot spot at distances of 3.5-3.7 Å and Θ1 angles of 100-120°. There is also a cluster of contacts featuring short distances (2.6-2.9 Å) but only nearly optimal Θ1 angles (140-160°). 51.3% of stabilizing residues are involved in building halogen bonds with the non-natural amino acids. We discovered three types of structural motifs significantly over-represented: beta-turn-ir, beta-turn-il and niche-4r. The halogen-bonding statistics of the dataset do not show any preference for α-helices (36%), ß-sheets (36%), or turns/coils (28%) structures. Most of the amino acid residues that were involved in halogen bonds prefer to be in the solvent excluded environment (buried). Furthermore, we have shown that in amino acid-protein complexes halogen atoms can sometimes be involved in hydrogen bonding interactions with hydrogen bonding-donors. The results from this study might be used for the rational design of halogenated ligands as inhibitors and drugs, and in biomolecular engineering.

Cation-π interactions in high resolution protein-RNA complex crystal structures.

In this work, we have analyzed the influence of cation-π interactions to the stability of 59 high resolution protein-RNA complex crystal structures. The total number of Lys and Arg are similar in the dataset as well as the number of their interactions. On the other hand, the aromatic chains of purines are exhibiting more cation-π interactions than pyrimidines. 35% of the total interactions in the dataset are involved in the formation of multiple cation-π interactions. The multiple cation-π interactions have been conserved more than the single interactions. The analysis of the geometry of the cation-π interactions has revealed that the average distance (d) value falls into distinct ranges corresponding to the multiple (4.28 Å) and single (5.50 Å) cation-π interactions. The G-Arg pair has the strongest interaction energy of -3.68 kcal mol(-1) among all the possible pairs of amino acids and bases. Further, we found that the cation-π interactions due to five-membered rings of A and G are stronger than that with the atoms in six-membered rings. 8.7% stabilizing residues are involved in building cation-π interactions with the nucleic bases. There are three types of structural motifs significantly over-represented in protein-RNA interfaces: beta-turn-ir, niche-4r and st-staple. Tetraloops and kink-turns are the most abundant RNA motifs in protein-RNA interfaces. Amino acids deployed in the protein-RNA interfaces are deposited in helices, sheets and coils. Arg and Lys, involved in cation-π interactions, prefer to be in the solvent exposed surface. The results from this study might be used for structure-based prediction and as scaffolds for future protein-RNA complex design.

Non-canonical interactions of porphyrins in porphyrin-containing proteins.

In this study we have described the noncanonical interactions between the porphyrin ring and the protein part of porphyrin-containing proteins to better understand their stabilizing role. The analysis reported in this study shows that the predominant type of non-canonical interactions at porphyrins are CH····O and CH····N interactions, with a small percentage of CH···π and noncanonical interactions involving sulfur atoms. The majority of non-canonical interactions are formed from side-chains of charged and polar amino acids, whereas backbone groups are not frequently involved. The main-chain noncanonical interactions might be slightly more linear than the side-chain interactions, and they have somewhat shorter median distances. The analysis, performed in this study, shows that about 44% of the total interactions in the dataset are involved in the formation of multiple (furcated) noncanonical interactions. The high number of porphyrin-water interactions show importance of the inclusion of solvent in protein-ligand interaction studies. Furthermore, in the present study we have observed that stabilization centers are composed predominantly from nonpolar amino acid residues. Amino acids deployed in the environment of porphyrin rings are deposited in helices and coils. The results from this study might be used for structure-based porphyrin protein prediction and as scaffolds for future porphyrin-containing protein design.

Peroxisome proliferator-activated receptors and atherosclerosis.

The peroxisome proliferator-activated receptors (PPARs) represent the family of 3 nuclear receptor isoforms-PPARα, -γ, and -δ/ß, which are encoded by different genes. As lipid sensors, they are primarily involved in regulation of lipid metabolism and subsequently in inflammation and atherosclerosis. Atherosclerosis considers accumulation of the cells and extracellular matrix in the vessel wall leading to the formation of atherosclerotic plaque, atherothrombosis, and other vascular complications. Besides existence of natural ligands for PPARs, their more potent synthetic ligands are fibrates and thiazolidindiones. Future investigations should now focus on the mechanisms of PPARs activation, which might present new approaches involved in the antiatherosclerotic effects revealed in this review. In addition, in this review we are presenting latest data from recent performed clinical studies which have focus on novel approach to PPARs agonists as potential therapeutic agents in the treatment of complex disease such as atherosclerosis.

Contribution of Non-Canonical Interactions to the Stability of Sm/LSm Oligomeric Assemblies.

The distinguishing property of Sm/LSm protein assemblies is their high stability. In order to better understand the nature of Sm/LSm protein oligomers in this study we have analyzed the contribution of non-canonical interactions to the stability of assemblies. The predominant types of non-canonical interactions at Sm/LSm protein interfaces are CH⋅⋅⋅O, and CH⋅⋅⋅N interactions represented at interfaces. Our results show low percentages of XH-π and non-canonical interactions involving sulfur atoms, while the backbone groups were less frequently involved. The data show a high percentage of non-canonical interactions in interfaces formed by charged residues with Lys and Arg, these being the major charged donors. The main chain non-canonical interactions might be slightly more linear than the side chain interactions, and they have somewhat shorter median distances. Comparing the stabilizing amino acid residues with amino acids which build non-canonical interactions at interfaces shows that certain amino acids like Phe, Pro, His and Tyr are involved with a high percentage. The high conservation score of amino acids that are involved in non-canonical interactions in protein interfaces is an additional strong argument for their importance in the stabilization of Sm/LSm protein assemblies.

Non-covalent interactions across subunit interfaces in Sm proteins.

The distinguishing property of Sm protein associations is their high stability. In order to understand this property, we analyzed the interface non-covalent interactions and compared the properties of the Sm protein interfaces with those of a test set, Binding Interface Database (BID). The comparison revealed that the main differences between interfaces of Sm proteins and those of the BID set are the content of charged residues, hydrogen bonds, salt bridges, and conservation scores of interface residues. In Sm proteins, the interfaces have more hydrophobic and fewer charged residues than the surface, which is also the case for the BID test set and other proteins. However, in the interfaces, the content of charged residues in Sm proteins (26%) is substantially larger than that in the BID set (22%). Both interfaces of Sm proteins and of test set have a similar number of hydrophobic interactions per 100Å(2). The interfaces of Sm proteins have substantially more hydrogen bonds than the interfaces in test set. The results show clearly that the interfaces of Sm proteins form more salt bridges compared with test set. On average, there are about 16 salt bridges per interface. The high conservation score of amino acids that are involved in non-covalent interactions in protein interfaces is an additional strong argument for their importance. The overriding conclusion from this study is that the non-covalent interactions in Sm protein interfaces considerably contribute to stability of higher order structures.

Protein subunit interfaces: A statistical analysis of hot spots in Sm proteins.

The distinguishing property of Sm protein associations is very high stability. In order to understand this property, we analyzed the interfaces and compared the properties of Sm protein interfaces with those of a test set, the Binding Interface Database (BID). The comparison revealed that the main differences between the interfaces of Sm proteins and those of the BID set are the content of charged residues, the coordination numbers of the residues, knowledge-based pair potentials, and the conservation scores of hot spots. In Sm proteins, the interfaces have more hydrophobic and fewer charged residues than the surfaces, which is also the case for the BID test set and other proteins. However, in the interfaces, the content of charged residues in Sm proteins (26%) is substantially larger than that in the BID set (22%). Hot spots are residues that make up a small fraction of the interfaces, but they contribute most of the binding energy. These residues are critical to protein-protein interactions. Analyses of knowledge-based pair potentials of hot spot and non-hot spot residues in Sm proteins show that they are significantly different; their mean values are 31.5 and 11.3, respectively. In the BID set, this difference is smaller; in this case, the mean values for hot spot and non-hot spot residues are 20.7 and 12.4, respectively. Hence, the pair potentials of hot spots differ significantly for the Sm and BID data sets. In the interfaces of Sm proteins, the amino acids are tightly packed, and the coordination numbers are larger in Sm proteins than in the BID set for both hot spots and non-hot spots. At the same time, the coordination numbers are higher for hot spots; the average coordination number of the hot spot residues in Sm proteins is 7.7, while it is 6.1 for the non-hot spot residues. The difference in the calculated average conservation score for hot spots and non-hot spots in Sm proteins is significantly larger than it is in the BID set. In Sm proteins, the average conservation score for the hot spots is 7.4. Hot spots are surrounded by residues that are moderately conserved (5.9). The average conservation score for the other interface residues is 5.6. The conservation scores in the BID set do not show a significant distinction between hot and non-hot spots: the mean values for hot and non-hot spot residues are 5.5 and 5.2, respectively. These data show that structurally conserved residues and hot spots are significantly correlated in Sm proteins.

A reexamination of correlations of amino acids with particular secondary structures.

Using the data from Protein Data Bank the correlations of primary and secondary structures of proteins were analyzed. The correlation values of the amino acids and the eight secondary structure types were calculated, where the position of the amino acid and the position in sequence with the particular secondary structure differ at most 25. The diagrams describing these results indicate that correlations are significant at distances between -9 and 10. The results show that the substituents on Cbeta or Cgamma atoms of amino acid play major role in their preference for particular secondary structure at the same position in the sequence, while the polarity of amino acid has significant influence on alpha-helices and strands at some distance in the sequence. The diagrams corresponding to polar amino acids are noticeably asymmetric. The diagrams point out the exchangeability of residues in the proteins; the amino acids with similar diagrams have similar local folding requirements.

XH/pi interactions with the pi system of porphyrin ring in porphyrin-containing proteins.

Searching structures of porphyrin-containing proteins from the Protein Data Bank revealed that the pi system of every porphyrin ring is involved in XH/pi interactions, with most of the porphyrins having several interactions. Both five-membered pyrrole rings and six-membered chelate rings are involved in XH/pi interactions; the number of interactions with five-membered rings is larger than the number of interactions with six-membered rings. We found interactions with C-H and N-H groups as hydrogen-atom donors; however, the number of CH/pi interactions is much larger than the number of NH/pi interactions. The amino acids involved in the interactions show a high conservation score. Our results that every porphyrin is involved in XH/pi interactions and that amino acids involved in these interactions are highly conserved demonstrate that XH/pi interactions play an important role in porphyrin-protein stability.