Biomedical subjects

N Gautham

Publications and source records attributed to N Gautham.

14 recordsLinked to original sources

PubMed2006

Protein structure prediction using mutually orthogonal Latin squares and a genetic algorithm.

We combine a new, extremely fast technique to generate a library of low energy structures of an oligopeptide (by using mutually orthogonal Latin squares to sample its conformational space) with a genetic algorithm to predict protein structures. The protein sequence is divided into oligopeptides, and a structure library is generated for each. These libraries are used in a newly defined mutation operator that, together with variation, crossover, and diversity operators, is used in a modified genetic algorithm to make the prediction. Application to five small proteins has yielded near native structures.

Algorithms↗

PubMed2006

Effect of DNA structural flexibility on promoter strength--molecular dynamics studies of E. coli promoter sequences.

To study possible correlations between promoter activity and the structural flexibility of the DNA helix, we have carried out unrestrained molecular dynamics simulations of the -10 consensus region sequence and five variants forming the -10 region of various Escherichia coli promoter sequences. Analyses of the trajectories obtained from the simulations show that the consensus sequence has a pattern of two structurally flexible nucleotide steps sandwiched between two stiff steps. In the other sequences, this pattern varies in consonance with the change in the sequence. The variations in the patterns show correlation with the promoter strength.

Base Sequence↗

PubMed2006

Principal component analysis of DNA oligonucleotide structural data.

The microstructure of a DNA helix is characterized by several base pair and base step parameters such as twist, rise, roll, propeller twist, etc., in addition to conformational parameters such as the backbone and the glycosidic torsion angles. Among these only a few, which are independent of all others and of each other, may be used to precisely characterize the helix. The problem however is to identify these independent parameters. We have used principal component analysis to identify a relatively small set of independent parameters, with which to characterize each DNA helix. We show that these principal components clearly discriminate between A and B DNA helical types. The calculations further suggest that the microstructure of a DNA helix is better characterized using dinucleotides.

Base Sequence↗

PubMed2005

Structure of d(TGCGCG).d(CGCGCA) in two crystal forms: effect of sequence and crystal packing in Z-DNA.

The sequence d(TGCGCG).d(CGCGCA) crystallized in two crystal forms, orthorhombic and hexagonal, in the presence of cobalt hexammine chloride, a known inducer of the left-handed Z-form of DNA. The crystal structures have been solved and refined at 1.71 A resolution in space group P2(1)2(1)2(1) and 2.0 A resolution in space group P6(5). The orthorhombic structure contains one Z-DNA hexamer duplex, while the hexagonal structure contains two hexamer duplexes in the structure. Of the latter, one is situated on a crystallographic sixfold screw axis, leading to disorder. This paper reports the effects of sequence and crystal packing on the structure of Z-type DNA. The structures lend additional support to the authors' earlier conclusion that a stretch of four C.G base pairs is sufficient to nucleate and define the regular model of the left-handed helix based on the structure of d(CGCGCG)2.

Base Pairing↗

PubMed2004

Cobalt hexammine induced tautomeric shift in Z-DNA: the structure of d(CGCGCA)*d(TGCGCG) in two crystal forms.

We report here the crystal structure of the DNA hexamer duplex d(CGCGCA)*d(TGCGCG) at 1.71 A resolution. The crystals, in orthorhombic space group, were grown in the presence of cobalt hexammine, a known inducer of the left-handed Z form of DNA. The interaction of this ion with the DNA helix results in a change of the adenine base from the common amino tautomeric form to the imino tautomer. Consequently the A:T base pair is disrupted from the normal Watson-Crick base pairing to a 'wobble' like base pairing. This change is accommodated easily within the helix, and the helical parameters are those expected for Z-DNA. When the cobalt hexammine concentration is decreased slightly in the crystallization conditions, the duplex crystallizes in a different, hexagonal space group, with two hexamer duplexes in the asymmetric unit. One of these is situated on a crystallographic 6-fold screw axis, leading to disorder. The tautomeric shift is not observed in this space group. We show that the change in inter-helix interactions that lead to the two different space groups probably arise from the small decrease in ion concentration, and consequently disordered positions for the ion.

Base Pairing↗

PubMed2004

Conformational studies on enkephalins using the MOLS technique.

Conformational studies of two linear enkephalin molecules, Met-enkephalin and Leu-enkephalin, have been carried using the mutually orthogonal Latin squares (MOLS) technique with the ECEPP/3 force field. This technique was developed recently in our laboratory to perform an unbiased search of the conformational space of peptides and to locate low energy conformations. The present study identified all the folds predicted by other studies, and in addition picked up other energetically favorable structures. The results suggest that the peptide backbone exists as a mixture of folded and unfolded forms (approximately 50% each). The study also provides information on the distribution of the low energy conformations that we have classified on the basis of structural motifs, backbone hydrogen-bonding patterns, and root mean square deviations in atomic positions.

Algorithms↗

PubMed2004

An application of experimental design using mutually orthogonal Latin squares in conformational studies of peptides.

We address the question-can we use experimental design methods to investigate peptide conformation and identify conformational parameters that may contribute more significantly to the potential energy than others? We used mutually orthogonal Latin square design to sample the conformational space of peptides and analysed the samples using analysis of variance. We examined the equality of the effect of the torsion angles on the conformational potential energy. The results showed that different torsion angles contributed differently to the conformational energy. We are able to identify those parameters that may have to be more carefully considered in conformational studies of peptides.

Algorithms↗

PubMed2004

Correlations between nucleotide frequencies and amino acid composition in 115 bacterial species.

We studied the correlations between amino acid composition and mononucleotide and dinucleotide frequencies in 115 bacterial genomes of varying G+C content. Observed amino acid frequencies were compared with those expected from the actual mononucleotide and dinucleotide frequencies. Both mononucleotide and dinucleotide frequencies correlate well with the amino acid frequency, with dinucleotide frequencies doing so better. Despite the strong correlations, some of the observed amino acid frequencies, in particular for Arg, Val, Asp, Glu, Ser, and Cys, were consistently different from predicted values in all genomes. We suggest that this variation from predicted values is a consequence of selection pressure at the level of amino acids, while the close correspondence to the predictions in residues such as Thr, Phe, Lys, and Asn arises only from mutation and selection pressure at the level of the nucleic acid sequences.

Amino Acid Sequence↗

PubMed2003

Enhanced sampling of the molecular potential energy surface using mutually orthogonal latin squares: application to peptide structures.

The computational identification of the optimal three-dimensional fold of even a small peptide chain from its sequence, without reference to other known structures, is a complex problem. There have been several attempts at solving this by sampling the potential energy surface of the molecule in a systematic manner. Here we present a new method to carry out the sampling, and to identify low energy conformers of the molecule. The method uses mutually orthogonal Latin squares to select (of the order of) n(2) points from the multidimensional conformation space of size m(n), where n is the number of dimensions (i.e., the number of conformational variables), and m specifies the fineness of the search grid. The sampling is accomplished by first calculating the value of the potential energy function at each one of the selected points. This is followed by analysis of these values of the potential energy to obtain the optimal value for each of the n-variables separately. We show that the set of the n-optimal values obtained in this manner specifies a low energy conformation of the molecule. Repeated application of the method identifies other low energy structures. The computational complexity of this algorithm scales as the fourth power of the size of the molecule. We applied this method to several small peptides, such as the neuropeptide enkephalin, and could identify a set of low energy conformations for each. Many of the structures identified by this method have also been previously identified and characterized by experiment and theory. We also compared the best structures obtained for the tripeptide (Ala)(3) by the present method, with those obtained by an exhaustive grid search, and showed that the algorithm is successful in identifying all the low energy conformers of this molecule.

Algorithms↗

PubMed2002

Structure of d(TGCGCA)2 at 293 K: comparison of the effects of sequence and temperature.

The crystal structure of a hexameric DNA fragment with the sequence d(TGCGCA)(2) has been solved and refined at 293 K at a resolution of 1.64 A. The molecule adopts a left-handed Z-type helical conformation which is common in alternating pyrimidine-purine sequences. The presence of A.T base pairs at the two terminals does not perturb the structure to any great degree. However, several sequence-specific microstructural changes are noticeable. The structure of the identical sequence determined at 120 K involving somewhat different crystallization conditions has been reported previously [Harper et al. (1998), Acta Cryst. D54, 1273-1284]. A comparison of the present structure with that at low temperature and with that of d(CGCGCG)(2) shows that the effect of the change in sequence is greater than the combined effect of changes in temperature and environment.

Base Pairing↗

PubMed1999

Crystal structure at 1.63 A resolution of the native form of porcine beta-trypsin: revealing an acetate ion binding site and functional water network.

The active center of a serine protease is the catalytic triad composed of His-57, Ser-195 and Asp-102. The existing crystal structure data on serine proteases have not fully answered a number of fundamental questions relating to the catalytic activity of serine proteases. The new high resolution native porcine beta-trypsin (BPT) structure is aimed at extending the knowledge on the conformation of the active site and the ordered water structure within and around the active site. The crystal structure of BPT has been determined at 1.63 A resolution. An acetate ion bound at the active site of a trypsin molecule by both classical hydrogen bonds and C-HellipsisO hydrogen bonds has been identified for the first time. A large network of water molecules extending from the recognition amino acid Asp-184 to the entry of the active site has been observed in the BPT structure. A detailed comparison with inhibitor complexes and autolysates indicates that the sulfate ion and the acetate ion bind at the same site of the trypsin molecule. The Ser-195 Cbeta-Ogamma-His-57 Nepsilon angle in the catalytic triad of BPT is intermediate between the corresponding values of the complex and native structure due to acetate ion binding. The network of waters from the recognition amino acid to the active site entry is probably the first ever complete picture of functional waters around the active site. Structural comparisons show that the functional waters involved in the binding of small molecule inhibitors and protease inhibitors are distinctly different.

Acetates↗

PubMed1998

Structure of d(CACGCG).d(CGCGTG) in crystals grown in the presence of ruthenium III hexammine chloride.

Hexammine ions are strong inducers of the transition from the B-form to the left-handed Z-form in DNA. Here the structure of d(CACGCG). d(CGCGTG) obtained from crystals grown from a drop containing [Ru(NH3)6]Cl3 is reported. The structure is clearly characterized as Z-DNA. When compared with the structure of d(CACGCG).d(CGCGTG)/MgCl2 and that of d(CGCGCG)2, subtle differences are seen, most noticeably in the water structure. Since stable well diffracting crystals grow easily in the presence of [Ru(NH3)6]Cl3 and since this ion is not visible in the electron density it is concluded that the ion plays a non-specific role in stabilizing Z-DNA.

Base Pairing↗

PubMed1995

Sequence-dependent microheterogeneity of Z-DNA: the crystal and molecular structures of d(CACGCG).d(CGCGTG) and d(CGCACG).d(CGTGCG).

We have solved and refined the crystal structures of the first two non-self-complementary hexadeoxyribonucleotide duplexes with Watson-Crick base pairs, namely d(CACGCG).d(CGCGTG) and d(CGCACG).d(CGTGCG). Both the hexamers crystallize in the left-handed Z-DNA conformation. The packing of the molecules is similar in the two crystals: the hexamers are stacked on top of each other to form columns of infinite length, which are arranged in a close-packed hexagonal pattern. In spite of the similar packing, crystals of the first duplex belong to the space group P2(1)2(1)2(1), isomorphous to previous Z-DNA hexamers, while crystals of the other duplex belong to the space group P2(1), which has not been observed before in this context. The molecular structures of the two duplexes are also different. The first is very similar in conformation to the other Z-DNA hexamers and to the idealized ZI model. The other duplex shows large differences in the stacking and the relative disposition of its bases, leading to its possible classification as a novel Z-DNA variant. We conclude that the differences in the structure under similar environmental conditions point to a sequence-dependent plasticity in the DNA molecule which is visible even under close-packed conditions.

Base Sequence↗

PubMed1994

Space-group degeneracy in the packing of a non-selfcomplementary Z-DNA hexamer.

The X-ray diffraction pattern of the crystals of the non-selfcomplementary hexadeoxyribonucleotide d(CGCACG).d(CGTGCG) can be indexed in four different space groups: (i) P6(5) and P2(1), with cell parameters a = 17.75 (1), b = 17.76 (1), c = 42.77 (3) A, alpha = 90, beta = 90, gamma = 120 degrees, and (ii) P2(1)2(1)2(1) and C2, with cell parameters a = 17.75 (1), b = 30.74 (2), c = 42.77 (3) A, alpha = 90, beta = 90, gamma = 90 degrees. While the R(merge) for the equivalent reflections in the different space groups indicates that P2(1) is the correct choice in the present case, it is demonstrated that the near degeneracy of the space groups arises out of the fact that the DNA molecule is nearly cylindrical. A perfect cylinder would show perfect degeneracy.

Journal Article↗