Functional interpretation and structural insights of Arabidopsis lyrata cytochrome P450 CYP71A13 involved in auxin synthesis.

Cytochrome P450 CYP71A13 of Arabidopsis lyrata is a heme protein involved in biosynthesis of indole-3-acetonitrile which leads to the formation of indolyl-3-acetic acid. It catalyzes a unique reaction: formation of a carbon-nitrogen triple bond and dehydration of indolyl-3-acetaldoxime. Homology model of this 57 kDa polypeptide revealed that the heme existed between H-helix and J- helix in the hydrophobic pocket, although both helixes are involved in catalytic activity, where Gly305 and Thr308, 311 of H- helix were involved in its stabilization. The substrate indole-3-acetaldoxime was tightly fitted into the substrate pocket with the aromatic ring being surrounded by amino acid residues creating a hydrophobic environment. The smaller size of the substrate binding pocket in cytochrome P450 CYP71A13 was due to the bulkiness of the two amino acid residues Phe182 and Trp315 pointing into the substrate binding cavity. The apparent role of the heme in cytochrome P450 CYP71A13 was to tether the substrate in the catalysis by indole-3-acetaldoxime dehydratase. Since the crystal structure of cytochrome P450 CYP71A13 has not yet been solved, the modeled structure revealed mechanism of substrate recognition and catalysis.

Purification studies on a thermo-active amidase of Geobacillus pallidus BTP-5x MTCC 9225 isolated from thermal springs of Tatapani (Himachal Pradesh).

An intracellular aliphatic amide degrading inducible thermo-active amidase produced by Geobacillus pallidus BTP-5x MTCC 9225 was purified to apparent homogeneity using anion exchange and gel filtration chromatography, giving a yield of 6.7 % and a specific activity of 30.49 units mg(-1). The purified protein migrated as a single band of estimated molecular mass of 158 kDa (homo-tetramer) in 8 % polyacrylamide gel electrophoresis and ∼38.5 kDa in 12 % sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Optima of pH and temperature varied widely in broad pH range (pH 6-9) and temperature range (45-70 °C). The purified amidase was stable up to 6 h at 50 °C, with a t (1/2) of 7 h at 55 °C. The multimeric nature of the holozyme (tetramer) contributed to protection of the enzyme against thermal denaturation. The enzyme showed resistance to metal chelating agents (EDTA, 8-hydroxyquinoline, and sodium azide), explaining its non-metallic nature, and is strongly inhibited by thiol reagents that means cysteine is involved in catalysis. The amidase of G. pallidus BTP-5x preferentially hydrolyzed only small aliphatic amides and has a narrow substrate spectrum. The K (M) value for acrylamide is 10.54 mM, V (max) 45.19 µmol(-1) min(-1) mg(-1) protein, and k (cat) 4.29 min(-1). The sequence of amino acids of the purified enzyme MRHGDISSSHDTVGI appears similar to thermophilic amidases. Sequence analysis of the amidase gene showed that the enzyme is 347 amino-acid-long with a molecular weight of 38.4 kDa (as observed in SDS-PAGE), theoretical pI 5.38, and show strong similarity to thermostable amidases, possessing unique restriction sites.