Molecular basis for the inability of an oxygen atom donor ligand to replace the natural sulfur donor heme axial ligand in cytochrome P450 catalysis. Spectroscopic characterization of the Cys436Ser CYP2B4 mutant.

All cytochrome P450s (CYPs) contain a cysteinate heme iron proximal ligand that plays a crucial role in their mechanism of action. Conversion of the proximal Cys436 to Ser in NH(2)-truncated microsomal CYP2B4 (ΔCYP2B4) transforms the enzyme into a two-electron NADPH oxidase producing H(2)O(2) without monooxygenase activity [K.P. Vatsis, H.M. Peng, M.J. Coon, J. Inorg. Biochem. 91 (2002) 542-553]. To examine the effects of this ligation change on the heme iron spin-state and coordination structure of ΔC436S CYP2B4, the magnetic circular dichroism and electronic absorption spectra of several oxidation/ligation states of the variant have been measured and compared with those of structurally defined heme complexes. The spectra of the substrate-free ferric mutant are indicative of a high-spin five-coordinate structure ligated by anionic serinate. The spectroscopic properties of the dithionite-reduced (deoxyferrous) protein are those of a five-coordinate (high-spin) state, and it is concluded that the proximal ligand has been protonated to yield neutral serine (ROH-donor). Low-spin six-coordinate ferrous complexes of the mutant with neutral sixth ligands (NO, CO, and O(2)) examined are also likely ligated by neutral serine, as would be expected for ferric complexes with anionic sixth ligands such as the hydroperoxo-ferric catalytic intermediate. Ligation of the heme iron by neutral serine vs. deprotonated cysteine is likely the result of the large difference in their acidity. Thus, without the necessary proximal ligand push of the cysteinate, although the ΔC436S mutant can accept two electrons and two protons, it is unable to heterolytically cleave the O-O bond of the hydroperoxo-ferric species to generate Compound I and hydroxylate the substrate.

E-2-ethylhexenal, E-2-ethyl-2-hexenol, mellein, and 4-hydroxymellein in Camponotus species from Brunei.

E-2-ethyl-2-hexen-1-ol (1), mellein (4), and 4-hydroxymellein (5) were identified as the major volatile compounds in the head and/or thorax of Camponotus quadrisectus. Neither 1 nor 5 have been previously detected in insects. Also identified were small amounts of m-cresol (2) and 6-methyl salicylic acid (3). E-2-ethylhexenal (6) and small amounts of 3 were identified in heads of Camponotus irritibilis from Kuala Belalong, Brunei. Compounds 2-4 occur in other Bornean camponotines with hypertrophied mandibular glands, and 4 is widespread in the tribe. The possibility of semiochemical parsimony (multiple functions) for these mandibular gland compounds is reviewed in the context of existing data on mandibular gland products of other camponotines, reported biological activities of the compounds, and secondary loss of metapleural glands in this ant group.

Venom chemistry of the ant Myrmicaria melanogaster from Brunei.

Analysis of the extracts of the ant Myrmicaria melanogaster from Brunei in the Indonesian archipelago by GC-MS and GC-IR revealed the presence of five new alkaloids, identified as (9Z)-3-propylindolizidine (1), cis- and trans-2-butyl-5-propylpyrrolidine (2 and 3, respectively), (10E)-3-butyllehmizidine (7), and (5Z,8Z,9Z)-3-butyl-5-propyl-8-hydroxyindolizidine (10a), whose structures were established by comparison with synthetic samples. In addition the monoterpene hydrocarbons beta-pinene, myrcene, and limonene were detected along with all four isomers of 3-butyl-5-methylindolizidine (4a-d), cis- and trans-2-butyl-5-(4-pentenyl)pyrrolidine (5a and 5b), trans-2-butyl-5-pentylpyrrolidine (6), (5Z,9Z)-3-butyl-5-propylindolizidine (8), and (5Z,9E)-3-butyl-5-propylindolizidine (9), alkaloids well known from ants and frogs, whose structures were established on the basis of published spectra or comparison with authentic samples. This study utilized vapor-phase infrared analysis for the assignment of stereochemistry using Bohlmann bands for the bicyclic alkaloids and, in the case of 10a, the detection of an intramolecular hydrogen bond. A biogenetic relationship between the mono- and bicyclic ring systems is proposed.

Spectroscopic characterization of five- and six-coordinate ferrous-NO heme complexes. Evidence for heme Fe-proximal cysteinate bond cleavage in the ferrous-NO adducts of the Trp-409Tyr/Phe proximal environment mutants of neuronal nitric oxide synthase.

Nitric oxide synthases (NOS) are a family of cysteine thiolate-ligated heme-containing monooxygenases that catalyze the NADPH-dependent two-step conversion of L-arginine to NO and L-citrulline. During the catalysis, a portion of the NOS heme forms an inhibitory complex with self-generated NO that is subsequently reverted back to NO-free active enzyme under aerobic conditions, suggesting a downstream regulator role of NO. Recent studies revealed that mutation of a conserved proximal tryptophan-409, which forms one of three hydrogen bonds to the heme-coordinated cysteine thiolate, to tyrosine or phenylalanine considerably increases the turnover number of neuronal NOS (nNOS). To further understand these properties of nNOS on its active site structural level, we have examined the oxygenase (heme-containing) domain of the two mutants in close comparison with that of wild-type nNOS with UV-visible absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopy. Among several oxidation and ligation states examined, only the ferrous-NO adducts of the two mutants exhibit spectra that are markedly distinct from those of parallel derivatives of the wild-type protein. The spectra of the ferrous-NO mutants are broadly similar to those of known five-coordinate ferrous-NO heme complexes, suggesting that these mutants are predominantly five coordinate in their ferrous-NO states. The present results are indicative of cleavage of the Fe-S bond in the nNOS mutants in their ferrous-NO state and imply a significant role of the conserved tryptophan in stabilization of the Fe-S bond.