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Heme oxygenase is a central enzyme of heme degradation and associated carbon monoxide biosynthesis. We have prepared the alpha-hydroxyheme-heme oxygenase complex, which is the first intermediate in the catalytic reaction. The active site structure of the complex was examined by optical absorption, EPR, and resonance Raman spectroscopies. In the ferric form of the enzyme complex, the heme iron is five coordinate high spin and the alpha-hydroxyheme group in the complex assumes a structure of an oxophlorin where the alpha-meso hydroxy group is deprotonated. In the ferrous form, the alpha-hydroxy group is protonated and consequently the prosthetic group assumes a porphyrin structure. The alpha-hydroxyheme group undergoes a redox-linked conversion between a keto and an enol form. The ferric alpha-hydroxyheme reacts with molecular oxygen to form a radical species. Reaction of the radical species with a reducing equivalent yields the verdoheme-heme oxygenase complex. Reaction of the ferrous alpha-hydroxyheme-heme oxygenase complex with oxygen also yields the verdoheme-enzyme complex. We conclude that the catalytic conversion of ferric alpha-hydroxyheme to verdoheme by heme oxygenase requires molecular oxygen and one reducing equivalent.

A cone snail venom peptide, μO §-conotoxin GVIIJ from Conus geographus, has a unique posttranslational modification, S-cysteinylated cysteine, which makes possible formation of a covalent tether of peptide to its target Na channels at a distinct ligandbinding site. μO §-conotoxin GVIIJ is a 35-aa peptide, with 7 cysteine residues; six of the cysteines form 3 disulfide cross-links, and one (Cys24) is S-cysteinylated. Due to limited availability of native GVIIJ, we primarily used a synthetic analog whose Cys24 was S-glutathionylated (abbreviated GVIIJ SSG ). The peptide-channel complex is stabilized by a disulfide tether between Cys24 of the peptide and Cys910 of rat (r) Na V 1.2. A mutant channel of rNa V 1.2 lacking a cysteine near the pore loop of domain II (C910L), was >10 3 -fold less sensitive to GVIIJ SSG than was wild-type rNa V 1.2. In contrast, although rNa V 1.5 was >10 4 -fold less sensitive to GVIIJ SSG than Na V 1.2, an rNa V 1.5 mutant with a cysteine in the homologous location, rNa V 1.5[L869C], was >10 3 -fold more sensitive than wildtype rNa V 1.5. The susceptibility of rNa V 1.2 to GVIIJ SSG was significantly altered by treating the channels with thiol-oxidizing or disulfide-reducing agents. Furthermore, coexpression of rNa V β2 or rNa V β4, but not that of rNa V β1 or rNa V β3, protected rNa V 1.1 to -1.7 (excluding Na V 1.5) against block by GVIIJ SSG . Thus, GVIIJrelated peptides may serve as probes for both the redox state of extracellular cysteines and for assessing which Na V β-and Na V α-subunits are present in native neurons.oltage-gated sodium channels (VGSCs) are responsible for the upstroke of action potentials in excitable tissues. Each VGSC is composed of a pore-and voltage sensor-bearing α-subunit and one or more auxiliary β-subunits. Mammals have nine α-subunit isoforms (Na V 1.1 to -1.9) and four β-subunit isoforms (Na V β1 to -β4) (1). An Na V 1 has about 2,000-aa residues arranged in four homologous domains, where each domain has six transmembrane spanning segments with an extracellular "pore" loop between segments 5 and 6 (1, 2); furthermore, each Na V 1 has about a dozen extracellular cysteine residues, all located in or near the pore loops. For the most part, not much is known about these cysteines (including whether they are disulfide bonded).Na V β-subunits can affect the function and cellular localization of Na V 1s (1, 3-5). Each Na V β-subunit has some 200-aa residues and consists of a single transmembrane segment with a large extracellular domain and a smaller intracellular domain (1). Na V β2-and Na V β4-subunits, unlike Na V β1-and Na V β3-subunits, are disulfide bonded to α-subunits (1, 6). A given neuron can have multiple isoforms of these subunits whose identities are challenging to appraise pharmacologically (7).Toxins that target VGSCs have been invaluable for probing the structure and function of these channels. Venoms are a rich source of such toxins. For example, in Conus snails, four families of neuroactive peptides have been characterized that target VGSCs:...

The O 2 and CO reactions with the heme, ␣-hydroxyheme, and verdoheme complexes of heme oxygenase have been studied. and protein residues in the heme pocket. The CO affinities estimated for both isoforms are only 1-6-fold higher than the corresponding O 2 affinities. Thus, heme oxygenase discriminates much more strongly against CO binding than either myoglobin or hemoglobin. The CO binding reactions with the ferrous ␣-hydroxyheme complex are similar to those of the protoheme complex, and hydroxylation at the ␣-meso position does not appear to affect the reactivity of the iron atom. In contrast, the CO affinities of the verdoheme complexes are >10,000 times weaker than those of the heme complexes because of a 100-fold slower association rate constant (k CO Ϸ 0.004 M ؊1 s ؊1) and a 300-fold greater dissociation rate constant (k CO Ϸ 3 s ؊1 ) compared with the corresponding rate constants of the protoheme and ␣-hydroxyheme complexes. The positive charge on the verdoporphyrin ring causes a large decrease in reactivity of the iron.

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