H2 Reaction Lab Report

Chapter 3: H2S Interactions with Hemes and Cytochrome C Oxidase There are three main ways for free hydrogen sulfide to interact with heme centers.2 This includes the binding of H2S to the metal center, the reduction of the metal center to produce a radical thiol species, and the direct covalent modification of the porphyrin.2

Type I interactions are typically stabilized by a distal imidazole, similar to O2 and CO binding onto hemes.2 Also, it is important to acknowledge that the iron remains in the ferric state when binding H2S, even if sulfur is a soft ligand, likely due to the higher valence of the iron prior to binding.2 Type I and type II interactions can be considered to be in equilibrium with each other, since a type II product is essentially the result of a homolytic cleavage of the Fe-S bond from a type I product.2 Type II products are not thermodynamically stable without a second equivalent of H2S, which complexes with the sulfide radical to create an H2S2•- species.2 This type of reaction can continue to create polysulfide chains.2 Lastly, type III reactions
As shown in Figure 12, at low H2S levels, a type I reaction proceeds, binding H2S to the heme a3 center (intermediates 1 and 2).2 As the concentration of endogenous H2S increases, the second equivalent becomes available, allowing for a type II reaction to occur.2 Intermediate 3, or the reduced form of cytochrome c oxidase, is capable of binding O2, and as a result, this level of H2S activates respiration.2 The process to intermediate 4 can then follow from an electron transfer from the heme a3 to the CuB site or from a type II homologue reaction with the copper in the resting state of the terminal oxidase.2 At even higher levels of H2S, a type I homologue causes H2S to bind reduced copper, inactivating the site, and causing copper to lose oxidizing capability.2 As a result, when H2S binds the iron center in the last type I reaction to intermediate 6, it stays bound and inhibits catalytic