5677-55-4Relevant articles and documents
A tightly bound quinone functions in the ubiquinone reaction sites of quinoprotein alcohol dehydrogenase of an acetic acid bacterium, Gluconobacter suboxydans
Matsushita, Kazunobu,Kobayashi, Yoshiki,Mizuguchi, Mitsuhiro,Toyama, Hirohide,Adachi, Osao,Sakamoto, Kimitoshi,Miyoshi, Hideto
, p. 2723 - 2731 (2008)
Quinoprotein alcohol dehydrogenase (ADH) of acetic acid bacteria is a membrane-bound enzyme that functions as the primary dehydrogenase in the ethanol oxidase respiratory chain. It consists of three subunits and has a pyrroloquinoline quinone (PQQ) in the active site and four heme c moieties as electron transfer mediators. Of these, three heme c sites and a further site have been found to be involved in ubiquinone (Q) reduction and ubiquinol (QH2) oxidation respectively (Matsushita et al., Biochim. Biophys. Acta, 1409, 154-164 (1999)). In this study, it was found that ADH solubilized and purified with dodecyl maltoside, but not with Triton X-100, had a tightly bound Q, and thus two different ADHs, one having the tightly bound Q (Q-bound ADH) and Q-free ADH, could be obtained. The Q-binding sites of both the ADHs were characterized using specific inhibitors, a substituted phenol PC16 (a Q analog inhibitor) and antimycin A. Based on the inhibition kinetics of Q2 reductase and ubiquinol-2 (Q2H2) oxidase activities, it was suggested that there are one and two PC16-binding sites in Q-bound ADH and Q-free ADH respectively. On the other hand, with antimycin A, only one binding site was found for Q2 reductase and Q2H2 oxidase activities, irrespective of the presence of bound Q. These results suggest that ADH has a high-affinity Q binding site (QH) besides low-affinity Q reduction and QH2 oxidation sites, and that the bound Q in the QH site is involved in the electron transfer between heme c moieties and bulk Q or QH2 in the low-affinity sites.
Rapid photochemical generation of ubiquinol through a radical pathway: An avenue for probing submillisecond enzyme kinetics
Schultz, Brian E.,Hansen, Kirk C.,Lin, Charles C.,Chan, Sunney I.
, p. 3244 - 3247 (2007/10/03)
-
The surprisingly high reactivity of phenoxyl radicals
Foti,Ingold,Lusztyk
, p. 9440 - 9447 (2007/10/02)
Rate constants have been measured in nonaqueous media for hydrogen atom abstraction by the phenoxyl radical from some biologically important phenols and related compounds. Although the thermochemistry for these reactions must be very similar to the thermochemistry for H atom abstraction from the same substrate by a peroxyl radical, the phenoxyl rate constants, k5, are ca. 100-300 times greater than the (already well-known) peroxyl rate constants, k1. For example, with α-tocopherol in benzene/di-tert-butyl peroxide (1:3, v/v) k5293K = 1.1 × 109 M-1 s-1 vs k1303K = 3.2 × 106 M-1 s-1 in a similar nonpolar medium, and with ubiquinol-10 in the same solvent mixture k5293k = 8.4 × 107 M-1 s-1, while the corresponding value for k1 is 3.5 × 105 M-1 s-1. The greater reactivity of the phenoxyl radical has been traced to the fact that the Arrhenius preexponential factors are much larger than for the corresponding peroxyl radical reactions, i.e., A5 ～ 102A1. For example, with α-naphthol log(A5/M-1 s-1) = 8.9 and E5 = 2.2 kcal/mol vs log(A1/M-1 s-1) = 6.4 and E1 = 1.7 kcal/mol. The preexponential factors for H-atom donors more reactive than α-naphthol are even greater; for example, with α-tocopherol in CH3CN/di-tert-butyl peroxide (1:2, v/v) log(A5/M-1 s-1) = 10.0 and E5 = 2.0 kcal/mol, and with ubiquinol-0 in benzene/di-tert-butyl peroxide (1:3, v/v) log(A5/M-1 s-1) = 10.5 and E5 = 3.5 kcal/mol. The role that intermediate hydrogen-bonded complexes between the reacting radical and the phenolic hydrogen donor may play in these reactions is discussed, and it is pointed out that our results are likely to be relevant to in vivo radical chemistry.
