The activation mechanism of human porphobilinogen synthase by 2-mercaptoethanol: intrasubunit transfer of a reserve zinc ion and coordination with three cysteines in the active center |
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Authors: | Nori Sawada Noriyuki Nagahara Tadashi Sakai Yoshiaki Nakajima Masayasu Minami Tomoyuki Kawada |
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Affiliation: | (1) Environmental Medicine, Graduate School of Medicine, Nippon Medical School, 1-1-5, Sendagi, Bunkyo-ku, Tokyo 113-8602, Japan;(2) Occupational Poisoning Center, Tokyo Rosai Hospital, 4-13-21, Ohmoriminami, Ohta-ku, Tokyo 143-0013, Japan |
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Abstract: | Human porphobilinogen synthase [EC.4.2.1.24] is a homo-octamer enzyme. In the active center of each subunit, four cysteines are titrated with 5,5-dithiobis(2-nitrobenzoic acid). Cys122, Cys124 and Cys132 are placed near two catalytic sites, Lys199 and Lys252, and coordinate a zinc ion, referred to as a proximal zinc ion, and Cys223 is placed at the orifice of the catalytic cavity and coordinates a zinc ion, referred to as a distal zinc ion, with His131 . When the wild-type enzymes C122A (Cys122Ala), C124A (Cys124Ala), C132A (Cys132Ala) and C223A (Cys223Ala) were oxidized by hydrogen peroxide, the levels of activity were decreased. Two cysteines were titrated with 5,5-dithiobis(2-nitrobenzoic acid) in the wild-type enzyme, while on the other hand, one cysteine was titrated in the mutant enzymes. When wild-type and mutant enzymes were reduced by 2-mercaptoethanol, the levels of activity were increased: four and three cysteines were titrated, respectively, suggesting that a disulfide bond was formed among Cys122, Cys124 and Cys132 under oxidizing conditions. We analyzed the enzyme-bound zinc ion of these enzymes using inductively coupled plasma mass spectrometry with gel-filtration chromatography. The results for C223A showed that the number of proximal zinc ions correlated to the level of enzymatic activity. Furthermore, zinc-ion-free 2-mercaptoethanol increased the activity of the wild-type enzyme without a change in the total number of zinc ions, but C223A was not activated. These findings suggest that a distal zinc ion moved to the proximal binding site when a disulfide bond among Cys122, Cys124 and Cys132 was reduced by reductants. Thus, in the catalytic functioning of the enzyme, the distal zinc ion does not directly contribute but serves rather as a reserve as the next proximal one that catalyzes the enzyme reaction. A redox change of the three cysteines in the active center accommodates the catch and release of the reserve distal zinc ion placed at the orifice of the catalytic cavity. |
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Keywords: | Inductively coupled plasma mass spectrometry Oxidation Reduction SH-group titration Zinc ion |
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