Theoretical investigations of the hydrolysis pathway of verdoheme to biliverdin Mahin Gheidi, Nasser Safari * , Homayoon Bahrami, Mansour Zahedi * Department of Chemistry, Faculty of Sciences, Shahid Beheshti University, Evin, 19839-63113, Tehran, Iran Received 9 November 2005; received in revised form 19 October 2006; accepted 27 October 2006 Available online 17 November 2006 Abstract Conversion of iron(II) verdoheme to iron biliverdin in the presence of OH À was investigated using B3LYP method. Both 3-21G and 6-31G* basis sets were employed for geometry optimization calculation as well as energy stabilization estimation. Calculation at 6-31G* level was found necessary for a correct spin state estimation of the iron complexes. Two possible pathways for the conversion of iron verdoheme to iron biliverdin were considered. In one path the iron was six-coordinate while in the other it was considered to be five- coordinate. In the six-coordinated pathway, the ground state of bis imidazole iron verdoheme is singlet while that for open chain iron biliverdin it is triplet state with 4.86 kcal/mol more stable than the singlet state. The potential energy surface suggests that a spin inver- sion take place during the course of reaction after TS. The ring opening process in the six-coordinated pathway is in overall À2.26 kcal/ mol exothermic with a kinetic barrier of 9.76 kcal/mol. In the five-coordinated pathway the reactant and product are in the ground triplet state. In this path, hydroxyl ion attacks the iron center to produce a complex, which is only 1.59 kcal/mol more stable than when OH À directly attacks the macrocycle. The activation barrier for the conversion of iron hydroxy species to the iron biliverdin complex by a rebound mechanism is estimated to be 32.68 kcal/mol. Large barrier for rebound mechanism, small barrier of 4.18 kcal/mol for ring opening process of the hydroxylated macrocycle, and relatively same stabilities for complexes resulted by the attack of nucleophile to the iron and macrocycle indicate that five-coordinated pathway with direct attack of nucleophile to the 5-oxo position of macrocycle might be the path for the conversion of verdoheme to biliverdin. Ó 2006 Elsevier Inc. All rights reserved. Keywords: B3LYP; Verdoheme; Biliverdin; Hydrolysis; OH À 1. Introduction Degradation of heme is an important physiological pro- cess that is carried out by O 2 /NADPH in the presence of heme oxygenase enzyme/HO in mammals [1–3]. The cou- pled oxidation in which heme degradation occurs by O 2 / reducing agent in a coordinating solvent has widely been used as a model for biological heme catabolism [4–6]. The occurrence of same intermediates in the physiological process and in the coupled oxidation can show a resem- blance in these two pathways (Scheme 1). The mechanism that converts verdoheme into biliverdin is the subject of some controversy. Two pathways have been proposed for the oxidation of verdoheme to biliverdin (Scheme 2). One path, the oxidative path, implicates fer- rous verdoheme and O 2 to produce ferric peroxide inter- mediate that further decays to iron(III) biliverdin. In this oxidative path, parallel routes exist for the release of oxi- dized iron ion either in the form of iron(V) or iron(III) with two electrons oxidized form of biliverdin, in a path similar to that for the cytochrom P450 catalytic cycle. The other path for the conversion of verdoheme into biliverdin involves hydrolysis of the 5-oxaporphryrin macrocycle [7–12]. Balch’s group have recently shown that treatment of iron(II), cobalt(II) and zinc(II) verdoheme [13,14] with 0162-0134/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2006.10.012 * Corresponding authors. Tel.: +98 21 2401765; fax: +98 21 2403041 (N. Safari). E-mail address: n-safari@cc.sbu.ac.ir (N. Safari). www.elsevier.com/locate/jinorgbio Journal of Inorganic Biochemistry 101 (2007) 385–395 JOURNAL OF Inorganic Biochemistry