Reaction Mechanism of Porphyrin Metallation Studied by Theoretical Methods

Abstract

We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ into a porphyrin ring with density functional calculations with large basis set and including solvation, zero-point and thermal effects. We have followed the reaction from the outer-sphere complex, in which the metal is coordinated with six water molecules and the porphyrin is doubly protonated, until the metal ion is inserted into the deprotonated porphyrin ring with only one water ligand remaining. This reaction involves the stepwise displacement of five water molecules and the removal of two protons from the porphyrin ring. In addition, a step seems to be necessary in which a porphyrin pyrrolenine nitrogen atom changes its interaction from a hydrogen bond to a metal-bound solvent molecule to a direct coordination to the metal ion. If the protons are taken up by a neutral imidazole molecule, the deprotonation reactions are exothermic with minimal barriers. However, with a water molecule as an acceptor, they are endothermic. The ligand exchange reactions were approximately thermoneutral (+/-20 kJ mol(-1), with one exception) with barriers of up to 72 kJ mol(-1) for Mg and 51 kJ mol(-1) for Fe. For Mg, the highest barrier was found for the formation of the first bond to the porphyrin ring. For Fe, a higher barrier was found for the formation of the second bond to the porphyrin ring, but this barrier is probably lower in solution. No evidence was found for an initial pre-equilibrium between a planar and a distorted porphyrin ring. Instead, the porphyrin becomes more and more distorted as the number of metal-porphyrin bonds increase (by up to 191 kJ mol(-1)). This strain is released when the porphyrin becomes deprotonated and the metal moves into the ring plane. Implications of these findings for the chelatase enzymes are discussed.

Evolution in a family of chelatases facilitated by the introduction of active site asymmetry and protein oligomerization

“…Theoretical calculations predict that metallation involves a number of steps including removal of water from the metal ion annulus and deprotonation of the modified tetrapyrrole nitrogens (26). The binding of cobalt to the Se-CbiK His145, His207, and Glu175 triad promotes the desolvation steps.…”

Chelatases: distort to select?

“…Subsequent studies in solution and with enzymatic systems led to the formulation of a general reaction mechanism for the metalation of porphyrins [12][13][14][15]. The steps for the mechanism include, first, deformation of the porphyrin ring; second, outer-sphere association of the solvated metal ion and the porphyrin; third, exchange of a coordinated solvent molecule with the first pyrrolenine nitrogen atom; fourth, chelate-ring closure with liberation of additional solvent molecules to form the 'sitting-atop complex'; fifth, first deprotonation of a pyrrole nitrogen atom; and last, deprotonation of the second pyrrole nitrogen with formation of the metalated porphyrin ( Figure Ib).…”

“…Recently, Shen and Ryde [14] assessed the importance of ligand exchange in the non-enzymatic incorporation of either Fe 2+ or Mg 2+ into a porphyrin ring and found that the ligand-exchange step is rate limiting for the reaction. The estimated activation energies associated with this step were ~51 and 72 kJ/mol for Fe 2+ and Mg 2+ , respectively.…”

“…For non-enzymatic porphyrin metalation, Shen and Ryde [14] also proposed that deprotonation might be facilitated by hydrogen-bond networks, which translocate the proton away from the 'sitting-atop complex' (Box 2). Although the nature of the catalytic base involved in proton abstraction in ferrochelatase or any other chelatase remains to be determined, it is expected that any amino acid involved in deprotonatation will be located on the side of the porphyrin ring opposite to the metal ion.…”

“…Although a model of the general reaction mechanism of porphyrin metalation has emerged [10][11][12][13][14][15], how chelatases bestow metal ion selectivity and specificity on porphyrin metalation remains unresolved.…”