Use of magnetic circular dichroism to study dinuclear metallohydrolases and the corresponding biomimetics

Abstract

Magnetic circular dichroism (MCD) is a convenient technique for providing structural and mechanistic insight into enzymatic systems in solution. The focus of this review is on aspects of geometric and electronic structure that can be determined by MCD, and how this method can further our understanding of enzymatic mechanisms. Dinuclear Co(II) systems that catalyse hydrolytic reactions were selected to illustrate the approach. These systems all contain active sites with similar structures consisting of two Co(II) ions bridged by one or two carboxylates and a water or hydroxide. In most of these active sites one Co(II) is five-coordinate and one is six-coordinate, with differing binding affinities. It is shown how MCD can be used to determine which binding site--five or six-coordinate--has the greater affinity. Importantly, zero-field-splitting data and magnetic exchange coupling constants may be determined from the temperature and field dependence of MCD data. The relevance of these data to the function of the enzymatic systems is discussed.

Kinetic, mechanistic, structural and spectroscopic investigations of Bimetallic Metallohydrolases

“…The calculations indicate that the six transitions originate from two six-coordinate Co 2+ species (Table 5.3). .12) were analysed using the dimer model (229) .…”

“…The excess of Co 2+ was removed and fitted to a minimum number of Gaussian peaks to achieve the final composite spectrum using GRAMS AI software package. The data were analysed as described elsewhere (22,(218)(219)(220)(229)(230)(231)(232) .…”

“…The MCD spectrum of resting AIM-1 ( Figure 5.11) could be fit to no fewer than six transitions between 450 and 600 nm. Calculations using the angular overlap model (AOM) were used to assign the individual transitions as described in detail elsewhere (218,220,(229)(230)(231)(232) . The coordinates for the calculations were taken from the crystal structure (PDB code: 4AWY (216) ) and each Co 2+ site was treated separately.…”

“…While CD requires an optically active sample and relies on structural features (chirality) to distribute the electronic charge, MCD arises purely from the electromagnetic interactions between the charges within the sample and the externally applied field. Furthermore, the A in the MCD results from a magnetic perturbation of the electronic states involved in the absorption transition by the Zeeman effect, defined by the magnetic optical rotation observed when a plane of linear polarized light is rotated in a collinear magnetic field in terms of B and C leading to Equation 6.1 (253,254) . Equation 6.1…”

“…In this chapter, I will focus on enzyme samples whose active sites contain reconstituted bimetallic Co 2+ centres. Apart from being paramagnetic, Co 2+ -reconstituted metal ion centres have been studied for a number of BMHs and related model complexes, and thus they provide a well-studied platform for structural comparisons (23,144,(254)(255)(256)(257)(258) . Co 2+ displays coordination flexibility, encompassing the ability to form octahedral, tetrahedral, square pyramidal, trigonal bipyramidal and trigonal prismatic complexes (259) .…”