title: Heme symmetry, vibronic structure, and dynamics in heme proteins: ferrous nicotinate horse myoglobin and soybean leghemoglobin.
abstract: We report the visible and Soret absorption bands, down to cryogenic temperatures, of the ferrous nicotinate adducts of native and deuteroheme reconstituted horse heart myoglobin in comparison with soybean leghemoglobin-a. The band profile in the visible region is analyzed in terms of vibronic coupling of the heme normal modes to the electronic transition in the framework of the Herzberg-Teller approximation. This theoretical approach makes use of the crude Born-Oppenheimer states and therefore neglects the mixing between electronic and vibrational coordinates; however, it takes into account the vibronic nature of the visible absorption bands and allows an estimate of the vibronic side bands for both Condon and non-Condon vibrational modes. In this framework, an x-y splitting of the Q transition for native and deuteroheme reconstituted horse myoglobin is clearly assessed and attributed to electronic perturbations that, in turn, are caused by a reduction of the typical D(4h) symmetry of the system due to heme distortions of B(1g)-type symmetry and/or to an x-y asymmetric position of the nicotinate ring; in deuteroheme reconstituted horse myoglobin the asymmetric heme peripheral substituents add to the above effect(s). On the contrary, in leghemoglobin-a no spectral splitting upon nicotinate binding is observed, pointing to a planar heme configuration in which only distortions of A(1g)-type symmetry are effective and to which the nicotinate ring is bound in an x - y symmetric position. The local dynamic properties of the heme pocket of the three proteins are investigated through the temperature dependence of spectral line broadening. Leghemoglobin-a behaves as a softer matrix with respect to horse myoglobin, thus validating the hypothesis of a looser heme pocket conformation in the former protein, which allows a nondistorted heme configuration and a symmetric binding of the bulky nicotinate ligand.
authors.affiliation: Istituto Nazionale di Fisica della Materia and Department of Physical and Astronomical Sciences, University of Palermo, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Sanfratello
authors.forename: A
authors.initials: A
authors.lastname: Boffi
authors.forename: A
authors.initials: A
authors.lastname: Cupane
authors.forename: M
authors.initials: M
authors.lastname: Leone
mesh_terms: D000818:Animals; D005296:Ferrous Compounds / Q000737:chemistry / Q000378:metabolism; D006418:Heme / Q000737:chemistry*; D006420:Hemeproteins / Q000737:chemistry*; D006736:Horses; D007874:Leghemoglobin / Q000737:chemistry; D009211:Myoglobin / Q000737:chemistry / Q000378:metabolism; D009539:Nicotinic Acids / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary; D013025:Glycine max; D013053:Spectrophotometry; D013696:Temperature; D014732:Vibration
chemical_list: D005296:Ferrous Compounds; D006420:Hemeproteins; D007874:Leghemoglobin; D009211:Myoglobin; D009539:Nicotinic Acids; D006418:Heme