Unusually Fast bis-Histidyl Coordination in a Plant Hemoglobin

International Journal of Molecular Sciences

AlexJ. Barker¹  Manuel Becana²  Carmen Pérez-Rontomé²  Irene Villar²  Stefania Abbruzzetti³  Cristiano Viappiani³  Stefano Bruno⁴  Giulio Cerullo⁵ 

[1] Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, via G. Pascoli 70/3, 20133 Milano, Italy;Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas (CSIC), Apartado 13034, 50080 Zaragoza, Spain;Dipartimento di Scienze Matematiche, Fisiche e Informatiche, Università di Parma, Parco Area delle Scienze 7/A, 43124 Parma, Italy;Dipartimento di Scienze degli Alimenti e del Farmaco, Parco Area delle Scienze 27/A, 43124 Parma, Italy;IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy;

关键词: Medicago truncatula;    ultrafast spectroscopy;    plant hemoglobins;    CO rebinding kinetics;    iron/heme hexacoordination;   

DOI  :  10.3390/ijms22052740

来源: DOAJ

【 摘 要 】

The recently identified nonsymbiotic hemoglobin gene MtGlb1-2 of the legume Medicago truncatula possesses unique properties as it generates four alternative splice forms encoding proteins with one or two heme domains. Here we investigate the ligand binding kinetics of MtGlb1-2.1 and MtGlb1-2.4, bearing one and two hemes, respectively. Unexpectedly, the overall time-course of ligand rebinding was unusually fast. Thus, we complemented nanosecond laser flash photolysis kinetics with data collected with a hybrid femtosecond–nanosecond pump–probe setup. Most photodissociated ligands are rebound geminately within a few nanoseconds, which leads to rates of the bimolecular rebinding to pentacoordinate species in the 10⁸ M^-1s^-1 range. Binding of the distal histidine to the heme competes with CO rebinding with extremely high rates (k_h~10⁵ s^-1). Histidine dissociation from the heme occurs with comparable rates, thus resulting in moderate equilibrium binding constants (K_H~1). The rate constants for ligation and deligation of distal histidine to the heme are the highest reported for any plant or vertebrate globin. The combination of microscopic rates results in unusually high overall ligand binding rate constants, a fact that contributes to explaining at the mechanistic level the extremely high reactivity of these proteins toward the physiological ligands oxygen, nitric oxide and nitrite.

【 授权许可】

Unknown