Insights into the function of two hemeproteins involved in denitrification and apoptosis, as revealed by ultrafast transient absorption spectroscopy.

by Dr Sofia Kapetanaki (Laboratoire d’Optique et Biosciences, CNRS, INSERM U696 Ecole Polytechnique, 91128 Palaiseau, France)

Thursday 16 Oct 2008, 14:00 → 16:00 Europe/Rome

Aula Corbino (Dip. di Fisica - Edificio E. Fermi)

The catalytic subunit of nitric oxide reductase (NOR) from Paracoccus denitrificans is evolutionarily related to that of heme-copper oxidases. With the aim of exploring the interactions of external ligands with NOR, using ultrafast transient absorption spectroscopy we investigated the dynamics of the physiological substrate NO (nitric oxide), and of CO (carbon monoxide), with the active site, which contains heme (heme b3) and non-heme iron (FeB). Our results reveal that heme-ligand recombination in this enzyme is considerably faster than in heme-copper oxidases and are consistent with a more confined configuration of the active site. The interaction of mitochondrial cytochrome c (cyt c) with cardiolipin (CL) is involved in the initial stages of apoptosis. This interaction can lead to destabilization of the heme-Met80 bond and peroxidase activity. Under these conditions CO binds to cyt c, with very high affinity in contrast to the native cyt c protein involved in respiratory electron shuttling that does not bind CO. Our studies on the ultrafast CO and NO binding dynamics reveal that CL leads to substantial changes in protein conformation and flexibility, allowing access of ligands to the heme and point towards a regulatory role of both gases in the cyt c-mediated apoptotic pathway.