Communication between L-galactono-1,4-lactone dehydrogenase and cytochrome c

l-galactono-1,4-lactone dehydrogenase (GALDH) catalyzes the terminal step of vitamin C biosynthesis in plant mitochondria. Here we investigated the communication between Arabidopsisthaliana GALDH and its natural electron acceptor cytochrome c (Cc). Using laser-generated radicals we observed the formation and stabilization of the GALDH semiquinone anionic species (GALDHSQ). GALDHSQ oxidation by Cc exhibited a nonlinear dependence on Cc concentration consistent with a kinetic mechanism involving proteinpartner association to form a transient bimolecular complex prior to the electron transfer step. Oxidation of GALDHSQ by Cc was significantly impaired at high ionic strength, revealing the existence of attractive chargecharge interactions between the two reactants. Isothermal titration calorimetry showed that GALDH weakly interacts with both oxidized and reduced Cc. Chemical shift perturbations for 1H and 15N nuclei of Cc, arising from the interactions with unlabeled GALDH, were used to map the interacting surface of Cc. For ArabidopsisCc and yeast Cc, similar residues are involved in the interaction with GALDH. These residues are confined to a single surface surrounding the heme edge. The range of chemical shift perturbations for the physiological ArabidopsisCcGALDH complex is larger than that of the non-physiological yeast CcGALDH complex, indicating that the former complex is more specific. In summary, the results point to a relatively low affinity GALDHCc interaction, similar for all partner redox states, involving proteinprotein dynamic motions. Evidence is also provided that Cc utilizes a conserved surface surrounding the heme edge for the interaction with GALDH and other redox partners. Database NMR assignment of the backbone amide resonances of ArabidopsisCcRED has been deposited in BMRB database (BMRB accession number 18828). L-galactono-1,4-lactone dehydrogenase (L-galactono-1,4-lactone: ferricytochrome c oxidoreductase, EC 1.3.2.3)