Probing the Role of a Conserved Phenylalanine in the Active Site of Thiocyanate Dehydrogenase

Copper-containing enzymes catalyze a broad spectrum of redox reactions. Thiocyanate dehydrogenase (TcDH) from Thioalkalivibrio paradoxus Arh1 enables the bacterium to use thiocyanate as a unique source of energy and nitrogen. Oxidation of thiocyanate takes place in the trinuclear copper center of TcDH with peculiar organization. Despite the TcDH crystal structure being established, a role of some residues in the enzyme active site has yet to be obscured. F436 residue is located in the enzyme active site and conserved among a number of TcDH homologs, however, its role in the copper center formation or the catalytic process is still not clear. To address this question, a mutant form of the enzyme with F436Q substitution (TcDHF436Q) was obtained, biochemically characterized, and its crystal structure was determined. The TcDHF436Q had an unaltered protein fold but did not possess enzymatic activity, whereas it contained all three copper ions, according to ICP-MS data. The structural data showed that the F436Q substitution resulted in a disturbance of hydrophobic interactions within the active site crucial for a correct transition between open/closed forms of the enzyme–substrate channel. Thus, we demonstrated that F436 does not participate in copper ion binding, but rather possesses a structural role in the TcDH active site

Probing the Role of a Conserved Phenylalanine in the Active Site of Thiocyanate Dehydrogenase

Copper-containing enzymes catalyze a broad spectrum of redox reactions. Thiocyanate dehydrogenase (TcDH) from Thioalkalivibrio paradoxus Arh1 enables the bacterium to use thiocyanate as a unique source of energy and nitrogen. Oxidation of thiocyanate takes place in the trinuclear copper center of TcDH with peculiar organization. Despite the TcDH crystal structure being established, a role of some residues in the enzyme active site has yet to be obscured. F436 residue is located in the enzyme active site and conserved among a number of TcDH homologs, however, its role in the copper center formation or the catalytic process is still not clear. To address this question, a mutant form of the enzyme with F436Q substitution (TcDHF436Q) was obtained, biochemically characterized, and its crystal structure was determined. The TcDHF436Q had an unaltered protein fold but did not possess enzymatic activity, whereas it contained all three copper ions, according to ICP-MS data. The structural data showed that the F436Q substitution resulted in a disturbance of hydrophobic interactions within the active site crucial for a correct transition between open/closed forms of the enzyme–substrate channel. Thus, we demonstrated that F436 does not participate in copper ion binding, but rather possesses a structural role in the TcDH active site

Structural framework for the understanding spectroscopic and functional signatures of the cyanobacterial Orange Carotenoid Protein families

The Orange Carotenoid Protein (OCP) is a unique photoreceptor crucial for cyanobacterial photoprotection. Best studied Synechocystis sp. PCC 6803 OCP belongs to the large OCP1 family. Downregulated by the Fluorescence Recovery Protein (FRP) in low-light, high-light-activated OCP1 binds to the phycobilisomes and performs non -photochemical quenching. Recently discovered families OCP2 and OCP3 remain structurally and functionally underexplored, and no systematic comparative studies have ever been conducted. Here we present two first crystal structures of OCP2 from morphoecophysiologically different cyanobacteria and provide their comprehensive structural, spectroscopic and functional comparison with OCP1, the recently described OCP3 and all-OCP ancestor. Structures enable correlation of spectroscopic signatures with the effective number of hydrogen and discovered here chalcogen bonds anchoring the ketocarotenoid in OCP, as well as with the rotation of the echinenone's beta-ionone ring in the CTD. Structural data also helped rationalize the observed differences in OCP/ FRP and OCP/phycobilisome functional interactions. These data are expected to foster OCP research and applications in optogenetics, targeted carotenoid delivery and cyanobacterial biomass engineering

Unusual Cytochrome c552 from Thioalkalivibrio paradoxus: Solution NMR Structure and Interaction with Thiocyanate Dehydrogenase

The search of a putative physiological electron acceptor for thiocyanate dehydrogenase (TcDH) newly discovered in the thiocyanate-oxidizing bacteria Thioalkalivibrio paradoxus revealed an unusually large, single-heme cytochrome c (CytC552), which was co-purified with TcDH from the periplasm. Recombinant CytC552, produced in Escherichia coli as a mature protein without a signal peptide, has spectral properties similar to the endogenous protein and serves as an in vitro electron acceptor in the TcDH-catalyzed reaction. The CytC552 structure determined by NMR spectroscopy reveals significant differences compared to those of the typical class I bacterial cytochromes c: a high solvent accessible surface area for the heme group and so-called “intrinsically disordered” nature of the histidine-rich N- and C-terminal regions. Comparison of the signal splitting in the heteronuclear NMR spectra of oxidized, reduced, and TcDH-bound CytC552 reveals the heme axial methionine fluxionality. The TcDH binding site on the CytC552 surface was mapped using NMR chemical shift perturbations. Putative TcDH-CytC552 complexes were reconstructed by the information-driven docking approach and used for the analysis of effective electron transfer pathways. The best pathway includes the electron hopping through His528 and Tyr164 of TcDH, and His83 of CytC552 to the heme group in accordance with pH-dependence of TcDH activity with CytC552

Unusual Cytochrome c552 from Thioalkalivibrio paradoxus: Solution NMR Structure and Interaction with Thiocyanate Dehydrogenase

The search of a putative physiological electron acceptor for thiocyanate dehydrogenase (TcDH) newly discovered in the thiocyanate-oxidizing bacteria Thioalkalivibrio paradoxus revealed an unusually large, single-heme cytochrome c (CytC552), which was co-purified with TcDH from the periplasm. Recombinant CytC552, produced in Escherichia coli as a mature protein without a signal peptide, has spectral properties similar to the endogenous protein and serves as an in vitro electron acceptor in the TcDH-catalyzed reaction. The CytC552 structure determined by NMR spectroscopy reveals significant differences compared to those of the typical class I bacterial cytochromes c: a high solvent accessible surface area for the heme group and so-called “intrinsically disordered” nature of the histidine-rich N- and C-terminal regions. Comparison of the signal splitting in the heteronuclear NMR spectra of oxidized, reduced, and TcDH-bound CytC552 reveals the heme axial methionine fluxionality. The TcDH binding site on the CytC552 surface was mapped using NMR chemical shift perturbations. Putative TcDH-CytC552 complexes were reconstructed by the information-driven docking approach and used for the analysis of effective electron transfer pathways. The best pathway includes the electron hopping through His528 and Tyr164 of TcDH, and His83 of CytC552 to the heme group in accordance with pH-dependence of TcDH activity with CytC552