Cloning, expression and purification of cytochrome c(6) from the brown alga Hizikia fusiformis and complete X-ray diffraction analysis of the structure

A High-Resolution Crystallographic Study of Cytochrome c6: Structural Basis for Electron Transfer in Cyanobacterial Photosynthesis

Cyanobacterial cytochrome c6 (Cyt c6) is crucial for electron transfer between the cytochrome b6f complex and photosystem I (PSI), playing a key role in photosynthesis and enhancing adaptation to extreme environments. This study investigates the high-resolution crystal structures of Cyt c6 from Synechococcus elongatus PCC 7942 and Synechocystis PCC 6803, focusing on its dimerization mechanisms and functional implications for photosynthesis. Cyt c6 was expressed in Escherichia coli using a dual-plasmid co-expression system and characterized in both oxidized and reduced states. X-ray crystallography revealed three distinct crystal forms, with asymmetric units containing 2, 4, or 12 molecules, all of which consist of repeating dimeric structures. Structural comparisons across species indicated that dimerization predominantly occurs through hydrophobic interactions within a conserved motif around the heme crevice, despite notable variations in dimer positioning. We propose that the dimerization of Cyt c6 enhances structural stability, optimizes electron transfer kinetics, and protects the protein from oxidative damage. Furthermore, we used AlphaFold3 to predict the structure of the PSI-Cyt c6 complex, revealing specific interactions that may facilitate efficient electron transfer. These findings provide new insights into the functional role of Cyt c6 dimerization and its contribution to improving cyanobacterial photosynthetic electron transport.

Alanine to serine substitutions drive thermal adaptation in a psychrophilic diatom cytochrome c6

In this study, we investigate the thermodynamic mechanisms by which electron transfer proteins adapt to environmental temperature by directly comparing the redox properties and folding stability of a psychrophilic cytochrome c and a mesophilic homolog. Our model system consists of two cytochrome c₆ proteins from diatoms: one adapted specifically to polar environments, the other adapted generally to surface ocean environments. Direct electrochemistry shows that the midpoint potential for the mesophilic homolog is slightly higher at all temperatures measured. Cytochrome c₆ from the psychrophilic diatom unfolds with a melting temperature 10.4 °C lower than the homologous mesophilic cytochrome c₆. Changes in free energy upon unfolding are identical, within error, for the psychrophilic and mesophilic protein; however, the chemical unfolding transition of the psychrophilic cytochrome c₆ is more cooperative than for the mesophilic cytochrome c₆. Substituting alanine residues found in the mesophile with serine found in corresponding positions of the psychrophile demonstrates that burial of the polar serine both decreases the thermal stability and decreases the midpoint potential. The mutagenesis data, combined with differences in the m-value of chemical denaturation, suggest that differences in solvent accessibility of the hydrophobic core underlie the adaptation of cytochrome c₆ to differing environmental temperature.

Insights into the relationship between the haem-binding pocket and the redox potential ofc6cytochromes: four atomic resolution structures ofc6andc6-like proteins fromSynechococcussp. PCC 7002

The structure of cytochromec6Cfrom the mesophilic cyanobacteriumSynechococcussp. PCC 7002 has been determined at 1.03 Å resolution. This is the first structural report on the recently discovered cyanobacterial cytochromec6-like proteins found in marine and nitrogen-fixing cyanobacteria. Despite high similarity in the overall three-dimensional fold between cytochromesc6andc6C, the latter shows saliently different electrostatic properties in terms of surface charge distribution and dipole moments. Its midpoint redox potential is less than half of the value for typicalc6cytochromes and results mainly from the substitution of one residue in the haem pocket. Here, high-resolution crystal structures of mutants of both cytochromesc6andc6Care presented, and the impact of the mutation of specific residues in the haem-binding pocket on the redox potential is discussed. These findings contribute to the elucidation of the structure–function relationship ofc6-like cytochromes.

Mannitol-1-phosphate dehydrogenase activity in Ectocarpus siliculosus, a key role for mannitol synthesis in brown algae

Mannitol represents a major end product of photosynthesis in brown algae (Phaeophyceae), and is, with the β-1,3-glucan laminarin, the main form of carbon storage for these organisms. Despite its importance, little is known about the genes and enzymes responsible for the metabolism of mannitol in these seaweeds. Taking benefit of the sequencing of the Ectocarpus siliculosus genome, we focussed our attention on the first step of the synthesis of mannitol (reduction of the photo-assimilate fructose-6-phosphate), catalysed by the mannitol-1-phosphate dehydrogenase (M1PDH). This activity was measured in algal extracts, and was shown to be regulated by NaCl concentration in the reaction medium. Genomic analysis revealed the presence of three putative M1PDH genes (named EsM1PHD1, EsM1PDH2 and EsM1PDH3). Sequence comparison with orthologs demonstrates the modular architecture of EsM1PHD1 and EsM1PDH2, with an additional N-terminal domain of unknown function. In addition, gene expression experiments carried out on samples harvested through the diurnal cycle, and after several short-term saline and oxidative stress treatments, showed that EsM1PDH1 is the most highly expressed of these genes, whatever the conditions tested. In order to assess the activity of the corresponding protein, this gene was expressed in Escherichia coli. Cell-free extracts prepared from bacteria containing EsM1PDH1 displayed higher M1PDH activity than bacteria transformed with an empty plasmid. Further characterisation of recombinant EsM1PDH1 activity revealed its very narrow substrate specificity, salt regulation, and sensitivity towards an inhibitor of SH-enzymes.

Structural basis for the oxidation of protein-bound sulfur by the sulfur cycle molybdohemo-enzyme sulfane dehydrogenase SoxCD

The sulfur cycle enzyme sulfane dehydrogenase SoxCD is an essential component of the sulfur oxidation (Sox) enzyme system of Paracoccus pantotrophus. SoxCD catalyzes a six-electron oxidation reaction within the Sox cycle. SoxCD is an α(2)β(2) heterotetrameric complex of the molybdenum cofactor-containing SoxC protein and the diheme c-type cytochrome SoxD with the heme domains D(1) and D(2). SoxCD(1) misses the heme-2 domain D(2) and is catalytically as active as SoxCD. The crystal structure of SoxCD(1) was solved at 1.33 Å. The substrate of SoxCD is the outer (sulfane) sulfur of Cys-110-persulfide located at the C-terminal peptide swinging arm of SoxY of the SoxYZ carrier complex. The SoxCD(1) substrate funnel toward the molybdopterin is narrow and partially shielded by side-chain residues of SoxD(1). For access of the sulfane-sulfur of SoxY-Cys-110 persulfide we propose that (i) the blockage by SoxD-Arg-98 is opened via interaction with the C terminus of SoxY and (ii) the C-terminal peptide VTIGGCGG of SoxY provides interactions with the entrance path such that the cysteine-bound persulfide is optimally positioned near the molybdenum atom. The subsequent oxidation reactions of the sulfane-sulfur are initiated by the nucleophilic attack of the persulfide anion on the molybdenum atom that is, in turn, reduced. The close proximity of heme-1 to the molybdopterin allows easy acceptance of the electrons. Because SoxYZ, SoxXA, and SoxB are already structurally characterized, with SoxCD(1) the structures of all key enzymes of the Sox cycle are known with atomic resolution.