X-ray structure of a vanadium-containing enzyme: chloroperoxidase from the fungus Curvularia inaequalis.

The chloroperoxidase (EC 1.11.1.-) from the fungus Curvularia inaequalis belongs to a class of vanadium enzymes that oxidize halides in the presence of hydrogen peroxide to the corresponding hypohalous acids. The 2.1 A crystal structure (R = 20%) of an azide chloroperoxidase complex reveals the geometry of the catalytic vanadium center. Azide coordinates directly to the metal center, resulting in a structure with azide, three nonprotein oxygens, and a histidine as ligands. In the native state vanadium will be bound as hydrogen vanadate(V) in a trigonal bipyramidal coordination with the metal coordinated to three oxygens in the equatorial plane, to the OH group at one apical position, and to the epsilon 2 nitrogen of a histidine at the other apical position. The protein fold is mainly alpha-helical with two four-helix bundles as main structural motifs and an overall structure different from other structures. The helices pack together to a compact molecule, which explains the high stability of the protein. An amino acid sequence comparison with vanadium-containing bromoperoxidase from the seaweed Ascophyllum nodosum shows high similarities in the regions of the metal binding site, with all hydrogen vanadate(V) interacting residues conserved except for lysine-353, which is an asparagine.

Biomethane production from macroalgae

Irish brown seaweeds have been identified as a potential bio-resource with potentially high specific methane yields. Anaerobic digestion is deemed the most feasible technology due to its commercial viability for handling such wet feedstock. However, the biomethane potential of seaweed is highly dependent on its chemical composition which can vary by species type, cultivation method, and time of harvest. This study aims to investigate and optimize the process for the production of biomethane from Irish brown seaweeds focusing on the key technology bottlenecks including for seaweed characterization, biomethane potential assessment, optimization of long-term anaerobic digestion and suitable pre-treatment technologies to enhance potential gas yields. Laminaria digitata and Ascophyllum nodosum were tested for seasonal variation. From the characterization and batch digestion of L. digitata, August was found to be the optimal month for harvest due to high organic matter content, low level of ash and ultimately highest biomethane yield. The specific methane yield of 53 m3 CH4 t-1 wwt in August was 4.5 times higher than the yield in December (12 m3 CH4 t-1 wwt), with ash content the key factor in seasonal variation. For A. nodosum, the optimal harvest month was October with polyphenol content found to be a more influential factor than ash. The gross energy yields from both species were evaluated in the range of 116-200 GJ ha-1 yr-1. Continuous digestion trials were subsequently designed for S. latissima and L. digitata to optimize the key digestion parameters. Results from mono-digestion and co-digestion with dairy slurry revealed that both seaweeds could be digested at maximum biomethane efficiency to a loading rate of 4 kg VS m-3 d-1. Accumulation of salt in the digesters was a concern for long term digestion and it was reasoned that suitable pretreatment may be required prior to digestion. Various pre-treatments were subsequently tested on L. digitata to enhance the gas yield. It was found that maceration after hot water washing yielded 25% more specific methane and up to 54% salt removal as compared to untreated L. digitata. The experiments undertaken aim to assist in providing a basic guideline for feasible design and operation of seaweed digesters in Ireland.