Structural Basis for the Thermostability of Ferredoxin from the Cyanobacterium Mastigocladus laminosus

• Published in: Journal of molecular biology Vol. 350; no. 3; pp. 599 - 608
• Main Authors: Fish, Alexander, Danieli, Tsafi, Ohad, Itzhak, Nechushtai, Rachel, Livnah, Oded
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15.07.2005
• Subjects: Amino Acid Sequence; Crystallography, X-Ray; Cyanobacteria - metabolism; Cyanophyta; DNA, Complementary - metabolism; Electrons; ferredoxin; Ferredoxins - chemistry; Hot Temperature; Hydrogen - chemistry; Hydrogen Bonding; Iron-Sulfur Proteins - chemistry; Mastigocladus laminosus; Models, Molecular; Molecular Sequence Data; Mutation; Oxygen - metabolism; photosynthesis; photosystem I; Protein Conformation; Protein Denaturation; Protein Structure, Secondary; Protein Structure, Tertiary; Salts - pharmacology; Sequence Homology, Amino Acid; Temperature; thermostability; X-ray structure; ferredoxin; photosynthesis; thermostability; PSI; photosystem I; mFd; X-ray structure; wt; ET; ASA
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2005.04.071

Plant-type ferredoxins (Fds) carry a single [2Fe-2S] cluster and serve as electron acceptors of photosystem I (PSI). The ferredoxin from the thermophilic cyanobacterium Mastigocladus laminosus displays optimal activity at 65 °C. In order to reveal the molecular factors that confer thermostability, the crystal structure of M. laminosus Fd (mFd) was determined to 1.25 Å resolution and subsequently analyzed in comparison with four similar plant-type mesophilic ferredoxins. The topologies of the plant-type ferredoxins are similar, yet two structural determinants were identified that may account for differences in thermostability, a salt bridge network in the C-terminal region, and the flexible L1,2 loop that increases hydrophobic accessible surface area. These conclusions were verified by three mutations, i.e. substitution of L1,2 into a rigid β-turn (ΔL1,2) and two point mutations (E90S and E96S) that disrupt the salt bridge network at the C-terminal region. All three mutants have shown reduced electron transfer (ET) capabilities and [2Fe-2S] stability at high temperatures in comparison to the wild-type mFd. The results have also provided new insights into the involvement of the L1,2 loop in the Fd interactions with its electron donor, the PSI complex.