Aldehyde ferredoxin oxidoreductase (AOR)
Pyrococcus furiosus (thermophile, archaea)
It is difficult for many organisms to survive under the circumstance with over 45 degrees Celsius. One of the reasons is that proteins which constitute organisms are usually denatured by the high temperature and lost their activities. But thermophiles, one kind of bacteria or archaea, are thriving under high temperature. Among thermophiles, there are organisms living at 100 degrees Celsius, such as a kind of hyperthermophile Pyrococcus furiosus. The aldehyde ferredoxin oxidoreductase (AOR), which is indispensable for their metabolism, can work under such high temperature without being denatured. This protein catalyzes the reversible oxidation of aldehydes to their corresponding carboxylic acids with accompanying reduction of the redox protein ferredoxin.
The structure shown here is AOR consists of two identical subunits. Each subunit is composed of three domains, and at the interfaces of the three domains, tungsten cofactor and Fe4S4 cluster are located, and two molybdopterin molecule, small organic compound (pterin) complexed with molybdenum, are coordinated to the tungsten. In this structure, pterin moiety is modified to have three ringed structure. The location of redox centers at subunit interfaces seems to be a general property of multi centered redox proteins. Domain 1 consists primarily of 12 beta strands arranged in two six-stranded, predominantly antiparallel beta sheets, on which the saddle-like tungsten pterin cofactor sits. Domain 2 and 3, primarily consisting of alpha helices, surround the opposite surface of the tungsten cofactor, and provide residues that form specific polar and ionic interaction with the different metal centers. The AOR structure demonstrates that hyper-thermostability can be achieved without requiring any new types of interactions to stabilize the folded conformation. The stability of AOR may reflect a number of subtle interactions that both minimize the surface energy which maximizing packing and electrostatic interactions.
Figure 1: AOR dimer in cartoon representation, complexed with iron/sulfur clusters, an iron atom, and tungstopterin cofactors in spacefill representation.
Figure 2: One subunit of AOR specified three domains and two beta sheets described above.
Figure 3: Seen from the opposite side of figure 2.
Protein Data Bank (PDB)
author: Sachiyo Nomura