Form follows function in molybdenum enzymes.
Molybdenum-containing enzymes have many roles in bacteria, from catalysing atmospheric nitrogen fixation to facilitating the sulphur chemistry used to survive in hydrothermal vents. In higher organisms, they fall into only two categories: xanthine dehydrogenases are involved in metabolic pathways; sulfite oxidases catalyse a small but important set of reactions involved in brain development. Unlike other metal-containing enzymes where the metal atom is held in place by amino acid residues, molybdenum is contained within a cofactor: a large organic molecule called a pyranopterin.
Joel Weiner's team in the biochemistry department at the University of Alberta examined the structure of 319 pyranopterins in 102 protein structures from bacteria, yeast, algae and animals. "When you look at the three-dimensional protein structure, you find that the pyranopterin shape changes based on what the enzyme is doing," says Weiner. "In the sulfite oxidases, the pyranopterin was always very flat, whereas in the xanthine dehydrogenases, it's skewed." In bacterial enzymes, molybdenum is contained using two pyranopterins; one flat and one skewed.
In a paper published in Proceedings of the National Academy of Sciences, the team theorized that skewed pyranopterins act only as a conduit for electrons during reactions. In contrast, flat pyranopterins work with the surrounding amino acids to tune the electrochemical potential of the molybdenum. Human enzymes can only do one or the other, but bacterial enzymes can do both, which explains their ability to catalyse a wider range of reactions. Understanding this form/function relationship could help medical researchers create pyranopterin analogues for people with genetic diseases where molybdenum-containing enzymes are malformed or not functional. It could also help chemists design artificial cofactor molecules and enzymes to catalyse reactions which are currently difficult or impossible.
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|Publication:||Canadian Chemical News|
|Date:||Nov 1, 2012|
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