Science generally gets reported as if it happens in big leaps, but in reality most of the time science progresses in small but satisfying steps. One example of this is another step in a story I have followed for several years from Professor David Britt’s lab in the UC Davis Department of Chemistry, published April 9 in the journal Nature Chemistry.
Britt’s lab is broadly interested in how to chemically split water to make hydrogen fuel. With current methods, it is quite hard to do that efficiently, but bacteria do it all the time. In the big picture, cheap and plentiful hydrogen would be a new clean fuel that would revolutionize energy production.
But that is a very big step. The smaller steps along the way involve figuring out exactly how bacteria split hydrogen. Britt’s lab has for several years been studying how hydrogenase, a bacterial enzyme that can split hydrogen, is assembled. Hydrogenase has an active site or “H-cluster” based around a cube of four atoms of iron and four of sulfur, and another two iron atoms with cyanide (carbon-nitrogen) and carbon monoxide attached.
(Both cyanide and carbon monoxide are, of course, extremely poisonous and not the kinds of things you want knocking around inside your cells uncontrolled.)
In 2013, Britt’s lab showed how the cyanide and carbon monoxide were taken from the amino acid tyrosine and attached to the iron-sulfur center. In a follow up paper the following year, they showed how cyanide and carbon monoxide remain bound to the same iron atom after the rest of the tyrosine is cleared away, and that this complex goes on to form the active site of the water-splitting hydrogenase.
In the latest instalment, they characterize the structure of “Complex A,” the precursor to the hydrogenase H-cluster. In 2016, researchers from the University of Southampton, England produced a crystal structure for the complex showing a cube with four iron atoms, four sulfur atoms and the fifth iron that Britt calls “a dangler.”
The new work shows that after tyrosine is cut and removed, the cyanide and carbon monoxide groups are attached to this dangler iron atom.
A second carbon monoxide binds to the dangler iron atom, sending the cyanide group off to the four iron structure, which will form the basis of the hydrogenase, and leaving the dangler and two carbon monoxides to form the two-iron cluster.
“But that’s another step in the story,” Britt says.
Unique chemistry in hydrogen catalysts (news release, October 2013)
Probing hydrogen catalyst assembly, part II (blog post, Jan. 2014)