Algae clue to 'green' hydrogen future

In September we reported how research into the hydrogen-making enzyme (iron-iron hydrogenase) in green algae revealed the mechanism by which oxygen irreversibly halts its hydrogen production.

It was a set-back for those hoping to use such photosynthetic microorganisms to make 'green' hydrogen fuel from just sunlight and water.

But in a recent paper in PNAS the same team, led by Fraser Armstrong of Oxford University's Department of Chemistry, report complementary research into the hydrogen-producing enzyme found in blue-green algae (nickel-iron hydrogenase) that gives important clues to how it can survive oxygen's onslaught.

I asked Fraser about his team's work and what it might mean for those working towards 'solar hydrogen farms':

OxSciBlog: How do these enzymes react differently to oxygen compared to those in green algae?
Fraser Armstrong: The enzymes we have studied contain a different type of active site: they are called nickel-iron [NiFe]-hydrogenases as opposed to iron-iron [FeFe] hydrogenases that occur in green algae.

The [FeFe] hydrogenase have an iron-sulphur cluster linked directly to the active site: iron sulphur clusters are rapidly degraded by oxygen or reactive oxygen species (such as superoxide) and this cluster is thought to be the group at which oxygen causes irreversible damage.

The [NiFe]-hydrogenases do not have a directly linked iron sulphur cluster, so when oxygen attacks the active site there need not be any permanent damage.

OSB: Why do we think these enzymes have a higher oxygen tolerance?
FA: We are not exactly sure, at the molecular level; but our electrochemical studies show that two properties/aspects are particularly important:

The first of these is that when oxygen attacks the active site, there are sufficient electrons available to ensure that the oxygen molecule can be reduced all the way to water molecules; this requires initially three electrons giving a harmless state known as Ni-B in which the Ni has been oxidised to the Ni(III) state and coordinates a hydroxide ion. 

The second aspect is that Ni-B can be rapidly reduced back to Ni(II) releasing the hydroxide and re-activating the enzyme. Rapid reduction is favoured by a high reduction potential for this step. The [NiFe]-hydrogenases that we regard as oxygen tolerant have a high reduction potential, and re-activation is therefore spontaneous, allowing the hydrogenase to function even in the presence of oxygen, as in air.

OSB: What new avenues of research do your findings suggest?
FA: They show how it could be possible to ‘design’ hydrogenases in cyanobacteria that have improved oxygen tolerance, and so use these genetically altered organisms for photosynthetic hydrogen production (where oxygen is produced).

The results also suggest that organisms operating at higher temperatures should be more successful because a determining factor for the hydrogenase’s oxygen tolerance is the rate of re-activation.

OSB: What do you hope might be the end result of research into similar oxygen-tolerant enzymes?
FA: Crystal structure studies of an oxygen tolerant hydrogenase are now crucial because they would provide a molecular understanding of the mechanism of oxygen tolerance, for example modification of the region around the active site that can steer oxygen reaction away from producing reactive oxygen species, and modifications of the electron transfer relay system (a series of iron-sulphur clusters) that enable it to ‘hold’ more electrons.

Professor Fraser Armstrong is based at Oxford University's Department of Chemistry