Photosynthesis

Very nice article about the photosynthesis photosystem II mechanism and how design principles of the system can be used to engineer similar systems to produce solar fuel.
Solar water-splitting into H2 and O2: design principles of photosystem II and hydrogenases

2.6 Summary: Principles of photosynthetic water-splitting

From the above text the following seven principles of photosynthetic water splitting can be extracted:

1. The components of the primary photo-reactions as well as the Mn4OxCa cluster are supported by protective components and, once destroyed, automatically replaced by the organism by a specific repair mechanism.
2. A multimeric transition metal complex (Mn4OxCa cluster) is employed to couple the very fast one–electron photochemistry with several orders of magnitude slower four electron water-splitting chemistry.
3. The water-splitting catalyst is located in a sequestered environment; channels exist for substrate entry and product release.
4. The matrix (protein) around the Mn4OxCa cluster is highly important for the coupling of proton and electron transfer reactions. This feature is essential for achieving about equal redox potentials for all oxidation steps that match the oxidizing potential of the light-generated primary oxidant.
5. Point 4 leads to a decoupling of the release of the two products O2 and H+ from the catalytic site.
6. The substrate water molecules are stepwise prepared for O–O bond formation by binding to the Mn4OxCa cluster and by (partial) deprotonation. The concerted oxidation of the activated substrate occurs then either in two 2 e− or one concerted 4 e− reaction step(s). This avoids high energy intermediates.
7. The Mn4OxCa cluster undergoes several structural changes during the Kok cycle, which are probably significant for the mechanism. The surrounding matrix therefore needs to be flexible enough to support such changes.

3.6 Design principles of hydrogenases

For a better understanding of the design principles of native hydrogenases a comparison of the two major hydrogenases is useful.

The two groups of hydrogenases have a completely different genetic background. Whereas the [NiFe] group is widely distributed in prokaryotes (mostly sulfur reducing bacteria), the [FeFe] group is less widely distributed but occurs in both prokaryots and eukaryots (algae, yeast). In fact, the genetic signature of the H-cluster is found in many higher organisms, even in homo sapiens. The [FeFe] hydrogenases are, in general, most active in H2 production while [NiFe] hydrogenases are more tuned to H2 oxidation. Both types are however bidirectional. Organisms employing [NiFe] hydrogenases are found in regions with higher oxygen levels than those using [FeFe] hydrogenase. This is because [FeFe] hydrogenases are extremely oxygen sensitive and will be inhibited irreversibly under O2. [NiFe] hydrogenases are, in general, more oxygen tolerant and some enzymes even evolve H2 under O2.

On the other hand, there are many similarities between the basic structures of the active site in both enzymes:

1. Both enzymes employ a bimetallic center where the chemistry is taking place.
2. Both active sites have a butterfly-shaped core in which the two metals are bridged by SR-ligands.
3. Only one of these metal atoms is redox active (Ni in [NiFe] and Fed in [FeFe] hydrogenase) and they both have a d7 configuration (Ni(III) and Fe(I), respectively) in their active states.
4. In both catalytic sites the Fe atom is kept at a low valence by the strongly donating ligands CN− and CO.
5. The metal-metal distance in both structures is short (2.5–2.9 ), indicating a metal–metal bond.
6. One metal with an open coordination site can be identified in both active states. This is the site where H2 is believed to bind or is being released.
7. The H/D-isotope effect shows that in both cases the H2 splitting is heterolytic
8. In both active sites a sulfur or nitrogen/oxygen ligand probably acts as base to accept or donate the H+.
9. For both enzymes the catalytic activity is often inhibited by O2 and CO.

These features can serve as guidelines for the construction of biomimetic hydrogenase models.

Emphasis mine.

Also, the photosystem II mechanism makes use of quantum mechanical computing principles, leading to an excellent quantum efficiency for water-splitting.

From Nature;Vol 446;12 April 2007: Quantum path to photosynthesis

Elsewhere in this issue, Engel et al. (page 782) take a close look at how nature, in the form of the green sulphur bacterium Chlorobium tepidum, manages to transfer and trap light’s energy so effectively. The key might be a clever quantum computation built into the photosynthetic algorithm.

The process is analogous to Grover’s algorithm in quantum computing, which has been proved to provide the fastest possible search of an unsorted information database.

And in the same issue: Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems

When viewed in this way, the system is essentially performing a single quantum computation, sensing many states simultaneously and selecting the correct answer, as indicated by the efficiency of the energy transfer.

A glimpse into the future of our own designs.