Investigating the Structure and Dynamics of Apo‐Photosystem II

Alteration of the O2-Producing Mn4Ca Cluster in Photosystem II by the Mutation of a Metal Ligand

The O₂-evolving Mn₄Ca cluster in photosystem II (PSII) is arranged as a distorted Mn₃Ca cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the Mn₄Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O-H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn₄Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn₄Ca cluster in the dark-stable S₁ state so that a high-spin form of the S₂ state is produced during the S₁-to-S₂ transition instead of the low-spin form that gives rise to the S₂ state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn₄Ca cluster without substantially eliminating O₂ evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn₄Ca cluster in the mechanism of O₂ formation.

Structural and dynamic insights into Mn4Ca cluster-depleted Photosystem II

In the first steps of natural oxygenic photosynthesis, sunlight is used to oxidize water molecules to protons, electrons and molecular oxygen. This reaction takes place on the Mn₄Ca cluster located in the reaction centre of Photosystem II (PSII), where the cluster is assembled and continuously repaired through a process known as photoactivation. Understanding the molecular details of such a process has important implications in different fields, in particular inspiring synthesis and repair strategies for artificial photosynthesis devices. In this regard, a detailed structural and dynamic characterization of Photosystem II lacking a Mn₄Ca cluster, namely apo PSII, is a prerequisite for the full comprehension of the photoactivation. Recently, the structure of the apo PSII was resolved at 2.55 Å resolution [Zhang et al., eLife, 2017, 6, e26933], suggesting a pre-organized structure of the protein cavity hosting the cluster. Anyway, the question of whether these findings are a feature of the method used remains open. Here, by means of classical Molecular Dynamics simulations, we characterized the structural and dynamic features of the apo PSII for different protonation states of the cluster cavity. Albeit an overall conformational stability common to all investigated systems, we found significant deviations in the conformation of the side chains of the active site with respect to the X-ray positions. Our findings suggest that not all residues acting as Mn ligands are pre-organized prior to the Mn₄Ca formation and previous local conformational changes are required in order to bind the first Mn ion in the high-affinity binding site.

Hybridization of Particulate Methane Monooxygenase by Methanobactin-Modified AuNPs

Particulate methane monooxygenase (pMMO) is a characteristic membrane-bound metalloenzyme of methane-oxidizing bacteria that can catalyze the bioconversion of methane to methanol. However, in order to achieve pMMO-based continuous methane-to-methanol bioconversion, the problems of reducing power in vitro regeneration and pMMO stability need to be overcome. Methanobactin (Mb) is a small copper-chelating molecule that functions not only as electron carrier for pMMO catalysis and pMMO protector against oxygen radicals, but also as an agent for copper acquisition and uptake. In order to improve the activity and stability of pMMO, methanobactin-Cu (Mb-Cu)-modified gold nanoparticle (AuNP)-pMMO nanobiohybrids were straightforwardly synthesized via in situ reduction of HAuCl4 to AuNPs in a membrane fraction before further association with Mb-Cu. Mb-Cu modification can greatly improve the activity and stability of pMMO in the AuNP-pMMO nanobiohybrids. It is shown that the Mb-Cu-modified AuNP-pMMO nanobiohybrids can persistently catalyze the conversion of methane to methanol with hydroquinone as electron donor. The artificial heterogeneous nanobiohybrids exhibited excellent reusability and reproducibility in three cycles of catalysis, and they provide a model for achieving hydroquinone-driven conversion of methane to methanol.