The Photosystem II D1-K238E mutation enhances electrical current production using cyanobacterial thylakoid membranes in a bio-photoelectrochemical cell

Abstract

The conversion of solar energy (SEC) to storable chemical energy by photosynthesis has been performed by photosynthetic organisms, including oxygenic cyanobacteria for over 3 billion years. We have previously shown that crude thylakoid membranes from the cyanobacterium Synechocytis sp. PCC 6803 can reduce the electron transfer (ET) protein cytochrome c even in the presence of the PSII inhibitor DCMU. Mutation of lysine 238 of the Photosystem II D1 protein to glutamic acid increased the cytochrome reduction rates, indicating the possible position of this unknown ET pathway. In this contribution, we show that D1-K238E is rather unique, as other mutations to K238, or to other residues in the same vicinity, are not as successful in cytochrome c reduction. This observation indicates the sensitivity of ET reactions to minor changes. As the next step in obtaining useful SEC from biological material, we describe the use of crude Synechocystis membranes in a bio-photovoltaic cell containing an N-acetyl cysteine-modified gold electrode. We show the production of significant current for prolonged time durations, in the presence of DCMU. Surprisingly, the presence of cytochrome c was not found to be necessary for ET to the bio-voltaic cell.

Hybrid bio-photo-electro-chemical cells for solar water splitting

“…Thylakoids have also been used without an electron mediator1920, though the reported photocurrents were lower compared with reports with an electron mediator. Larom et al 21. reported the production of an unmediated photocurrent density of 16 μA cm −2 using crude Synechocystis membranes and N-acetyl cysteine-modified gold electrode.…”

Redesigning the QA binding site of Photosystem II allows reduction of exogenous quinones

“…Substitution of D1-K238 by glutamate in Synechocystis was shown to promote the reduction of cytochrome c by Q A ° −8 . Thylakoids of this mutant strain generated a greater photo-induced current with respect to WT in the presence of exogenous cytochrome c , but its addition as a redox mediator between PSII and the electrode surprisingly turned out to be dispensable78. Here, our engineering specifications excluded any purification step thereby imposing the use of an amphiphilic redox mediator able to cross the cell membranes.…”

“…Alternatively, single amino acid substitutions were made within photosystems to change their surface charge density and favour electron transfer to the electrode. As an example, the D1-K238 mutation on the stromal surface of PSII led to higher photocurrent in Synechocystis 78.…”

Photosynthetic fuel for heterologous enzymes: the role of electron carrier proteins

“…The lumenal localization of plastocyanin and cytochrome c 6 insulates them from most of the biosynthetic machinery in the stroma of chloroplasts or the cytoplasm of cyanobacteria, and their positive midpoint potentials of +340 to +390 mV (Antal et al 2013) render them mostly unusable for metabolic engineering. However, a mutant of photosystem II containing a single Lys-to-Glu mutation near the Q A site on the luminal surface of the PSII complex was recently shown to pass electrons directly to a (positively charged) mitochondrial cytochrome c (Larom et al 2010(Larom et al , 2015. While cytochrome c is an unsuitable carrier to support many biosynthetic reactions, the fact that this novel tapping point harbors a potential of −30 to −80 mV (Antal et al 2013) and allows electron transfer to positively charged carrier proteins makes it possible to assemble entirely novel redox chains from this point.…”