Katsumi Amako

Functional Analysis of the AKR4C Subfamily of Arabidopsis thaliana: Model Structures, Substrate Specificity, Acrolein Toxicity, and Responses to Light and [CO2]

In Arabidopsis thaliana, the aldo-keto reductase (AKR) family includes four enzymes (The AKR4C subfamily: AKR4C8, AKR4C9, AKR4C10, and AKR4C11). AKR4C8 and AKR4C9 might detoxify sugar-derived reactive carbonyls (RCs). We analyzed AKR4C10 and AKR4C11, and compared the enzymatic functions of the four enzymes. Modeling of protein structures based on the known structure of AKR4C9 found an (α/β)8-barrel motif in all four enzymes. Loop structures (A, B, and C) which determine substrate specificity, differed among the four. Both AKR4C10 and AKR4C11 reduced methylglyoxal. AKR4C10 reduced triose phosphates, dihydroxyacetone phosphate (DHAP), and glyceraldehydes 3-phosphate (GAP), the most efficiently of all the AKR4Cs. Acrolein, a lipid-derived RC, inactivated the four enzymes to different degrees. Expression of the AKR4C genes was induced under high-[CO2] and high light, when photosynthesis was enhanced and photosynthates accumulated in the cells. These results suggest that the AKR4C subfamily contributes to the detoxification of sugar-derived RCs in plants.

Dimethylbenzoquinone-mediated photoelectrochemical oxidation of water at a carbon paste electrode coated with photosystem II membranes

Abstract Photosystem II (PS II)-enriched membranes from spinach chloroplasts were immobilized behind a dialysis membrane on a carbon paste electrode containing 2-6-dimethylbenzoquinone (DMQ). The PS-II-membrane-coated electrode produced an anodic steady-state current under illumination that was attributable to a PS II reaction with DMQ as an electron-transfer mediator between the PS II and the electrode. The magnitude of the steady-state photocurrent depended upon the applied potential, the intensity of the illumination, the amount of DMQ mixed in the carbon paste electrode and the amount of immobilized PS II membranes. The DMQ-mediated photoelectrochemical behavior could be described by an equation of the steady-state kinetics of the PS-II reaction linked to the electrode reaction of DMQ. 2-Methylbenzoquinone, tetramethylbenzoquinone and phenylbenzoquinone, which are sparingly soluble or practically insoluble in aqueous solution, also worked as the mediators when mixed in the carbon paste electrodes.