Biochemical Society Transactions. 2014 Apr; 42(2):543−47.

Why don’t plants have diabetes? Systems for scavenging reactive carbonyls in photosynthetic organisms.

ShimakawaG, Suzuki M, Yamamoto E, Saito R, Iwamoto T, Nishi A, Miyake C.

Department of Biological and Environmental Sciences, Faculty of Agriculture, Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan

 

ABSTRACT

In the present paper, we review the toxicity of sugar- and lipid-derived RCs (reactive carbonyls) and the RC-scavenging systems observed in photosynthetic organisms. Similar to heterotrophs, photosynthetic organisms are exposed to the danger of RCs produced in sugar metabolism during both respiration and photosynthesis. RCs such as methylglyoxal and acrolein have toxic effects on the photosynthetic activity of higher plants and cyanobacteria. These toxic effects are assumed to occur uniquely in photosynthetic organisms, suggesting that RC-scavenging systems are essential for their survival. The aldo-keto reductase and the glyoxalase system mainly scavenge sugar-derived RCs in higher plants and cyanobacteria. 2-Alkenal reductase and alkenal/alkenone reductase catalyze the reduction of lipid-derived RCs in higher plants. In cyanobacteria, medium-chain dehydrogenases/reductases are the main scavengers of lipid-derived RCs.

PMID: 24646276

 

SUPPLEMENTARY

Reactive carbonyls (RCs) including dicarbonyls (methylglyoxal, glyoxal, and 3-deoxyglucosone) and α,β-unsaturated carbonyls (acrolein and malondialdehyde) are toxic carbonyl compounds that are inevitably produced in sugar metabolism. These RCs induce modifications of amino acid residues such as lysine, arginine and cysteine in proteins to produce advanced glycation and lipoxidation end-products (AGEs and ALEs). The accumulation of AGEs and ALEs stimulates the oxidation of cell components and the inactivation of proteins, which causes diabetic complications.

Photosynthetic organisms possess not only the glycolysis but also the photosynthetic CO2-assimilation cycle (Calvin cycle) as the sugar-metabolism. Thus, these organisms would be exposed to higher levels of toxicity derived from RCs than heterotrophs. In fact, it has been found that photosynthesis in spinach chloroplasts stimulates the production of RCs. The production of RCs and the protein carbonylation by the RCs are accelerated under high-[CO2] conditions, where photosynthesis is promoted. We termed the cellular damage derived from RCs in photosynthetic organisms “plant diabetes”.

Photosynthetic organisms have scavenging systems for RCs. In our recent study, we constructed the model of production and detoxification of RCs in cyanobacteria known to be the ancestor of chloroplasts of higher plants. The cyanobacterium Synechocystis sp. PCC 6803 has aldo-keto reductase and short- and medium-chain dehydrogenases/reductases as the NADPH-dependent system, and the glyoxalase system as the glutathione-dependent system for RCs. On the other hand, in a higher plant Arabidopsis thaliana, 2-alkenal reductase and alkenal/alkenone reductase have the reducing activities for α,β-unsaturated carbonyls. Elucidation and characterization of the physiological functions of these systems for alleviation of plant diabetes are required to ensure the success of agriculture in future when atmospheric [CO2] reaches high level.

Abstract Illust

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