Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.6.5.4 (SOR)
 720 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Ascorbic acid (AsA) maintains redox homeostasis by scavenging reactive oxygen species from prokaryotes to eukaryotes, especially plants. The enzyme monodehydroascorbate reductase (MDHAR) regenerates AsA by catalysing the reduction of monodehydroascorbate, using NADH or NADPH as an electron donor. The detailed recycling mechanism of MDHAR remains unclear due to lack of structural information. Here, we present the crystal structures of MDHAR in the presence of cofactors, nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+), and complexed with AsA as well as its analogue, isoascorbic acid (ISD). The overall structure of MDHAR is similar to other iron-sulphur protein reductases, except for a unique long loop of 63-80 residues, which seems to be essential in forming the active site pocket. From the structural analysis and structure-guided point mutations, we found that the Arg320 residue plays a major substrate binding role, and the Tyr349 residue mediates electron transfer from NAD(P)H to bound substrate via FAD. The enzymatic activity of MDHAR favours NADH as an electron donor over NADPH. Our results show, for the first time, structural insights into this preference. The MDHAR-ISD complex structure revealed an alternative binding conformation of ISD, compared with the MDHAR-AsA complex. This implies a broad substrate (antioxidant) specificity and resulting greater protective ability of MDHAR.
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PMID:Structure and catalytic mechanism of monodehydroascorbate reductase, MDHAR, from Oryza sativa L. japonica. 2765 77

During evaluations of the ecophysiological adaptations of floating and submerged leaves of Potamogeton nodosus Poir, investigations were carried to assess their antioxidant status. Floating leaves possessed a significantly higher level of C skeletons per unit of area compared with submerged leaves as they possessed greater PSI and PSII activity (hence had superior potential to harness absorbed light energy and generate assimilatory power) and carboxylase activity of Rubisco (hence superior potential to fix CO2) compared with the latter. Interestingly, submerged leaves possessed ~2 times higher H2O2 levels compared with floating leaves. In contrast, the activity of all antioxidant enzymes tested (catalase, guaiacol peroxidase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) were significantly higher in floating leaves than in submerged leaves. Amazingly, catalase activity (a H2O2 detoxifying enzyme) was over fourfold higher in floating leaves than in submerged leaves. Among the nonenzymatic antioxidants, although levels of phenolics, ascorbate and thiols did not vary significantly between floating and submerged leaves, the level of total carotenoids was significantly higher in the former than the latter. In summary, floating leaves possess superior and efficient photosynthetic machinery for light and dark reactions, and also possess strong and superior enzymatic antioxidant machinery for scavenging reactive oxygen species and maintenance of the NAD(P)H to NAD(P)+ ratio compared with submerged leaves. Accordingly, floating leaves possessed superior potential to withstand photodamage compared with submerged leaves. We believe that excess H2O2 provides an ideal defence tool for submerged leaves to counter predators, pests and pathogens.
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PMID:Floating and submerged leaves of Potamogeton nodosus exhibit distinct variation in the antioxidant system as an ecophysiological adaptive strategy. 3248 Apr 66


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