Structural and kinetic characterization of a maize aldose reductase

Identification and functional characterization of a novel aldo-keto reductase from Aloe vera

MAIN CONCLUSION

The present investigation profoundly asserted the catalytic potential of plant-based aldo-ketoreductase, postulating its role in polyketide biosynthesis and providing new insights for tailored biosynthesis of vital plant polyketides for therapeutics. Plants hold great potential as a future source of innovative biocatalysts, expanding the possibilities within chemical reactions and generating a variety of benefits. The aldo-keto reductase (AKR) superfamily includes a huge collection of NAD(P)H-dependent oxidoreductases that carry out a variety of redox reactions essential for biosynthesis, detoxification, and intermediary metabolism. The present study involved the isolation, cloning, and purification of a novel aldo-ketoreductase (AvAKR) from the leaves of Aloe vera (Aloe barbadensis Miller) by heterologous gene expression in Escherichia coli based on the unigene sequences of putative ketoreductase and cDNA library screening by oligonucleotide hybridization. The in-silico structural analysis, phylogenetic relationship, and molecular modeling were outranged to approach the novelty of the sequence. Additionally, agroinfiltration of the candidate gene tagged with a green fluorescent protein (GFP) was employed for transient expression in the Nicotiana benthamiana to evaluate the sub-cellular localization of the candidate gene. The AvAKR preferred cytoplasmic localization and shared similarities with the known plant AKRs, keeping the majority of the conserved active-site residues in the AKR superfamily enzymes. The enzyme facilitated the NADPH-dependent reduction of various carbonyl substrates, including benzaldehyde and sugars, proclaiming a broad spectrum range. Our study successfully isolated and characterized a novel aldo-ketoreductase (AvAKR) from Aloe vera, highlighting its versatile NADPH-dependent carbonyl reduction proficiency therewith showcasing its potential as a versatile biocatalyst in diverse redox reactions.

Natural variations in the Sl-AKR9 aldo/keto reductase gene impact fruit flavor volatile and sugar contents

The unique flavors of different fruits depend upon complex blends of soluble sugars, organic acids, and volatile organic compounds. 2-Phenylethanol and phenylacetaldehyde are major contributors to flavor in many foods, including tomato. In the tomato fruit, glucose, and fructose are the chemicals that most positively contribute to human flavor preferences. We identified a gene encoding a tomato aldo/keto reductase, Sl-AKR9, that is associated with phenylacetaldehyde and 2-phenylethanol contents in fruits. Two distinct haplotypes were identified; one encodes a chloroplast-targeted protein while the other encodes a transit peptide-less protein that accumulates in the cytoplasm. Sl-AKR9 effectively catalyzes reduction of phenylacetaldehyde to 2-phenylethanol. The enzyme can also metabolize sugar-derived reactive carbonyls, including glyceraldehyde and methylglyoxal. CRISPR-Cas9-induced loss-of-function mutations in Sl-AKR9 significantly increased phenylacetaldehyde and lowered 2-phenylethanol content in ripe fruit. Reduced fruit weight and increased soluble solids, glucose, and fructose contents were observed in the loss-of-function fruits. These results reveal a previously unidentified mechanism affecting two flavor-associated phenylalanine-derived volatile organic compounds, sugar content, and fruit weight. Modern varieties of tomato almost universally contain the haplotype associated with larger fruit, lower sugar content, and lower phenylacetaldehyde and 2-phenylethanol, likely leading to flavor deterioration in modern varieties.

Comparison of Glyphosate-Degradation Ability of Aldo-Keto Reductase (AKR4) Proteins in Maize, Soybean and Rice

Genes that participate in the degradation or isolation of glyphosate in plants are promising, for they endow crops with herbicide tolerance with a low glyphosate residue. Recently, the aldo-keto reductase (AKR4) gene in Echinochloa colona (EcAKR4) was identified as a naturally evolved glyphosate-metabolism enzyme. Here, we compared the glyphosate-degradation ability of theAKR4 proteins from maize, soybean and rice, which belong to a clade containing EcAKR4 in the phylogenetic tree, by incubation of glyphosate with AKR proteins both in vivo and in vitro. The results indicated that, except for OsALR1, the other proteins were characterized as glyphosate-metabolism enzymes, with ZmAKR4 ranked the highest activity, and OsAKR4-1 and OsAKR4-2 exhibiting the highest activity among the AKR4 family in rice. Moreover, OsAKR4-1 was confirmed to endow glyphosate-tolerance at the plant level. Our study provides information on the mechanism underlying the glyphosate-degradation ability of AKR proteins in crops, which enables the development of glyphosate-resistant crops with a low glyphosate residue, mediated by AKRs.

Versatile roles of sorbitol in higher plants: luxury resource, effective defender or something else?

Sorbitol metabolism plays multiple roles in many plants, including energy and carbon enrichment, effective defence against various stresses and other emerging specific roles. The underlying mechanisms are, however, incompletely understood. This review provides the current state-of-the-art, highlights missing knowledge and poses several remaining questions. The basic properties of sugar alcohols are summarised and pathways of sorbitol metabolism, including biosynthesis, degradation and key enzymes are described. Sorbitol transport within the plant body is discussed and individual roles of sorbitol in different organs, specific cells or even cellular compartments, are elaborated, clarifying the critical importance of sorbitol allocation and distribution. In addition to plants that accumulate and transport significant quantities of sorbitol (usual producers), there are some that synthesize small amounts of sorbitol or only possess sorbitol metabolising enzymes (non-usual producers). Modern analytical methods have recently enabled large amounts of data to be acquired on this topic, although numerous uncertainties and questions remain. For a long time, it has been clear that enriching carbohydrate metabolism with a sorbitol branch improves plant fitness under stress. Nevertheless, this is probably valid only when appropriate growth and defence trade-offs are ensured. Information on the ectopic expression of sorbitol metabolism genes has contributed substantially to our understanding of the sorbitol roles and raises new questions regarding sorbitol signalling potential. We finally examine strategies in plants producing sorbitol compared with those producing mannitol. Providing an in-depth understanding of sugar alcohol metabolism is essential for the progress in plant physiology as well as in targeted, knowledge-based crop breeding.

Constitutive expression of aldose reductase 1 from Zea mays exacerbates salt and drought sensitivity of transgenic Escherichia coli and Arabidopsis

Aldose reductases (ARs) have been considered to play important roles in sorbitol biosynthesis, cellular detoxification and stress response in some plants. ARs from maize are capable of catalyzing the oxidation of sorbitol to glucose. However, little is known how maize ARs response to abiotic stresses. In this work, we cloned one isoform of maize ARs (ZmAR1), and furthermore we analyzed the roles of ZmAR1 in response to salt and drought stresses at both prokaryotic and eukaryotic levels. ZmAR1 encodes a putative 35 kDa protein that contains 310 amino acids. Under normal growth conditions, ZmAR1 was expressed in maize seedlings, and the highest expression level was found in leaves. But when seedlings were subjected to drought or salt treatment, the expression levels of ZmAR1 were significantly reduced. The constitutive expression of ZmAR1 increased the sensitivity of recombinant E. coli cells to drought and salt stresses compared with the control. Under salt and drought stresses, transgenic Arabidopsis lines displayed lower seed germination rate, shorter seedling root length, lower chlorophyll content, lower survival rate and lower antioxidant enzyme activity than wild type (WT) plants, but transgenic Arabidopsis had higher relative conductivity, higher water loss rate, and more MDA content than WT. Meanwhile, the introduction of ZmAR1 into Arabidopsis changed the expression levels of some stress-related genes. Taken together, our results suggested that ZmAR1 might act as a negative regulator in response to salt and drought stresses in Arabidopsis by reducing the sorbitol content and modulating the expression levels of some stress-related genes.

Structure-function study of AKR4C14, an aldo-keto reductase from Thai jasmine rice (Oryza sativa L. ssp. indica cv. KDML105)

Aldo-keto reductases (AKRs) are NADPH/NADP⁺-dependent oxidoreductase enzymes that metabolize an aldehyde/ketone to the corresponding alcohol. AKR4C14 from rice exhibits a much higher efficiency in metabolizing malondialdehyde (MDA) than do the Arabidopsis enzymes AKR4C8 and AKR4C9, despite sharing greater than 60% amino-acid sequence identity. This study confirms the role of rice AKR4C14 in the detoxification of methylglyoxal and MDA, and demonstrates that the endogenous contents of both aldehydes in transgenic Arabidopsis ectopically expressing AKR4C14 are significantly lower than their levels in the wild type. The apo structure of indica rice AKR4C14 was also determined in the absence of the cofactor, revealing the stabilized open conformation. This is the first crystal structure in AKR subfamily 4C from rice to be observed in the apo form (without bound NADP⁺). The refined AKR4C14 structure reveals a stabilized open conformation of loop B, suggesting the initial phase prior to cofactor binding. Based on the X-ray crystal structure, the substrate- and cofactor-binding pockets of AKR4C14 are formed by loops A, B, C and β1α1. Moreover, the residues Ser211 and Asn220 on loop B are proposed as the hinge residues that are responsible for conformational alteration while the cofactor binds. The open conformation of loop B is proposed to involve Phe216 pointing out from the cofactor-binding site and the opening of the safety belt. Structural comparison with other AKRs in subfamily 4C emphasizes the role of the substrate-channel wall, consisting of Trp24, Trp115, Tyr206, Phe216, Leu291 and Phe295, in substrate discrimination. In particular, Leu291 could contribute greatly to substrate selectivity, explaining the preference of AKR4C14 for its straight-chain aldehyde substrate.

Proteomic analysis of heterotic seed germination in maize using F1 hybrid DHM 117 and its parental inbreds

A number of differentially expressed proteins (DEPs) were identified in comparative two-dimensional gel electrophoresis analysis of dry and 24-h water-imbibed seeds of maize F1 hybrid DHM 117 (BML 6 × BML 7) and its parental inbreds. Of the DEPs, 53.4% (86/161) in dry seeds and 58% (127/219) in water-imbibed seeds exhibited a nonadditive pattern in the F1 hybrid as compared to parental inbreds. A total of 30 DEPs were categorized into different biological processes, most of which were related to metabolism and energy (34%), followed by storage proteins (27%), stress response (23%), transcription and translation (7%), cell cycle (3%), and hormone biosynthesis (3%). The transcript accumulation pattern of 8 selected genes corresponding to DEPs was examined using qRTPCR. Interestingly, LEA protein Rab28 showed higher accumulation in dry seeds at both protein and transcript levels, whereas indole3-acetaldehyde oxidase showed lower accumulation in water-imbibed seeds of the F1 hybrid than the female parent at the protein level. Thus, the DEPs particularly involved in metabolic and energy processes, as well as hormone biosynthesis in the F 1 hybrid, might be responsible for heterotic seed germination in the F1 hybrid. The DEPs identified in this study provide a scope for improving the seed germination trait of agricultural crops.

A comparative structural analysis reveals distinctive features of co-factor binding and substrate specificity in plant aldo-keto reductases

Plant aldo-keto reductases of the AKR4C subfamily play key roles during stress and are attractive targets for developing stress-tolerant crops. However, these AKR4Cs show little to no activity with previously-envisioned sugar substrates. We hypothesized a structural basis for the distinctive cofactor binding and substrate specificity of these plant enzymes. To test this, we solved the crystal structure of a novel AKR4C subfamily member, the AKR4C7 from maize, in the apo form and in complex with NADP(+). The binary complex revealed an intermediate state of cofactor binding that preceded closure of Loop B, and also indicated that conformational changes upon substrate binding are required to induce a catalytically-favorable conformation of the active-site pocket. Comparative structural analyses of homologues (AKR1B1, AKR4C8 and AKR4C9) showed that evolutionary redesign of plant AKR4Cs weakened interactions that stabilize the closed conformation of Loop B. This in turn decreased cofactor affinity and altered configuration of the substrate-binding site. We propose that these structural modifications contribute to impairment of sugar reductase activity in favor of other substrates in the plant AKR4C subgroup, and that catalysis involves a three-step process relevant to other AKRs.

Plant aldo-keto reductases (AKRs) as multi-tasking soldiers involved in diverse plant metabolic processes and stress defense: A structure-function update

The aldo-keto reductase (AKR) superfamily comprises of a large number of primarily monomeric protein members, which reduce a broad spectrum of substrates ranging from simple sugars to potentially toxic aldehydes. Plant AKRs can be broadly categorized into four important functional groups, which highlight their roles in diverse plant metabolic reactions including reactive aldehyde detoxification, biosynthesis of osmolytes, secondary metabolism and membrane transport. Further, multiple overlapping functional aspects of plant AKRs including biotic and abiotic stress defense, production of commercially important secondary metabolites, iron acquisition from soil, plant-microbe interactions etc. are discussed as subcategories within respective major groups. Owing to the broad substrate specificity and multiple stress tolerance of the well-characterized AKR4C9 from Arabidopsis thaliana, protein sequences of all the homologues of AKR4C9 (A9-like proteins) from forty different plant species (Phytozome database) were analyzed. The analysis revealed that all A9-like proteins possess strictly conserved key catalytic residues (D-47, Y-52 and K-81) and belong to the pfam00248 and cl00470 AKR superfamilies. Based on structural homology of the three flexible loops of AKR4C9 (Loop A, B and C) responsible for broad substrate specificity, A9-like proteins found in Brassica rapa, Phaseolus vulgaris, Cucumis sativus, Populus trichocarpa and Solanum lycopersicum were predicted to have a similar range of substrate specificity. Thus, plant AKRs can be considered as potential breeding targets for developing stress tolerant varieties in the future. The present review provides a consolidated update on the current research status of plant AKRs with an emphasis on important functional aspects as well as their potential future prospects and an insight into the overall structure-function relationships of A9-like proteins.

Structural characterization and functional validation of aldose reductase from the resurrection plant Xerophyta viscosa

Aldose reductases are key enzymes in the detoxification of reactive aldehyde compounds like methylglyoxal (MG) and malondialdehyde. The present study describes for first time the preliminary biochemical and structural characterization of the aldose reductase (ALDRXV4) enzyme from the resurrection plant Xerophyta viscosa. The ALDRXV4 cDNA was expressed in E. coli using pET28a expression vector, and the protein was purified using affinity chromatography. The recombinant protein showed a molecular mass of ~36 kDa. The K M (1.2 mM) and k cat (14.5 s(-1)) of the protein determined using MG as substrate was found to be comparable with other reported homologs. Three-dimensional structure prediction based on homology modeling suggested several similarities with the other aldose reductases reported from plants. Circular dichroism spectroscopy results supported the bioinformatic prediction of alpha-beta helix nature of aldose reductase proteins. Subcellular localization studies revealed that the ALDRXV4-GFP fusion protein was localized both in the nucleus and the cytoplasm. The E. coli cells overexpressing ALDRXV4 exhibited improved growth and showed tolerance against diverse abiotic stresses induced by high salt (500 mM NaCl), osmoticum (10 % PEG 6000), heavy metal (20 mM CdCl2), and MG (5 mM). Based on these results, we propose that ALDRXV4 gene from X. viscosa could be a potential candidate for developing stress-tolerant crop plants.

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

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 systems mainly scavenge sugar-derived RCs in higher plants and cyanobacteria. 2-Alkenal reductase and alkenal/alkenone reductase catalyse the reduction of lipid-derived RCs in higher plants. In cyanobacteria, medium-chain dehydrogenases/reductases are the main scavengers of lipid-derived RCs.

Functional analysis of the AKR4C subfamily of Arabidopsis thaliana: model structures, substrate specificity, acrolein toxicity, and responses to light and [CO(2)]

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.

Cloning and characterization of AKR4C14, a rice aldo-keto reductase, from Thai Jasmine rice

Aldo-keto reductase (AKR) is an enzyme superfamily whose members are involved in the metabolism of aldehydes/ketones. The AKR4 subfamily C (AKR4C) is a group of aldo-keto reductases that are found in plants. Some AKR4C(s) in dicot plants are capable of metabolizing reactive aldehydes whereas, such activities have not been reported for AKR4C(s) from monocot species. In this study, we have screened Indica rice genome for genes with significant homology to dicot AKR4C(s) and identified a cluster of putative AKR4C(s) located on the Indica rice chromosome I. The genes including OsI_04426, OsI_04428 and OsI_04429 were successfully cloned and sequenced by qRT-PCR from leaves of Thai Jasmine rice (KDML105). OsI_04428, later named AKR4C14, was chosen for further studies because it shares highest homology to the dicot AKR4C(s). The bacterially expressed recombinant protein of AKR4C14 was successfully produced as a MBP fusion protein and his-tagged protein. The recombinant AKR4C14 were capable of metabolizing sugars and reactive aldehydes i.e. methylglyoxal, a toxic by-product of the glycolysis pathway, glutaraldehyde, and trans-2-hexenal, a natural reactive 2-alkenal. AKR4C14 was highly expressed in green tissues, i.e. leaf sheets and stems, whereas flowers and roots had a significantly lower level of expression. These findings indicated that monocot AKR4C(s) can metabolize reactive aldehydes like the dicot AKR4C(s) and possibly play a role in detoxification mechanism of reactive aldehydes.