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evolution
the enzyme belongs to the beta-HAD (beta-hydroxyacid dehydrogenase) protein family. AtHPR2 and AtHPR3 are 45% identical to each other at the amino acid level, but only 19-25% identical to AtHPR1, the NADH-dependent form, and 8-9% identical to the AtGLYRs. None of the AtHPRs contains the active-site residues conserved in AtGLYR1 and AtGLYR2, indicating that the sites responsible for reducing glyoxylate differ greatly between the AtGLYRs and AtHPRs
evolution
HPR1 and HPR2 are the major hydroxypyruvate-reducing enzymes in leaves
evolution
no hydroxyphenylpyruvate reductase (HPPR) activity by isozyme HPPR4 from Arabidopsis thaliana. Isozyme HPPR2 mainly shows hydroxypyruvate reductase (HPR) activity, while isozyme HPPR3 mainly shows 4-hydroxyphenylpyruvate reductase (EC 1.1.1.237) activity. Enzyme HPPR2 belongs to the family of D-isomer-specific 2-hydroxyacid dehydrogenases, group II
evolution
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HPR1 and HPR2 are the major hydroxypyruvate-reducing enzymes in leaves
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malfunction
Arabidopsis thaliana mutants defective in either HPPR2 or HPPR3 isozyme contain lower amounts of pHPL and are impaired in conversion of tyrosine to pHPL. Furthermore, a loss-of-function mutation in tyrosine aminotransferase (TAT) also reduces the pHPL accumulation in plants
malfunction
deletion of HPR2 results in elevated levels of hydroxypyruvate and other metabolites in leaves. Photosynthetic gas exchange is slightly altered, especially under long-day conditions. Otherwise, the mutant closely resembles wild-type plants. The combined deletion of both HPR1 and HPR2 results in distinct air-sensitivity and a dramatic reduction in photosynthetic performance. Knockout of both HPR1 and HPR2 alters steady-state metabolite profiles. Knockout of either HPR1 or HPR2 alters photosynthetic gas exchange
malfunction
deletion of HPR3 results in slightly altered leaf concentrations of the photorespiratory intermediates HP, glycerate, and glycine, indicating a disrupted photorespiratory flux, but not in visible alteration of the phenotype. The combined deletion of HPR1, HPR2, and HPR3 causes increased growth retardation, decreased photochemical efficiency, and reduced oxygen-dependent gas exchange in comparison with the hpr1xhpr2 double mutant. HPR mutants show impaired growth and contain less chlorophyll, phenotypes, detailed overview
malfunction
deletion of photorespiratory enzymes typically leads to a strong air sensitivity of the respective mutants, which can be fully recovered by elevated-CO2 conditions. While this is a distinctive feature of most photorespiratory mutants, Arabidopsis thaliana HPR1 knockout mutants grow nearly normally in ambient air with moderate photoperiods and show only minor changes in photosynthetic and metabolic parameters under these conditions. The additional deletion of the cytosolic HPR2 distinctly elevates the oxygen sensitivity, but this hpr1xhpr2 double mutant can still survive long-term exposure to ambient air
malfunction
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Arabidopsis thaliana mutants defective in either HPPR2 or HPPR3 isozyme contain lower amounts of pHPL and are impaired in conversion of tyrosine to pHPL. Furthermore, a loss-of-function mutation in tyrosine aminotransferase (TAT) also reduces the pHPL accumulation in plants
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malfunction
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deletion of HPR2 results in elevated levels of hydroxypyruvate and other metabolites in leaves. Photosynthetic gas exchange is slightly altered, especially under long-day conditions. Otherwise, the mutant closely resembles wild-type plants. The combined deletion of both HPR1 and HPR2 results in distinct air-sensitivity and a dramatic reduction in photosynthetic performance. Knockout of both HPR1 and HPR2 alters steady-state metabolite profiles. Knockout of either HPR1 or HPR2 alters photosynthetic gas exchange
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malfunction
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deletion of photorespiratory enzymes typically leads to a strong air sensitivity of the respective mutants, which can be fully recovered by elevated-CO2 conditions. While this is a distinctive feature of most photorespiratory mutants, Arabidopsis thaliana HPR1 knockout mutants grow nearly normally in ambient air with moderate photoperiods and show only minor changes in photosynthetic and metabolic parameters under these conditions. The additional deletion of the cytosolic HPR2 distinctly elevates the oxygen sensitivity, but this hpr1xhpr2 double mutant can still survive long-term exposure to ambient air
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malfunction
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deletion of HPR3 results in slightly altered leaf concentrations of the photorespiratory intermediates HP, glycerate, and glycine, indicating a disrupted photorespiratory flux, but not in visible alteration of the phenotype. The combined deletion of HPR1, HPR2, and HPR3 causes increased growth retardation, decreased photochemical efficiency, and reduced oxygen-dependent gas exchange in comparison with the hpr1xhpr2 double mutant. HPR mutants show impaired growth and contain less chlorophyll, phenotypes, detailed overview
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metabolism
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NADH-HPR is extensively involved in carbon metabolism
metabolism
in vitro characterization of the recombinant proteins reveals that HPPR2 has both hydroxypyruvate reductase (HPR EC 1.1.1.81, main activity) and hydroxyphenylpyruvate reductase (HPPR, EC 1.1.1.237) activities, whereas HPPR3 has a strong preference for pHPP, and both enzymes are localized in the cytosol. In Arabidopsis thaliana, HPPR2 and HPPR3, together with tyrosine aminotransferase 1 (TAT1), constitute to a probably conserved biosynthetic pathway from tyrosine to 4-hydroxyphenyllactic acid (pHPL), from which some specialized metabolites, such as rosmarinic acid (RA), can be generated in specific groups of plants. Role of HPPR in the tyrosine conversion pathway, overview
metabolism
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in vitro characterization of the recombinant proteins reveals that HPPR2 has both hydroxypyruvate reductase (HPR EC 1.1.1.81, main activity) and hydroxyphenylpyruvate reductase (HPPR, EC 1.1.1.237) activities, whereas HPPR3 has a strong preference for pHPP, and both enzymes are localized in the cytosol. In Arabidopsis thaliana, HPPR2 and HPPR3, together with tyrosine aminotransferase 1 (TAT1), constitute to a probably conserved biosynthetic pathway from tyrosine to 4-hydroxyphenyllactic acid (pHPL), from which some specialized metabolites, such as rosmarinic acid (RA), can be generated in specific groups of plants. Role of HPPR in the tyrosine conversion pathway, overview
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physiological function
deletion of any of the core enzymes of the photorespiratory cycle, one of the major pathways of plant primary metabolism, results in severe air-sensitivity of the respective mutants with the exception of the peroxisomal enzyme hydroxypyruvate reductase, HPR1, due to the existence of a second hydroxypyruvate reductase, HPR2, in the cytosol, overview. The enzyme provides a cytosolic bypass to the photorespiratory core cycle in Arabidopsis thaliana
physiological function
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in the dark, cytokinins mimic the regulatory effect of light upon algal cell division, metabolite content and stimulate carbon recycling for Calvin cycle reactions by enhancing of light-dependent NADH-HPR activity, regulation, overview
physiological function
deletion of isoform HPR3 results in slightly altered leaf concentrations of the photorespiratory intermediates HP, glycerate, and glycine, indicating a disrupted photorespiratory flux, but not in visible alteration of the phenotype.The combined deletion of of isoforms HPR1, HPR2, and HPR3 causes increased growth retardation, decreased photochemical efficiency, and reduced oxygen-dependent gas exchange in comparison with the hpr1hpr2 double mutant. Isoform HPR3 could provide a compensatory bypass for the reduction of hydroxypyruvate and glyoxylate within the chloroplast
physiological function
upregulation of glyoxylate reductase/hydroxypyruvate reductase is associated with intestinal epithelial cells apoptosis in trinitrobenzenesulfonic acid-induced colitis
physiological function
hydroxypyruvate reductase activity is important in the recycling of metabolites derived during photorespiration
physiological function
Arabidopsis mutants defective in either isoform HPPR2 or HPPR3 contain lower amounts of 4-hydroxyphenyllactic acid and are impaired in conversion of tyrosine to 4-hydroxyphenyllactic acid
physiological function
HPR3 is the third enzyme in Arabidopsis (Arabidopsis thaliana), which also reduces 4-hydroxypyruvate (HP) to glycerate and shows even more activity with glyoxylate, a more upstream intermediate of the photorespiratory cycle. In silico analysis and proteomic studies target HPR3 to the chloroplast, the enzyme might provide a compensatory bypass for the reduction of HP and glyoxylate within this compartment
physiological function
hydroxyphenylpyruvate reductase (HPPR), which catalyzes the reduction of 4-hydroxyphenylpyruvic acid (pHPP) to 4-hydroxyphenyllactic acid (pHPL), is the key enzyme in the biosynthesis of rosmarinic acid (RA) from tyrosine and, so far, HPPR activity is reported only from the RA-accumulating plants
physiological function
the enzyme activity shows that photorespiratory metabolism is not confined to chloroplasts, peroxisomes, and mitochondria but also extends to the cytosol. The extent to which cytosolic reactions contribute to the operation of the photorespiratory cycle in varying natural environments might be dynamically regulated by the availability of NADH in the context of peroxisomal redox homeostasis. But isozyme HPR1 plays the dominant role in photorespiration
physiological function
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Arabidopsis mutants defective in either isoform HPPR2 or HPPR3 contain lower amounts of 4-hydroxyphenyllactic acid and are impaired in conversion of tyrosine to 4-hydroxyphenyllactic acid
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physiological function
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hydroxyphenylpyruvate reductase (HPPR), which catalyzes the reduction of 4-hydroxyphenylpyruvic acid (pHPP) to 4-hydroxyphenyllactic acid (pHPL), is the key enzyme in the biosynthesis of rosmarinic acid (RA) from tyrosine and, so far, HPPR activity is reported only from the RA-accumulating plants
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physiological function
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the enzyme activity shows that photorespiratory metabolism is not confined to chloroplasts, peroxisomes, and mitochondria but also extends to the cytosol. The extent to which cytosolic reactions contribute to the operation of the photorespiratory cycle in varying natural environments might be dynamically regulated by the availability of NADH in the context of peroxisomal redox homeostasis. But isozyme HPR1 plays the dominant role in photorespiration
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physiological function
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HPR3 is the third enzyme in Arabidopsis (Arabidopsis thaliana), which also reduces 4-hydroxypyruvate (HP) to glycerate and shows even more activity with glyoxylate, a more upstream intermediate of the photorespiratory cycle. In silico analysis and proteomic studies target HPR3 to the chloroplast, the enzyme might provide a compensatory bypass for the reduction of HP and glyoxylate within this compartment
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