title: The hydroxypyruvate-reducing system in Arabidopsis: multiple enzymes for the same end.
abstract: Hydroxypyruvate (HP) is an intermediate of the photorespiratory pathway that originates in the oxygenase activity of the key enzyme of photosynthetic CO(2) assimilation, Rubisco. In course of this high-throughput pathway, a peroxisomal transamination reaction converts serine to HP, most of which is subsequently reduced to glycerate by the NADH-dependent peroxisomal enzyme HP reductase (HPR1). In addition, a NADPH-dependent cytosolic HPR2 provides an efficient extraperoxisomal bypass. The combined deletion of these two enzymes, however, does not result in a fully lethal photorespiratory phenotype, indicating even more redundancy in the photorespiratory HP-into-glycerate conversion. Here, we report on a third enzyme, HPR3 (At1g12550), in Arabidopsis (Arabidopsis thaliana), which also reduces HP to glycerate and shows even more activity with glyoxylate, a more upstream intermediate of the photorespiratory cycle. The deletion of HPR3 by T-DNA insertion mutagenesis 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. On the other hand, 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. Since in silico analysis and proteomic studies from other groups indicate targeting of HPR3 to the chloroplast, this enzyme could provide a compensatory bypass for the reduction of HP and glyoxylate within this compartment.
authors.affiliation: Department of Plant Physiology, University of Rostock, D-18059 Rostock, Germany. stefan.timm@uni-rostock.de
authors.forename: Stefan
authors.initials: S
authors.lastname: Timm
authors.forename: Alexandra
authors.initials: A
authors.lastname: Florian
authors.forename: Kathrin
authors.initials: K
authors.lastname: Jahnke
authors.forename: Adriano
authors.initials: A
authors.lastname: Nunes-Nesi
authors.forename: Alisdair R
authors.initials: AR
authors.lastname: Fernie
authors.forename: Hermann
authors.initials: H
authors.lastname: Bauwe
mesh_terms: D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002734:Chlorophyll / Q000032:analysis; D017353:Gene Deletion; D055786:Gene Knockout Techniques; D005988:Glyceric Acids / Q000378:metabolism; D006038:Glyoxylates / Q000378:metabolism; D055432:Metabolomics; D016254:Mutagenesis, Insertional; D010788:Photosynthesis; D011773:Pyruvates / Q000378:metabolism*; D018749:RNA, Plant / Q000235:genetics; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D005988:Glyceric Acids; D006038:Glyoxylates; D011773:Pyruvates; D018749:RNA, Plant; D011994:Recombinant Proteins; D002734:Chlorophyll; C042971:glyceric acid; C012375:hydroxypyruvic acid; C031150:glyoxylic acid