An Arabidopsis Oxalyl-CoA Decarboxylase, AtOXC, Is Important for Oxalate Catabolism in Plants.

Justin Foster,Vincent Paris,Jin Wang,Paul A Nakata,Xiaoqiang Wang,Ninghui Cheng,Lingfei Wang

doi:10.3390/ijms22063266

Abstract

Considering the widespread occurrence of oxalate in nature and its broad impact on a host of organisms, it is surprising that so little is known about the turnover of this important acid. In plants, oxalate oxidase is the most well-studied enzyme capable of degrading oxalate, but not all plants possess this activity. Recently, acyl-activating enzyme 3 (AAE3), encoding an oxalyl-CoA synthetase, was identified in Arabidopsis. This enzyme has been proposed to catalyze the first step in an alternative pathway of oxalate degradation. Since this initial discovery, this enzyme and proposed pathway have been found to be important to other plants and yeast as well. In this study, we identify, in Arabidopsis, an oxalyl-CoA decarboxylase (AtOXC) that is capable of catalyzing the second step in this proposed pathway of oxalate catabolism. This enzyme breaks down oxalyl-CoA, the product of AtAAE3, into formyl-CoA and CO2. AtOXC:GFP localization suggested that this enzyme functions within the cytosol of the cell. An Atoxc knock-down mutant showed a reduction in the ability to degrade oxalate into CO2. This reduction in AtOXC activity resulted in an increase in the accumulation of oxalate and the enzyme substrate, oxalyl-CoA. Size exclusion studies suggest that the enzyme functions as a dimer. Computer modeling of the AtOXC enzyme structure identified amino acids of predicted importance in co-factor binding and catalysis. Overall, these results suggest that AtOXC catalyzes the second step in this alternative pathway of oxalate catabolism.

Highlights

Oxalate is the simplest of the dicarboxylic acids
Our previous work proposed the existence of an oxalate catabolism pathway in dicot plants such as Arabidopsis thaliana (Figure 1) and demonstrated that an oxalyl-CoA synthetase (AAE3) acted as a key rate-limiting enzyme in this pathway [21]
Subcellular localization of GFP-AtOXC in Arabidopsis showed that the enzyme functioned in the cytosol of cells (Figure 6). This finding is consistent with localization studies that place the AtAAE3 [21], as well as the activating enzyme 3 (AAE3) from other plants [24,25], within the cytosolic compartment. These findings suggest that AtAAE3 and AtOXC could act in a stepwise fashion catalyzing the first and the second reactions in the proposed CoA-dependent pathway of oxalate catabolism (Figure 1)

Summary

Introduction

Oxalate is the simplest of the dicarboxylic acids. Its biosynthesis in plants has been proposed to occur via multiple pathways. Isocitrate, glycollate, glyoxylate, oxaloacetate, and ascorbate have all been suggested as possible precursors to this organic acid [1] Of these precursors, ascorbate has been considered the primary substrate for the biosynthesis of oxalate utilized in the formation of the calcium oxalate crystal [1]. The ability to produce ample amounts of oxalate can provide many beneficial functions to the plant, uncontrolled or prolonged exposure to this strong organic acid can cause multiple physiological problems. Such problems can result from a disruption of membrane integrity, disruption of mitochondrial metabolism, metal precipitation, and free radical formation [4]

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Results

Conclusion