Recent progress on the characterization of aldonolactone oxidoreductases

Abstract

l-Ascorbic acid (ascorbate, AsA, vitamin C) is essential for animal and plant health. Despite our dependence on fruits and vegetables to fulfill our requirement for this vitamin, the metabolic network leading to its formation in plants is just being fully elucidated. There is evidence supporting the operation of at least four biosynthetic pathways leading to AsA formation in plants. These routes use d-mannose/l-Galactose, l-gulose, d-galacturonate, and myo-inositol as the main precursors. This review focuses on aldonolactone oxidoreductases, a subgroup of the vanillyl alcohol oxidase (VAO; EC 1.1.3.38) superfamily, enzymes that catalyze the terminal step in AsA biosynthesis in bacteria, protozoa, animals, and plants. In this report, we review the properties of well characterized aldonolactone oxidoreductases to date. A shared feature in these proteins is the presence of a flavin cofactor as well as a thiol group. The flavin cofactor in many cases is bound to the N terminus of the enzymes or to a recently discovered HWXK motif in the C terminus. The binding between the flavin moiety and the protein can be either covalent or non-covalent. Substrate specificity and subcellular localization differ among the isozymes of each kingdom. All oxidases among these enzymes possess dehydrogenase activity, however, exclusive dehydrogenases are also found. We also discuss recent evidence indicating that plants have both l-gulono-1,4-lactone oxidases and l-Galactono-1,4-lactone dehydrogenases involved in AsA biosynthesis.

Characterization of a L-Gulono-1,4-Lactone Oxidase Like Protein in the Floral Nectar of Mucuna sempervirens, Fabaceae

“…However, unlike MsGulLO, MsGLDH was predicted to be a mitochondrial protein, containing no signal peptide but a FAD-binding motif in the N terminus. This prediction is in agreement with plant GLDHs being localized in the mitochondria ( Aboobucker and Lorence, 2016 ). MsGLDH had an instability index (II) of 44.97, which indicated that it is not theoretically stable.…”

“…The full amino acid sequence of MsGulLO was deduced from the cDNA sequence, and subjected to a Pfam search which revealed an ALO family domain at positions 375−520 ( E -value: 5.6e −13 ) and an FAD binding domain at positions 2−133 ( E -value: 6.3e −22 ). However, in common with other plant GulLOs, MsGulLO doesn’t have a FAD-binding motif in its N terminus ( Leferink et al, 2008 ; Aboobucker and Lorence, 2016 ). The mature MsGulLO protein had a neutral isoelectric point of 6.72, and a predicted molecular mass of 61,962.52 Da which is consistent with the 2-DE results ( Figure 2 ).…”

“…The presence or absence of FAD from the system had no effect on MsGulLO’s oxidase activity (data not shown). The FAD-binding motif is not present in the protein sequence of MsGulLO, AtGulLOs, or other so-called plant GulLOs ( Leferink et al, 2008 ; Aboobucker and Lorence, 2016 ). This indicated that plant GulLOs might have different activity and/or a distinct catalysis mechanism from animal GulLOs.…”

“…This also suggests that Plant GLDHs and GulLOs carry out different functions in plants. Fourthly, even though plant GulLOs share high sequence similarity with each other, the identity between plant GulLOs and animal GulLOs is very low, less than 30% ( Aboobucker and Lorence, 2016 ). Phylogenetic analysis also showed that plant GLDHs are far closer to animal GulLOs than either are to plant GulLOs, which indicates that plant GulLOs probably have a different evolutionary origin to either, and perform a different physiological function.…”

Characterization of Two Arabidopsis L-Gulono-1,4-lactone Oxidases, AtGulLO3 and AtGulLO5, Involved in Ascorbate Biosynthesis

“…Interestingly, M. tuberculosis GulLDH had higher activity when phenazine methosulfate was used as an electron acceptor similar to the findings of Mapson and Breslow [16]. No flavin prosthetic group is found when aldonolactone oxidoreductases—known to have a covalent FAD but not non-covalent FAD—are expressed recombinantly in E. coli [61, 62]. However, no problem with the attachment of the flavin group was experienced when aldonolactone oxidoreductases were expressed in eukaryotic systems [63‒65].…”