Kinetic Basis for the Conjugation of Auxin by a GH3 Family Indole-acetic Acid-Amido Synthetase

Abstract

The GH3 family of acyl-acid-amido synthetases catalyze the ATP-dependent formation of amino acid conjugates to modulate levels of active plant hormones, including auxins and jasmonates. Initial biochemical studies of various GH3s show that these enzymes group into three families based on sequence relationships and acyl-acid substrate preference (

Modulating plant hormones by enzyme action

“…Asparagine, the next best amino acid substrate, shows a 45-fold reduction in k cat /K m compared to aspartate and other amino acids are more than 100-fold lower. 40 These results suggest that under biological conditions, it is highly unlikely that OsGH3-8 accepts amino acids other than aspartate as a substrate. AtGH3-12 is the only biochemically studied group III enzyme.…”

“…14 Sequence comparisons detected low amino acid similarity between AtGH3-11 and the firefly luciferase superfamily of IAA. 40 Similarly, the enzyme is also highly specific for aspartate. Asparagine, the next best amino acid substrate, shows a 45-fold reduction in k cat /K m compared to aspartate and other amino acids are more than 100-fold lower.…”

“…38 The OsGH3-8 enzyme specifically conjugates IAA and aspartate to form IAA-aspartate. [38][39][40] Less is known about the other two identified rice GH3 genes. Overexpression of OsGH3-1 in rice causes dwarfism, significantly reduces free auxin levels and cell elongation, but increases resistance to fungal pathogens.…”

“…Addressing this point, will help to define IAA-Asp formation, the kinetic parameters for the overall reaction catalyzed by OsGH3-8 were determined. 39,40 Each assay yields comparable kinetic values, 39,40 as summarized in Table 2. Importantly, these assays provide a quantitative assessment of GH3 protein acyl acid and amino acid specificities.…”

“…Other auxin-analogs, including phenylacetic acid, indole butyric acid and napthanlenacid acid, were also substrates with catalytic efficiencies (i.e., k cat /K m ) 1.4-to 9-fold lower than that observed for substrates for the enzyme. 42 Surprisingly, the kinetic parameters for only two GH3 proteins (OsGH3-8, 39,40 and AtGH3-12, 45 ) have been determined to date.…”