Submitted to: Biochimica et Biophysica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2006
Publication Date: 3/31/2006
Citation: Shrestha, R., Kim, S., Dyer, J.M., Dixon, R.A., Chapman, K.D. 2006. Plant fatty acid (ethanol) amide hydrolases. Biochimica et Biophysica Acta. 1761(3):324-334.
Interpretive Summary: It has recently been identified that plants have a class of hormones that may be similar to mammals, but little is known about how these hormones are regulated in plants. In collaboration with scientists at the University of North Texas and the Samuel Roberts Noble Foundation, an ARS scientist has helped to identify and characterize an enzyme that is involved in breaking down this particular class of hormones in plants. Breakdown of the hormone effectively stops the function of the hormone, which is an essential component of its regulation. The enzyme responsible for the breakdown of the hormone was identified in distantly related plant species, indicating that this hormonal pathway is present in many different types of plants. Comparison of the plant enzymes to a similar enzyme from mammals demonstrated that they were highly similar, which suggests that this hormonal pathway is indeed similar between plants and animals. This information will be highly instructive to scientists who are interested in understanding how plants sense their environment.
Technical Abstract: Fatty acid amide hydrolase (FAAH) plays a central role in modulating endogenous N-acylethanolamine (NAE) levels in vertebrates, and, in part, constitutes an “endocannabinoid” signaling pathway that regulates diverse physiological and behavioral processes in animals. Recently, an Arabidopsis FAAH homologue was identified which catalyzed the hydrolysis of NAEs in vitro suggesting a FAAH-mediated pathway exists in plants for the metabolism of endogenous NAEs. Here, we provide evidence to support this concept by identifying candidate FAAH genes in monocots (Oryza sativa) and legumes (Medicago truncatula), which have similar, but not identical, exon–intron organizations. Corresponding M. truncatula and rice cDNAs were isolated and cloned into prokaryotic expression vectors and expressed as recombinant proteins in Escherichia coli. NAE amidohydrolase assays confirmed that these proteins indeed catalyzed the hydrolysis of 14C-labeled NAEs in vitro. Kinetic parameters and inhibition properties of the rice FAAH were similar to those of Arabidopsis and rat FAAH, but not identical. Sequence alignments and motif analysis of plant FAAH enzymes revealed a conserved domain organization for these members of the amidase superfamily. Five amino-acid residues determined to be important for catalysis by rat FAAH were absolutely conserved within the FAAH sequences of six plant species. Homology modeling of the plant FAAH proteins using the rat FAAH crystal structure as a template revealed a conserved protein core that formed the active site of each enzyme. Collectively, these results indicate that plant and mammalian FAAH proteins have similar structure/activity relationships despite limited overall sequence identity. Defining the molecular properties of NAE amidohydrolase enzymes in plants will help to better understand the metabolic regulation of NAE lipid mediators.