Engineering Alfalfa to Produce 2-O-Caffeoyl-L-Malate (Phaselic Acid) for Preventing Post-harvest Protein Loss via Oxidation by Polyphenol Oxidase

Hydroxycinnamoyl-coenzyme A: tetrahydroxyhexanedioate hydroxycinnamoyl transferase (HHHT) from Phaseolus vulgaris L.: phylogeny, expression pattern, kinetic parameters, and active site analysis

BAHD acyl-coenzyme A (CoA) transferases comprise a large family of enzymes in plants which transfer an acyl group from a CoA thioester to hydroxyl or amine groups to form esters or amides, respectively. Clade Vb of this family primarily utilizes hydroxycinnamoyl-CoA as the acyl donor. These enzymes are involved in biosynthesis of diverse specialized metabolites with functions such as structure (e.g., lignin formation) and biotic/abiotic stress mitigation. The diversity of these enzymes has arisen from both divergent and convergent evolution, making it difficult to predict substrate specificity or enzyme function based on homology, and relatively few BAHD transferases have been characterized biochemically with respect to substrate specificity. We previously identified a hydroxycinnamoyl-CoA: tetrahydroxyhexanedioate hydroxycinnamoyl transferase (HHHT) from common bean capable of transferring hydroxycinnamic acids to mucic or saccharic acid to form the corresponding esters. Here, to better understand the structure/function relationships of this enzyme, we have further characterized it with respect to expression pattern, kinetic parameters, and predicted three-dimensional (3-D) structure and active site interactions with acceptor substrates. The hhht gene was expressed predominantly in leaves and to a lesser extent flowers and shoots. K M values did not vary greatly among donor or among acceptor substrates (generally less than two-fold), while k cat values were consistently higher for saccharic acid as substrate compared to mucic acid, leading to higher catalytic efficiency (as k cat/K M) for saccharic acid. Both acceptors had similar binding poses when docked into the active site, and the proximity of multiple hydroxyl groups to the catalytic His 150, especially for saccharic acid, might provide some insights into regiospecificity. These findings provide a foundation for better understanding how the 3-D structure of BAHD transferases relates to their substrate specificity, as we explore the chemistry of the active site and interactions with ligands. This could ultimately lead to better prediction of their function and ability to rationally design BAHD transferases to make useful and novel products.

Phenolic Composition and α-Glucosidase Inhibition of Leaves from Chilean Bean Landraces

The MeOH:H₂O (7:3) extracts of leaves from Chilean bean landraces were assessed for total phenolic (TP), total flavonoid (TF), total proanthocyanidin (TPA) content, antioxidant capacity (ORAC, FRAP, TEAC, CUPRAC, DPPH) and the inhibition of enzymes associated with metabolic syndrome (α-glucosidase, α-amylase, pancreatic lipase). The chemical profiles were analyzed by HPLC-DAD. Higher antioxidant activity in the ORAC and CUPRAC assay was found for the landrace Coscorrón, and the best effect in the TEAC for Sapito, respectively. The main phenolics were flavonol glycosides and caffeic acid derivatives. The extracts presented strong activity against α-glucosidase, but were inactive towards α-amylase and pancreatic lipase. The leaf extract from the Sapito landrace was fractionated to isolate the main α-glucosidase inhibitors, leading to caffeoylmalic acid with an IC₅₀ of 0.21 μg/mL. The HPLC fingerprints of the leaves differentiate three groups of chemical profiles, according to the main phenolic content. A significant correlation was found between the α-glucosidase inhibition, the content of caffeoylmalic acid (r = -0.979) and kaempferol 3-O-β-D-glucoside (r = 0.942) in the extracts. The presence of α-glucosidase inhibitors in the leaves of Chilean beans support their potential as a source of bioactive compounds.

Red Clover HDT, a BAHD Hydroxycinnamoyl-Coenzyme A:L-3,4-Dihydroxyphenylalanine (L-DOPA) Hydroxycinnamoyl Transferase That Synthesizes Clovamide and Other N-Hydroxycinnamoyl-Aromatic Amino Acid Amides

Red clover leaves accumulate high levels (up to 1 to 2% of dry matter) of two caffeic acid derivatives: phaselic acid (2-O-caffeoyl-L-malate) and clovamide [N-caffeoyl-L-3,4-dihydroxyphenylalanine (L-DOPA)]. These likely play roles in protecting the plant from biotic and abiotic stresses but can also help preserve protein during harvest and storage of the forage via oxidation by an endogenous polyphenol oxidase. We previously identified and characterized, a hydroxycinnamoyl-coenzyme A (CoA):malate hydroxycinnamoyl transferase (HMT) from red clover. Here, we identified a hydroxycinnamoyl-CoA:L-DOPA hydroxycinnamoyl transferase (HDT) activity in unexpanded red clover leaves. Silencing of the previously cloned HMT gene reduced both HMT and HDT activities in red clover, even though the HMT enzyme lacks HDT activity. A combination of PCR with degenerate primers based on BAHD hydroxycinnamoyl-CoA transferase sequences and 5' and 3' rapid amplification of cDNA ends was used to clone two nearly identical cDNAs from red clover. When expressed in Escherichia coli, the encoded proteins were capable of transferring hydroxycinnamic acids (p-coumaric, caffeic, or ferulic) from the corresponding CoA thioesters to the aromatic amino acids L-Phe, L-Tyr, L-DOPA, or L-Trp. Kinetic parameters for these substrates were determined. Stable expression of HDT in transgenic alfalfa resulted in foliar accumulation of p-coumaroyl- and feruloyl-L-Tyr that are not normally present in alfalfa, but not derivatives containing caffeoyl or L-DOPA moieties. Transient expression of HDT in Nicotiana benthamiana resulted in the production of caffeoyl-L-Tyr, but not clovamide. Coexpression of HDT with a tyrosine hydroxylase resulted in clovamide accumulation, indicating the host species' pool of available amino acid (and hydroxycinnamoyl-CoA) substrates likely plays a major role in determining HDT product accumulation in planta. Finally, that HDT and HMT proteins share a high degree of identity (72%), but differ substantially in substrate specificity, is promising for further investigation of structure-function relationships of this class of enzymes, which could allow the rational design of BAHD enzymes with specific and desirable activities.