Many, if not most, enzymes can promiscuously catalyze reactions, or act on substrates, other than those for which they evolved. Here, we discuss the structural, mechanistic, and evolutionary implications of this manifestation of infidelity of molecular recognition. We define promiscuity and related phenomena and also address their generality and physiological implications. We discuss the mechanistic enzymology of promiscuity—how enzymes, which generally exert exquisite specificity, catalyze other, and sometimes barely related, reactions. Finally, we address the hypothesis that promiscuous enzymatic activities serve as evolutionary starting points and highlight the unique evolutionary features of promiscuous enzyme functions.

Enzyme Promiscuity: A Mechanistic and Evolutionary Perspective

Volatile compounds act as a language that plants use for their communication and interaction with the surrounding environment. To date, a total of 1700 volatile compounds have been isolated from more than 90 plant families. These volatiles, released from leaves, flowers, and fruits into the atmosphere and from roots into the soil, defend plants against herbivores and pathogens or provide a reproductive advantage by attracting pollinators and seed dispersers. Plant volatiles constitute about 1% of plant secondary metabolites and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. In this review we focus on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.

Plant Volatiles: Recent Advances and Future Perspectives

Plants synthesize an amazing diversity of volatile organic compounds (VOCs) that facilitate interactions with their environment, from attracting pollinators and seed dispersers to protecting themselves from pathogens, parasites and herbivores. Recent progress in -omics technologies resulted in the isolation of genes encoding enzymes responsible for the biosynthesis of many volatiles and contributed to our understanding of regulatory mechanisms involved in VOC formation. In this review, we largely focus on the biosynthesis and regulation of plant volatiles, the involvement of floral volatiles in plant reproduction as well as their contribution to plant biodiversity and applications in agriculture via crop-pollinator interactions. In addition, metabolic engineering approaches for both the improvement of plant defense and pollinator attraction are discussed in light of methodological constraints and ecological complications that limit the transition of crops with modified volatile profiles from research laboratories to real-world implementation.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.12145

Biosynthesis, function and metabolic engineering of plant volatile organic compounds.

Plants have the capacity to synthesize, accumulate and emit volatiles that may act as aroma and flavor molecules due to interactions with human receptors. These low-molecular-weight substances derived from the fatty acid, amino acid and carbohydrate pools constitute a heterogenous group of molecules with saturated and unsaturated, straight-chain, branched-chain and cyclic structures bearing various functional groups (e.g. alcohols, aldehydes, ketones, esters and ethers) and also nitrogen and sulfur. They are commercially important for the food, pharmaceutical, agricultural and chemical industries as flavorants, drugs, pesticides and industrial feedstocks. Due to the low abundance of the volatiles in their plant sources, many of the natural products had been replaced by their synthetic analogues by the end of the last century. However, the foreseeable shortage of the crude oil that is the source for many of the artificial flavors and fragrances has prompted recent interest in understanding the formation of these compounds and engineering their biosynthesis. Although many of the volatile constituents of flavors and aromas have been identified, many of the enzymes and genes involved in their biosynthesis are still not known. However, modification of flavor by genetic engineering is dependent on the knowledge and availability of genes that encode enzymes of key reactions that influence or divert the biosynthetic pathways of plant-derived volatiles. Major progress has resulted from the use of molecular and biochemical techniques, and a large number of genes encoding enzymes of volatile biosynthesis have recently been reported.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2008.03446.x

Biosynthesis of plant-derived flavor compounds.

https://www.cell.com/molecular-plant/pdf/S1674-2052(14)60547-5.pdf

New insights into the shikimate and aromatic amino acids biosynthesis pathways in plants.

Plant phenylacetaldehyde synthase is a bifunctional homotetrameric enzyme that catalyzes phenylalanine decarboxylation and oxidation.

Rose flowers, like flowers and fruits of many other plants, produce and emit the aromatic volatiles 2-phenylacetaldehyde (PAA) and 2-phenylethylalchohol (PEA) which have a distinctive flowery/rose-like scent. Previous studies in rose have shown that, similar to petunia flowers, PAA is formed from l-phenylalanine via pyridoxal-5′-phosphate-dependent l-aromatic amino acid decarboxylase. Here we demonstrate the use of a Saccharomyces cerevisiaearo10∆ mutant to functionally characterize a Rosa hybrida cv. Fragrance Cloud sequence (RhPAAS) homologous to petunia phenylacetaldehyde synthase (PhPAAS). Volatile headspace analysis of the aro10∆ knockout strain showed that it produces up to eight times less PAA and PEA than the WT. Expression of RhPAAS in aro10∆ complemented the yeast’s mutant phenotype and elevated PAA levels, similar to petunia PhPAAS. PEA production levels were also enhanced in both aro10∆ and WT strains transformed with RhPAAS, implying an application for metabolic engineering of PEA biosynthesis in yeast. Characterization of spatial and temporal RhPAAS transcript accumulation in rose revealed it to be specific to floral tissues, peaking in mature flowers, i.e., coinciding with floral scent production and essentially identical to other rose scent-related genes. RhPAAS transcript, as well as PAA and PEA production in flowers, displayed a daily rhythmic behavior, reaching peak levels during the late afternoon hours. Examination of oscillation of RhPAAS transcript levels under free-running conditions suggested involvement of the endogenous clock in the regulation of RhPAAS expression in rose flowers.

Identification of rose phenylacetaldehyde synthase by functional complementation in yeast

We have isolated and characterized Petunia hybrida cv. Mitchell phenylacetaldehyde synthase (PAAS), which catalyzes the formation of phenylacetaldehyde, a constituent of floral scent. PAAS is a cytosolic homotetrameric enzyme that belongs to group II pyridoxal 5′-phosphate-dependent amino-acid decarboxylases and shares extensive amino acid identity (∼65%) with plant l-tyrosine/3,4-dihydroxy-l-phenylalanine and l-tryptophan decarboxylases. It displays a strict specificity for phenylalanine with an apparent Km of 1.2 mm. PAAS is a bifunctional enzyme that catalyzes the unprecedented efficient coupling of phenylalanine decarboxylation to oxidation, generating phenylacetaldehyde, CO2, ammonia, and hydrogen peroxide in stoichiometric amounts.

Orly Lavie

Papers

Plant phenylacetaldehyde synthase is a bifunctional homotetrameric enzyme that catalyzes phenylalanine decarboxylation and oxidation.

Identification of rose phenylacetaldehyde synthase by functional complementation in yeast

Plant Phenylacetaldehyde Synthase Is a Bifunctional Homotetrameric Enzyme That Catalyzes Phenylalanine