Improving Functional Annotation in the DRE-TIM Metallolyase Superfamily through Identification of Active Site Fingerprints

Chromosome-scale reference genome of broccoli (Brassica oleracea var. italica Plenck) provides insights into glucosinolate biosynthesis

Broccoli (Brassica oleracea var. italica Plenck) is an important vegetable crop, as it is rich in health-beneficial glucosinolates (GSLs). However, the genetic basis of the GSL diversity in Brassicaceae remains unclear. Here we report a chromosome-level genome assembly of broccoli generated using PacBio HiFi reads and Hi-C technology. The final genome assembly is 613.79 Mb in size, with a contig N50 of 14.70 Mb. The GSL profile and content analysis of different B. oleracea varieties, combined with a phylogenetic tree analysis, sequence alignment, and the construction of a 3D model of the methylthioalkylmalate synthase 1 (MAM1) protein, revealed that the gene copy number and amino acid sequence variation both contributed to the diversity of GSL biosynthesis in B. oleracea. The overexpression of BoMAM1 (BolI0108790) in broccoli resulted in high accumulation and a high ratio of C4-GSLs, demonstrating that BoMAM1 is the key enzyme in C4-GSL biosynthesis. These results provide valuable insights for future genetic studies and nutritive component applications of Brassica crops.

The domestication-associated L1 gene encodes a eucomic acid synthase pleiotropically modulating pod pigmentation and shattering in soybean

Pod coloration is a domestication-related trait in soybean, with modern cultivars typically displaying brown or tan pods, while their wild relative, Glycine soja, possesses black pods. However, the factors regulating this color variation remain unknown. In this study, we cloned and characterized L1, the classical locus responsible for black pods in soybean. By using map-based cloning and genetic analyses, we identified the causal gene of L1 and revealed that it encodes a hydroxymethylglutaryl-coenzyme A (CoA) lyase-like (HMGL-like) domain protein. Biochemical assays showed that L1 functions as a eucomic acid synthase and facilitates the synthesis of eucomic acid and piscidic acid, both of which contribute to coloration of pods and seed coats in soybean. Interestingly, we found that L1 plants are more prone to pod shattering under light exposure than l1 null mutants because dark pigmentation increases photothermal efficiency. Hence, pleiotropic effects of L1 on pod color and shattering, as well as seed pigmentation, likely contributed to the preference for l1 alleles during soybean domestication and improvement. Collectively, our study provides new insights into the mechanism of pod coloration and identifies a new target for future de novo domestication of legume crops.

Structural Studies of Aliphatic Glucosinolate Chain-Elongation Enzymes

Plants evolved specialized metabolic pathways through gene duplication and functional divergence of enzymes involved in primary metabolism. The results of this process are varied pathways that produce an array of natural products useful to both plants and humans. In plants, glucosinolates are a diverse class of natural products. Glucosinolate function stems from their hydrolysis products, which are responsible for the strong flavors of Brassicales plants, such as mustard, and serve as plant defense molecules by repelling insects, fighting fungal infections, and discouraging herbivory. Additionally, certain hydrolysis products such as isothiocyanates can potentially serve as cancer prevention agents in humans. The breadth of glucosinolate function is a result of its great structural diversity, which comes from the use of aliphatic, aromatic and indole amino acids as precursors and elongation of some side chains by up to nine carbons, which, after the formation of the core glucosinolate structure, can undergo further chemical modifications. Aliphatic methionine-derived glucosinolates are the most abundant form of these compounds. Although both elongation and chemical modification of amino acid side chains are important for aliphatic glucosinolate diversity, its elongation process has not been well described at the molecular level. Here, we summarize new insights on the iterative chain-elongation enzymes methylthioalkylmalate synthase (MAMS) and isopropylmalate dehydrogenase (IPMDH).

Biochemical characterization of 2-phosphinomethylmalate synthase from Streptomyces hygroscopicus: A member of the DRE-TIM metallolyase superfamily

2-Phosphinomethylmalate synthase (PMMS) from Streptomyces hygroscopicus catalyzes the first step in the biosynthesis of the herbicide bialophos using 3-phosphinopyruvic acid and acetyl coenzyme A as substrates to form 2-phosphinomethylmalic acid and coenzyme A. PMMS belongs to the Claisen condensation-like (CC-like) subgroup of the DRE-TIM metallolyase superfamily, which uses conserved active site architecture to catalyze a functionally-diverse set of reactions. Analysis of a sequence similarity network for the CC-like subgroup identified PMMS and the related R-citrate synthase in an early-diverging cluster suggesting that this group of sequences are more distinct in relation to other Claisen-condensation subgroup members. To better understand the structure/function landscape of the CC-like subgroup PMMS was recombinantly expressed in Escherichia coli, purified, and characterized with respect to its enzymatic properties. Using oxaloacetate as a substrate analog, the recombinantly-produced enzyme exhibited improved Michaelis constants relative to the previously reported natively-produced enzyme. Results from pH rate profiles and kinetic isotope effects were consistent with results from other members of the CC-like subgroup supporting acid-base chemistry and hydrolysis of the direct Claisen-condensation product as the rate-determining step. Results of site-directed mutagenesis experiments indicate that PMMS uses an active-site architecture similar to homocitrate synthase to select for a dicarboxylic acid substrate.

Changing substrate specificity and iteration of amino acid chain elongation in glucosinolate biosynthesis through targeted mutagenesis of Arabidopsis methylthioalkylmalate synthase 1

Methylthioalkylmalate synthases catalyse the committing step of amino acid chain elongation in glucosinolate biosynthesis. As such, this group of enzymes plays an important role in determining the glucosinolate composition of Brassicaceae species, including Arabidopsis thaliana. Based on protein structure modelling of MAM1 from A. thaliana and analysis of 57 MAM sequences from Brassicaceae species, we identified four polymorphic residues likely to interact with the 2-oxo acid substrate. Through site-directed mutagenesis, the natural variation in these residues and the effect on product composition were investigated. Fifteen MAM1 variants as well as the native MAM1 and MAM3 from A. thaliana were characterised by heterologous expression of the glucosinolate chain elongation pathway in Escherichia coli. Detected products derived from leucine, methionine or phenylalanine were elongated with up to six methylene groups. Product profile and accumulation were changed in 14 of the variants, demonstrating the relevance of the identified residues. The majority of the single amino acid substitutions decreased the length of methionine-derived products, while approximately half of the substitutions increased the phenylalanine-derived products. Combining two substitutions enabled the MAM1 variant to increase the number of elongation rounds of methionine from three to four. Notably, characterisation of the native MAMs indicated that MAM1 and not MAM3 is responsible for homophenylalanine production. This hypothesis was confirmed by glucosinolate analysis in mam1 and mam3 mutants of A. thaliana.

'Democratized' genomic enzymology web tools for functional assignment

The protein databases contain an exponentially growing number of sequences as a result of the recent increase in ease and decrease in cost of genome sequencing. The rate of data accumulation far exceeds the rate of functional studies, producing an increase in genomic 'dark matter', sequences for which no precise and validated function is defined. Publicly accessible, that is 'democratized,' genomic enzymology web tools are essential to leverage the protein and genome databases for discovery of the in vitro activities and in vivo functions of novel enzymes and proteins belonging to the dark matter. In this review, we discuss the use of web tools that have proven successful for functional assignment. We also describe a mechanism for ensuring the capture of published functional data so that the quality of both curated and automated annotations transfer can be improved.

Genomic Enzymology: Web Tools for Leveraging Protein Family Sequence-Function Space and Genome Context to Discover Novel Functions

The exponentially increasing number of protein and nucleic acid sequences provides opportunities to discover novel enzymes, metabolic pathways, and metabolites/natural products, thereby adding to our knowledge of biochemistry and biology. The challenge has evolved from generating sequence information to mining the databases to integrating and leveraging the available information, i.e., the availability of “genomic enzymology” web tools. Web tools that allow identification of biosynthetic gene clusters are widely used by the natural products/synthetic biology community, thereby facilitating the discovery of novel natural products and the enzymes responsible for their biosynthesis. However, many novel enzymes with interesting mechanisms participate in uncharacterized small-molecule metabolic pathways; their discovery and functional characterization also can be accomplished by leveraging information in protein and nucleic acid databases. This Perspective focuses on two genomic enzymology web tools that assist the discovery novel metabolic pathways: (1) Enzyme Function Initiative-Enzyme Similarity Tool (EFI-EST) for generating sequence similarity networks to visualize and analyze sequence–function space in protein families and (2) Enzyme Function Initiative-Genome Neighborhood Tool (EFI-GNT) for generating genome neighborhood networks to visualize and analyze the genome context in microbial and fungal genomes. Both tools have been adapted to other applications to facilitate target selection for enzyme discovery and functional characterization. As the natural products community has demonstrated, the enzymology community needs to embrace the essential role of web tools that allow the protein and genome sequence databases to be leveraged for novel insights into enzymological problems.