Chlorinated Natural Products and Related Halogenases

Abstract

Halogenated compounds are well‐known for their diverse bioactivities and wide applications in pharmaceutical industry, chemical engineering and other fields. Novel halometabolites isolated from nature inspire scientists to synthesize these “chance compounds”, which were, however, usually hampered by the extreme catalytic conditions and severe selectivity challenges. In the past several decades, enzymatic reactions have been brought into attention for their efficiency, selectivity, and mild reaction conditions. Synthetic biology which is in compliance with the laws of nature to a large extent shed light on the rational biosynthesis of halogenated compounds. This article summarizes several representative categories of halogenated natural products and halogenases to seek clues to the relationship between the structures and halogenase mechanisms. Some types of halogenase convert the basic building blocks to diverse early intermediates during natural product biosynthetic processes, while others are involved in the late tailoring steps to afford the final structures of halogenated molecules. The broad substrate specificity and strong regioselectivity exhibited in some halogenases are irreplaceable advantages for biosynthesis. In addition, tremendous efforts and encouraging results enable us to engineer the enzymes or techniques to synthesize “unnatural” halogenated natural products. More reaction mechanisms and structural data are expected to be revealed exponentially in this rapid growing realm.

Chlorinated Guaiane-Type Sesquiterpene Lactones as Cytotoxic Agents against Human Tumor Cells

“…Linichlorin A (6) has been shown to display substantial, selective antiproliferative activity against cancer and transformed cells in the micromolar range. The IC 50 values reported for linichlorin A (6) in HeLa (human cervix carcinoma), tsFT210 (mouse tumor cells) and NIH3T3 (mouse immortalized cells) were 3.2, 1.6 and 12.7 µM, respectively, as determined by WST-8 assay for 48 h. However, little is known about the mechanism of cell death induced by linichlorin A (6).…”

“…Human B-cell lymphoma-2 (Bcl-2) is a key protein involved in apoptosis inhibition through its regulation of mitochondrial membrane potential and cytochrome c release needed for the activation of caspase-9. Therefore, it should be interesting to clarify whether this protein is able to protect cells against the increase of apoptotic cells triggered by chlorohyssopifolins A (1) and D (4) and linichlorin A (6). To this end, we tested the cell line overexpressing Bcl-2 (U-937/Bcl-2) and observed that the overexpression of this survival factor did not block apoptosis triggered by chlorohyssopifolin A (1) or linichlorin A (6), but weakly decreased the percentage of apoptotic cells induced by chlorohyssopifolin D (4) (Figure 6a,b).…”

“…These results indicate that, in addition to apoptosis, other mechanisms that induce cell growth inhibition in U-937/Bcl-2, such as cell cycle arrest, may be involved by these guaianolides. To define which caspases were involved during apoptosis induced by the selected guaianolides, the enzymatic activity of lysates on tetrapeptide substrates LEHD-pNA (for caspase-9), IETD-pNA (for caspase-8) and DEVD-pNA (for caspase-3/7) was analyzed after 24-h exposure to chlorohyssopifolins A (1) and D (4) and linichlorin A (6). As shown in Figure 7a, there was an approximately twofold increase in caspase-9 activity in response to chlorohyssopifolins A (1) and D (4) and a fourfold increase after treatment with linichlorin A (6).…”

Halogenating Enzymes for Active Agent Synthesis: First Steps Are Done and Many Have to Follow

“…In recent years, these enzymes have become even more prominent and the various scientific advances in this field have already been presented several times in an overview. These reviews also provide an up-to-date overview of the different enzymes, their substrate scope and biotechnological developments as well as the diversity of halo-compounds from all kingdoms of life [5,6,7,8,9,10,11,12,13,14]. The aim of this review is—among other things—to include a further point of view.…”

Halogenases for biosynthetic pathway engineering: Toward new routes to naturals and non-naturals

“…There are several excellent reviews on mechanism and classification, engineering biohalogenases, halogenases from natural product pathways, application and development of halogenases etc. [20,23,[60][61][62][63][64] However, the use of halogenase enzymes for targeted installation of halogen moiety into both natural and synthetic scaffolds via pathway generation is rarely discussed. This is an attractive prospect for many specific synthetic biology applications and for the creation of bio-renewable routes to both naturals and non-naturals.…”