Nature and nurture: confluence of pathway determinism with metabolic and chemical serendipity diversifies Monascus azaphilone pigments

Abstract

Covering: up to June 2018 Understanding the biosynthetic mechanisms that generate the astounding structural complexity and variety of fungal secondary metabolites (FSMs) remains a challenge. As an example, the biogenesis of the Monascus azaphilone pigments (MonAzPs) has remained obscure until recently despite the significant medical potential of these metabolites and their long history of widespread use as food colorants. However, a considerable progress has been made in recent years towards the elucidation of MonAzPs biosynthesis in various fungi. In this highlight, we correlate a unified biosynthetic pathway with the diverse structures of the 111 MonAzPs congeners reported until June 2018. We also discuss the origins of structural diversity amongst MonAzPs analogues and summarize new research directions towards exploring novel MonAzPs. The case of MonAzPs illuminates the various ways that FSMs metabolic complexity emerges by the interplay of biosynthetic pathway determinism with metabolic and chemical serendipity.

Fig.1. MonAzPs biosynthetic gene clusters from the genera of Monascus and Talaromyces.

A. Comparison of the structures of MonAzPs gene clusters among Monascus and Talaromyces strains. Orthologous genes are highlighted by the same colors and assigned the same one-letter code. B. Biosynthetic and ancillary proteins encoded in the Monascus and Talaromyces MonAzPs clusters. Annotations of the encoded proteins A-P are based on the M. ruber M7 nomenclature. Numbers show the percent identities of the orthologous proteins in other strains as compared with those in M. ruber M7. Proteins encoded by genes absent in M. ruber M7: R, cytochrome P450; S, MFS transporter; A1, hypothetical protein; A2, GTP-binding protein; A3, transcription factor; A4, Kelch repeat protein; A5, iron-regulated transporter; A6, hypothetical protein. —, Genes encoding similar proteins are not present in the gene cluster. This figure was adapted from Chen et al., Chem. Sci., 2017, 8, 4917–4925 – published by The Royal Society of Chemistry - with some modification.

Scheme 1. The proposed trunk pathway for MonAzPs.

Shunt pathways (see text for details) are shown to branch from node compounds of the trunk pathway. P1-P6 in square brackets are proposed intermediates that may be too reactive to isolate, or not fully characterized. The names of the enzymes are those in M. ruber M7. This figure was adapted from Chen et al., Chem. Sci., 2017, 8, 4917–4925 – published by The Royal Society of Chemistry - with some modification.

Scheme 2

Shunt 1 yields the Stage I compounds 7–10

Scheme 3

Shunt 2 yields the Stage II compound 13

Scheme 4

Shunt 3 yields the Stage III compounds 14–20

Scheme 5

Shunt 4 yields the Stage III MonAzPs 21–23

Scheme 6

Shunt 5 yields the Stage III MonAzPs 24–37

Scheme 7

Shunt 6 yields the Stage IV MonAzPs 40–50

Scheme 8. Shunt 7 yields the Stage IV MonAzPs 5–6, 51–111.

Additional structures are shown in Supplementary Table 1.

Scheme 9. Regioselectivity of Knoevenagel condensations during MonAzPs biosynthesis.

Red crosses indicate that these reactions are seldom if ever observed.

Scheme 10. A CazP-like enzyme for the reduction of key azaphilone intermediates.

A. Substrate promiscuity of dehydrogenase CazP during chaetoviridin production in Chaetomium spp. . B. Reduction of MonAzPs intermediates may be catalyzed by the CazP-like enzyme GME3457 in M. ruber M7.