Carotenoids and Their Biosynthesis in Fungi

Abstract

Carotenoids represent a class of pigmented terpenoids. They are distributed in all taxonomic groups of fungi. Most of the fungal carotenoids differ in their chemical structures to those from other organisms. The general function of carotenoids in heterotrophic organisms is protection as antioxidants against reactive oxygen species generated by photosensitized reactions. Furthermore, carotenoids are metabolized to apocarotenoids by oxidative cleavage. This review presents the current knowledge on fungal-specific carotenoids, their occurrence in different taxonomic groups, and their biosynthesis and conversion into trisporic acids. The outline of the different pathways was focused on the reactions and genes involved in not only the known pathways, but also suggested the possible mechanisms of reactions, which may occur in several non-characterized pathways in different fungi. Finally, efforts and strategies for genetic engineering to enhance or establish pathways for the production of various carotenoids in carotenogenic or non-carotenogenic yeasts were highlighted, addressing the most-advanced producers of each engineered yeast, which offered the highest biotechnological potentials as production systems.

Keywords: carotenogenic pathways; carotenoid biosynthesis; carotenoid distribution; carotenoid pathway engineering; reaction mechanisms; trisporic acids.

Figure 1

Carotenoid biosynthesis pathway of fungi to β-carotene and torulene with gene products involved.

Figure 2

Late steps to neurosporaxanthin in Neurospora crassa, with corresponding fungal gene products.

Figure 3

Biosynthesis pathway and gene products for astaxanthin biosynthesis in Xanthophyllomyces dendrorhous.

Figure 4

Precursors and structures of oxygenated fungal carotenoids. Identical 3′4′-didehydro-1′,2′-dihydro-1′-hydroxy-2′-one-ψ-end groups of different carotenoids are boxed.

Figure 5

Proposed reaction mechanisms for the modification of acyclic end groups of fungal carotenoids leading to the formation of a 3′4′-didehydro-1′,2′-dihydro-1′-hydroxy-2′-one ψ-end group (A), carboxylic acid (B) and allylic 1′,2′-dihydro-1′,16′-didehydro 2′-ol (C). R indicates the carotenoid residue from six ionone terpenoid building blocks.

Figure 6

Synthesis of trisporic acids from β-carotene by interactions of different mating types in Mucoraceae. (A) Pathway to trisporic acid B; (B) Final step in trisporic acid C formation.

Figure 7

Selected examples of genetic engineering of carotenoids with highest yields in flask cultures on basic medium of different non-conventional yeasts by expression of existing genes, or genes for novel pathway (indicated as dotted arrows). (A) Lipomyces starkeyi [80] or Candida utilis [81,82] or Pichia pastoris [83,84]. (B) Yarrowia lipolytica [85,86,87,88]. (C) Rhodotorula utilis [89]. (D) Xanthophyllomyces dendrorhous [90,91,92]. The chosen uniform assignment of genes of same function also stands for corresponding fungal genes, as listed in Figure 1. Vertical arrow 1 marks inactivation of the phytoene desaturase gene; vertical arrow 2, inactivation of the astaxanthin synthase gene. ^a without gene HMG; ^b without crtE; ^c without squalene synthase inactivation; ^d without ERG8 gene.