Fungal Δ9-fatty acid desaturase: a unique enzyme at the core of lipid metabolism in Aspergillus fumigatus and a promising target for the search for antifungal strategies

Abstract

Aspergillus fumigatus, Candida albicans, and Cryptococcus neoformans are the leading fungal pathogens that cause life-threatening deep mycosis, posing significant challenges to immunocompromised patients and increasing healthcare costs worldwide. Lipid metabolism is crucial for the growth and development of all organisms. Increasing evidence highlights that complex structural lipids in the fungal cell membrane emerge as important factors involved in cell signaling, stress response, and immune recognition. Membrane fluidity is primarily regulated by the ratio of saturated and unsaturated fatty acids (UFAs), structural components of membrane phospholipids, and sphingolipids, which comprise UFAs with varying degrees of unsaturation. A notable group of UFA found in these molecules contains a cis double bond located at the C9 position of the carbon chain. The synthesis of such molecules is dependent on Δ9-fatty acid (FA) desaturase enzymes. In the absence of Δ9-FA desaturase, fungal cells become auxotrophic for palmitoleic and oleic acids (C16 and C18 UFA, respectively), suggesting that this essential enzyme family is fundamental for fungal physiology and virulence. However, the extent of phenotypes and especially the biochemical properties of fungal Δ9-FA desaturases remain poorly understood. In this manuscript, we summarize the current information and fundamental findings on Δ9-FA desaturase, gathered from functional studies on relevant fungal pathogens with a focus on A. fumigatus or deduced from model organisms, including yeasts and their mammalian counterparts. We also discuss its unique domain organization and its implications for the catalytic mechanism and the potential of fungal Δ9-FA desaturase as a chemotherapeutic target.

Keywords: Aspergillus fumigatus; OLE1; antifungals; cytochrome b5; sdeA; unsaturated fatty acid; Δ9-fatty acid desaturase.

Fig 1

Comparative phylogenetic and biochemical model of mammalian stearoyl-CoA desaturases and fungal SdeA^Ole1 desaturases. (A) Phylogenetic relationship of Δ9-FA desaturase in homologous mammals and selected pathogenic fungi. Maximum likelihood tree based on protein sequences from fungi and humans reveals distinct clades for SdeA and SdeB homologs, yeast Ole1 enzymes, and human SCD1 and SCD5 outgroup. Bootstrap values (based on 1,000 replicates; Mega11 software) are indicated at the nodes. (B) Scheme of the unsaturated fatty acid biosynthetic pathway in mammals (SCD1) (C) and the proposed model for fungal (SdeA^Ole1) desaturase. The orange arrows indicate the necessity of direct interactions between the desaturase (SCD1), CytB5, and cytochrome B5 reductase (CytB5R). The dotted arrows represent the electron flux during the desaturation process. Reduced and oxidized molecules and iron atoms (Fe²⁺ and Fe³⁺) are shown in blue and red, respectively. The histidine-rich motifs comprise HXXXXH, HXXHH, and QXXHH, which contain the His residues that coordinate a nonheme di-iron center within the catalytic site of the desaturase domain and are essential for catalysis. The CytB5 domain contains the heme iron center represented by the simplified heme structure where the iron atoms are coordinated and cycle between Fe²⁺ and Fe³⁺ during the electron transfer. Question marks indicate that the CytB5R is not identified in fungi. MUFA, monounsaturated fatty acid; E, elongases; Δ12, Δ6, Δ5, desaturases that introduce double bonds at carbons C12, C6, or C5 of the fatty acid acyl chain. Created in BioRender (https://BioRender.com/w1ilztm).

Fig 2

SdeA monomer from A. fumigatus predicted by Alphafold3. (A) SdeA structure (residues 19–409) colored by pLDDT confidence. The structure is composed of two domains: a desaturase domain (SdeA), ranging from residues 19 to 309, and a cytochrome b5 domain, ranging from residues 330 to 409. These domains are separated by a 21-residue-long linker. (B) Desaturase domain of SdeA (residues 37–275), colored in golden. Stearoyl-CoA molecule was docked in the structure by aligning SdeA against the human SDC1 crystal structure (PDB 4zyo). Stearoyl-CoA is depicted in red, with a 3 Å density around it. Iron ions are depicted in magenta. On the lower panel, the nine histidines belonging to three conserved histidine-rich motifs (HXXXXH, HXXHH, and QXXHH) in the catalytic site are depicted in blue, close to the aligned iron ions. (C) Cytochrome b5 domain of SdeA (residues 330–409), depicted in green. The heme prosthetic group is colored in cyan with oxygen atoms in red, nitrogen in blue, and iron in orange. (D) Predicted aligned error plot for SdeA monomer. Two clear domains are depicted, whereas the desaturase domain is highlighted in yellow, and the CytB5 domain is highlighted in blue. (E) SdeA dimer (residues 19–409), with protomer 1 depicted in gold and protomer 2 in green arranged in the stable dimeric structure. Stearoyl-CoA, depicted in red, was docked into the protomers by aligning SdeA against the human SDC1 crystal structure (PDB 4zyo). The heme prosthetic groups are depicted in cyan, while iron ions are depicted in magenta (SdeA domain) and red (heme). The lower panel shows the arrangement of the molecules involved in SdeA catalysis. Upon dimerization, the heme group of protomer 2 will be positioned in proximity to the desaturase catalytic site of protomer 1 and vice versa. Distances between the heme, the iron ions, and the docked stearoyl-CoA are depicted as black dotted lines. Heme group is colored in blue with oxygen atoms in red and iron in orange, stearoyl-CoA depicted in red, iron ions in SdeA are colored magenta, and SdeA residues in green and blue, where blue color indicates the nitrogen atoms of the coordinating histidines.