Fungal Lipid Biochemistry

Fats and oils, which are present in a wide variety of foods, are classified as lipids, a group of biomolecules. In addition to storing energy, lipids serve a diversity of biological purposes. Lipids are not characterized by the presence of specific functional groups, such as carbohydrates, but by their physical property and solubility. Multiple compounds obtained from body tissues are categorized as lipids if they are more soluble in nature in organic solvents. Thus, the lipid classification includes not only oils and fats, which are esters of the trihydroxy alcohol glycerol and fatty acids, but also compounds that merged functional groups derived from carbohydrates, phosphoric acid, or amino alcohols, in addition to steroid compounds such as cholesterol. In this chapter, we discussed the various kinds of lipids by considering classification and pointing out structural similarities, history, and nomenclature.

Lipids are considered a heterogeneous group of organic compounds which contain fats and their derivatives. This chapter achieved the data available on the nature and composition of lipids in filamentous fungi, and their distribution within the cell. The chapter describes some aspects of lipid metabolism, including fatty acid biosynthesis, lipid accumulation mechanisms, and different fermentation strategies. The lipid content of vegetative hyphae varies between 1% and more than 50%, of spores between 1% and 35%, and of yeast cells between 7% and approximately 15% of the tissue dry weights. The amount of lipids produced by a given species of fungus depends on the developmental stage of the growth and on the culture conditions. Culture parameters that influence the growth and the lipid contents of fungi have been found to be temperature, carbon and nitrogen sources, pH, inorganic salts, and others. The qualitative and quantitative nature of the extracellular lipids is influenced by the different growth parameters. The extracellular lipids known in a large number of oleaginous strains include polyol fatty acid esters, glycolipids, hydroxy fatty acids, sugar alcohols, acetylated sphingosines, and acetylated fatty acids. The main purpose of this chapter was to explain the biochemistry behind fungal lipid accumulation in oleaginous filamentous fungi, their distribution and functions, and the current applications of fungal fermentation strategies.

Fatty acids, in terms of their chemical structure, are aliphatic monocarboxylic acid that represents the most abundant class of lipids in nature. The major roles of fatty acids in the biological system are they constitute the building blocks of cell membranes, serve as reservoirs of energy, and their derivatives act as signaling molecules with various effects and functions. The type and composition of fatty acids vary from organism to organism. Recent research has revealed that cellular fatty acid profiles can be utilized to distinguish and identify yeast and yeast-like organism genera, species, and strains. Fatty acids commonly range from C:14 to C:20, and are predominant and most common in all organisms. Palmitic acid is the predominant saturated fatty acid of most organisms, and oleic, linoleic, and linolenic acids are the major unsaturated acids. Furthermore, several fungi have been suggested as potential sources for biodiesel production. As such, this chapter will focus on fatty acid metabolism in fungi and its characteristics that will broaden the fatty acid metabolism in fungi biology.

Fatty acid biosynthesis is a fundamental process that occurs in all living organisms and involves multiple reaction steps. Thus, a systematic transfer of the intermediates between the different catalytic sites is highly required for the efficient regulation of a pathway as well as for sustaining growth. Multienzyme complex, a protein complex that comprises a group of interacting enzymes in a specific metabolic pathway, has been identified to catalyze numerous metabolic pathways, including fatty acid synthesis. The existence of a lipogenic multienzyme complex that involves protein interaction between numerous enzymes that took part in fatty acid biosynthesis plays a key fundamental role in channelling the intermediate substrates. Herein, the growing evidence for the formation of multienzyme complexes in fatty acid synthesis and the properties of the complex will be elucidated in this chapter. 

Over the past decades, fungi have been increasingly recognized as the potential source of lipids that can be applied in various sectors, including nutraceutical and biofuel. Thus, many studies have been conducted to understand the structural and functional roles of lipid molecules, particularly in the potential oleaginous strains. Lipids produced by oleaginous fungi comprise different classes, and acylglycerol, which are esters formed from different fatty acids and alcohols such as glycerol, represent the major components of the lipid. The biosynthesis of acylglycerol in fungi involved a series of reactions involving the central carbon and glycerol-3-phosphate (G-3-P), while its catabolism in vivo generally involved the degradation of triacylglycerol (TAG) by intracellular lipase resulting in the release of fatty acids and glycerol. The resulting glycerol will be phosphorylated, oxygenated, and enter glycolysis, whereas the fatty acids will undergo β-oxidation into acetyl-CoA and be used for various physiological functions. On the other hand, several fungi species, particularly from the Mucoralles sp, have been documented to be able to utilize the oil and fat as the alternative substrate for growth and reproduction due to its capability to produce extracellular lipases which hydrolyze the ester bond of the TAG. This chapter will comprehensively discuss the functional role of acylglycerol in fungi, its biosynthesis, as well as in vivo and ex vivo degradation in fungi, which will be a bridge toward the development of the industrial application.

Triacylglycerols and steryl esters are the most common nonpolar lipids found in Fungi. When cells are provided with an abundance of carbon-rich nutrients, these storage lipids accumulate. Nonpolar lipids are sequestered from the cytosolic environment in lipid droplets (LDs) because they cannot be incorporated into biomembranes in large quantities. Triacylglycerol lipases and steryl ester hydrolases mobilize lipids from this compartment upon demand. The degradation products act as energy sources or building blocks for membrane formation. This chapter covers the mechanisms of triacylglycerol and steryl ester synthesis, storage of these lipids in lipid droplets, and subsequent mobilization in oleaginous fungi. The information on fungi’s LD biology, like size and distribution, their composition, mechanism and dynamics of formation, and their role in cell physiology, is important from a physiological and biotechnological point of view that will facilitate these organisms as model systems and also promote biofuel development. 

Glycerophospholipids are defined as phosphatidyl esters attached to the terminal carbon of glycerol in the triglyceride structure. Glycerophospholipids are the most abundant phospholipids, which form essential structural components of cellular or vesicle membranes. This chapter defines the mechanisms and regulations of glycerophospholipid synthesis, such as phosphatidic acids, phosphatidylcholines, phosphatidylethanolamines, phosphatidylserine, and phosphatidylinositol, and the roles of glycerophospholipids in fungi and yeast lipid metabolism. Some essential enzymes that catalyze the degradation of phospholipids were also discussed, such as phospholipase C, phospholipase D and phospholipase B.

Sphingolipids are a class of lipids containing the backbone of long-chain amino-alcohol bases in their structure, which are synthesized in the endoplasmic reticulum. Modification of this base gives rise to a variety of such lipids ranging from simple to complex sphingolipids that play a significant structural and functional role in membrane biology as well as regulate various cellular processes. Sphingosine, dihydrosphingosine and phytosphingosine are nature's most frequently occurring bases. Ceramides are the simplest sphingolipids after the backbone. These fatty acids are amide-linked derivatives of sphingoid bases and central intermediates of sphingolipid metabolism. Ceramides perform various biological functions and constitute the hydrophobic backbone of all complex sphingolipids. The best-characterized sphingolipids in fungi and yeast are glycosphingolipids (GSLs), which could be categorized into two groups, neutral GSLs (glucosyl and galactosylceramide) and acidic GSLs, (glycosylinositol-phosphorylceramides). Due to the several important functions of sphingolipids in cell biology, it is crucial to understand the regulation and metabolism of sphingolipids. Despite the diversity of structure and function of sphingolipids, their synthesis and degradation are governed by common synthetic and catabolic pathways. In recent years, significant progress in the field of sphingolipids has been made. Recent developments in sphingolipid biology, including the construction of analytical and genetic tools and the development of computer visualization techniques for sphingolipids analysis, have highlighted the role of sphingolipids in developing anticancer and antifungal therapeutics. Recent advances in sphingolipid biology continue to provoke and inspire vigorous investigations in sphingolipidology

Aliphatic compounds are carbon and hydrogen-containing hydrocarbon complexes and are present in almost every plant, animal and microorganism. In 1929, aliphatic hydrocarbons were first discovered by crude chemical methods. These observations were later confirmed and expanded by more sophisticated instruments, such as gas-liquid chromatography and GLC-mass spectrometry. Aliphatic hydrocarbons were detected in the wax of most studied organisms and mainly contained n-alkanes, but may also include n-alkenes, saturated and unsaturated, cyclic alkanes, and isoprenoid hydrocarbons. Similarly, surfaces of higher plants contain a complex waxy coating that consists of primary and secondary fatty alcohols, long chains of fatty acids, ketones, aldehyde, terpenes, diols, waxy esters, glycerides, etc. The chemical composition of aliphatic hydrocarbons, chain length predominance, branching, the unsaturation of surface wax and their variation among various organisms, such as plants, algae, bacteria, animals and particularly in fungi have been described in the current chapter. The hydrocarbon distribution in animals is reported to be slightly similar to that of higher plants, while in bacteria, a complex mixture of normal, single- double branched, saturated, or unsaturated structural isomers are reported. A brief description of the biotechnological production of various aliphatic compounds using genetic engineering has also been presented in this chapter. The biosynthesis of aliphatic hydrocarbons by two common routes, “elongation decarboxylation” and “head-to-head condensation,” has been studied well in plants and bacteria and are discussed here in detail. Pathways involved in the degradation of hydrocarbons by aerobic and anaerobic microbes and the enzymes involved are also described in this chapter. Aliphatic compounds with different chain lengths have been of biotechnological interest for the past few decades as they perform various biological functions in living organisms apart from their role as the chief component of diesel and jet fuels. The current chapter also highlighted the biological importance of these aliphatic hydrocarbons. 

Isoprenoid compounds are a family of compounds constructed with isoprene as the basic unit but with very different structures, including monoterpenes, diterpenes and polyterpenes. Isoprenoid compounds mainly include ergosterol, steroids, carotene, carotenoids, polyisoprene, and their structures range from relatively simple linear hydrocarbon chains to highly complex cyclic structures, in which the cyclic structure is cyclized by terpenoids. Enzymes, also known as terpene synthase catalyzed by them, are also called cyclic terpenes. Isoprenoids are widely distributed in archaea, bacteria, and eukaryotes, and a variety of isoprenoids are essential components of the biological mechanism of the organism. For example, in mammals, β-carotene (β-carotene) is the precursor substance of Vitamin A. β-carotene has the function of preventing oxidation reactions, and can inhibit and eliminate oxygen free radicals in the body, and has various effects such as slowing down aging and improving resistance. At the same time, carotenoids can be combined with protein. Astaxanthin and protein combine to form astaxanthin, which makes aquatic animals appear body color and has a certain protective effect. Therefore, ergosterol, steroids, carotene, carotenoids and polyisoprene, which are relatively large in terpenoids, are all important products with commercial value and have been widely used in food, medicine, and daily chemical products.

 Sterols are essential lipid components for the cell membrane. In addition to their structural roles, they are critical signaling molecules that regulate metabolism, development, and homeostasis. Due to the functions of sterols being concentrationdependent, the biosynthesis of sterols is tightly controlled. Here, we reviewed the biosynthesis processes of sterols (squalene, lanosterol, ergosterol, carotenoid, and polyprenols) and analyzed the key and limited enzymes in these processes. Although various sterols are identified in nature, their basic synthesis pathways appear to be conserved. Squalene is the key intermediate in the biosynthesis of sterols, and the cyclization of squalene into lanosterol (animals and fungi) or cycloartenol (plants), producing various types of terpenoids. In addition to the synthesis processes of sterols, how to enhance sterols production was also discussed, which provides the strategy for the industrial production of sterol products. 

Fungal biotechnology has enormously contributed to the growing bio- and circular economy. With the capabilities of individual fungal strains in diverse applications, the fundamentals of their growth development and metabolic traits significantly impact the process development of industrial production. Lipids are cellular biomolecules that play dynamic functions during vegetative growth and development, and environmental adaptation. Regarding the structural and functional roles of lipid molecules, intensive studies have been given to understanding the physiological and molecular regulations in the lipid metabolism of filamentous fungi, particularly in the potential oleaginous strains. Hence, a link between fungal growth, morphological development and lipid phenotypes, is presented. Vegetative growth phases of fungi are distinguishable based on their lipid content and profile. Cell morphology can be controlled by physical and genetic manipulations. Through multidimensional technologies and emerging tools, more biological insights into a systematic regulation underlying lipid metabolisms, precursors, and other related metabolites are described. In the end, a correlation of phenotypic and genotype characteristics in growth and lipid dynamics on various substrate and culture conditions is elaborated. The informative data bridging towards industrial biotechnology for the establishment of fungal bio-manufacturing platforms are discussed not only for diversified lipid production but also for developing the eco-friendly and economically feasible production process.