## 2 Structural Characteristics, Classification, and Nomenclature of GSLs  
Because of the existing variations in structure in both the ceramide and carbohydrate parts, GSLs are characterized by great complexity and diversity. The GSL glycan core expresses its specific species, while GSLs with different ceramide structures but the same glycan are considered different _lipoforms_ (lipid forms) of the same species of GSL [69, 70]. A single lipoform, homogeneous regarding the fatty acid, sphingoid base, and glycan, is considered equivalent to what is sometimes stated as a \"molecular species\" in GSL literature. Every species of GSL can present a multitude of lipoforms, varied in the ceramide structure, such as the fatty acid chain length (C14 to C30 or greater, although the fatty acids most common in mammalian GSL ceramides are C18:0, C16:0, and C20:0), unsaturation degree, branching pattern, and hydroxylation possibility for both the fatty acid and the sphingoid base. Removal from the ceramide moiety of the fatty acid residue, most commonly under physiological conditions using acid ceramidases, results in the formation of lyso-GSLs, which can be connected to various human diseases [71]. The basic chemical structure for the sphingoid base is represented by sphinganine or dihydrosphingosine, denoted as d18:0 (d stands for 'di' (two) hydroxyl groups at positions 1 and 3, 18 represents the number of carbon atoms, and 0 the C-C double bond number). In mammals, the most commonly encountered sphingoid base is, however, sphingosine (d18:1), possessing a double bond between C-4 and C-5, in addition to the structure of sphinganine. Phytosphingosine (t18:0) possesses an additional hydroxyl on C-4 while lacking the double bond. In animals, ceramides which contain sphinganine and phytosphingosine are less abundant, while those which present phytosphingosine are found very widely in the GSLs of plants and fungi [1, 72, 73, 74, 75, 76, 77]. Other sphingoid bases containing a different number of C atoms, d20:0, d20:1, d16:0, d16:1, are also present in eukaryotes [65, 78, 79] and additional double bonds of carbon occur regularly at different positions in the hydrocarbon chain, generating a variety of sphingetrienes and sphingedienes. Several modifications of sphingosine, such as creating hydroxyl or oxo groups at different positions in the carbon backbone by the oxidation of carbons, or the addition of methylene or methyl groups to form rings or branches, are usually found within the tree of life's low species [70]. Even though ceramide variations result in a substantial variety of GSL structures, most of the important structural and even functional classifications are owing to the structure of the carbohydrate core. The carbohydrate part can contain different types and numbers of monosaccharide residues, different linkages connecting them, or can be modified with various functional groups. In vertebrates, the first monosaccharide attached to ceramide can be glucose (Glc-Cer) or galactose (Gal-Cer), but relatively reduced numbers of Gal-Cer-derived GSLs are found because, usually, extending the Gal-Cer glycan is constrained. The result is that the majority of mammalian GSLs are generated from Glc-Cer, while in invertebrates Gal-Cer derivatives are found. GSLs have been divided into two classes on the basis of their glycans' physicochemical properties: acidic GSLs and neutral (nonionic) GSLs. Acidic GSLs are mainly made up of two groups: the sialosyl-GSLs (which contain one or more than one sialic acid residue) or gangliosides and the sulfo-GSLs (which contain sulfate monoesters) or sulfatides [7]. In humans, only two forms of sialic acid usually exist, namely, N-glycolylheuraminic (Neu5 Gc) and N-acetylneuraminic (Neu5 Ac), the former only being present in trace amounts, originating either from diet [80, 81], or produced by some malignant cells [82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96].