## 2 Structural Characteristics, Classification, and Nomenclature of GSLs
An unusual and rare form of sialic acid, deaminoneuraminic (KDN), and its glycoconjugates are only abundant in pathogenic bacteria and lower vertebrates; nevertheless, KDN was identified in some human tumors and in different animal organs, its presence being enhanced in hypoxic conditions [97; 98; 99]. GSLs are further classified according to the number, sequence, configuration, and linkages between the constituent monosaccharides as _ganglio-_, _isoganglio-_, _globo-_, _isoglobo-_, _lacto-_, _neolato-_, _lactoganglio-_, _muco-_, _gala-_, _neogala-_, _molu-_, _arthro-_, _schisto-_, and _spironeto_-series [100]. GSLs are divided into three key groups in vertebrates, containing the _lacto-_/_neolato-_series, the _globo-_/_isoglobo-_series, and the _ganglio-_/_isoganglio-_series, and are expressed in tissue-specific patterns (Table 1). This diversity probably reveals important differences in the functions of GSLs. Conventionally, all sialylated GSLs are called \"gangliosides\" if they are derived not just from the _ganglio-_series, but also from the _lacto-_ and _globo-_series of neutral GSLs [101; 102]. In invertebrates, the GSLs found are in the _molu-_ and _arthro-_series (Table 1). In addition to the high structural variety of GSLs as a result of the existent variations in their glycan and lipid portions, chemical modifications such as \(O\)-acetylation, fucosylation, lactonization [64; 103; 104], and the uncommon \(O\)-ketalation [105] may also occur in the structure of glycans. In 1997, the IUPAC-IUB Joint Commission on Biochemical Nomenclature proposed a nomenclature for GSL based on a set of core structures which are pre-defined (characterizing the linkages between the monosaccharide components and the composition of the glycan) for serving as a base name [106] and includes all modifications within the glycan headgroup (Table 2). An example is the name III2-\(\alpha\)-Fuc-Gb3 Cer (d18:1/18:0) for the GSL having the composition Fuc\(\alpha\)1-2 Gal\(\alpha\)1-4 Gal\(\beta\)1-4 Glc\(\beta\)1-\({}^{1}\)Cer. In this example, the carbohydrate core structure Gal\(\alpha\)1-4 Gal\(\beta\)1-4 Glc\(\beta\)1-1\({}^{\prime}\) is denoted Gb3 (the base name). Since the GSL contains a monosaccharide extending beyond this core structure, it is used as prefix to this name, which contains (1) a Roman numeral indicating the monosaccharide which is modified, counting the monosaccharide which is closest to the ceramide moiety as \"I\" (in this case Fuc is attached to Gal at position III); (2) a superscript on the Roman numeral indicating which hydroxyl on that monosaccharide is modified (2 in the above example); (3) the configuration of the linkage (\(\alpha\)); and (4) the abbreviated name of the attached monosaccharide unit (Fuc). While this terminology resolves confusion within the chemical literature, particularly when addressing numerous GSL isomers, it proves overly intricate for everyday application and lacks a framework for naming the lipid component. The established approach for GSL naming, initially introduced by Svennerholm and still prevalent today [107; 108], relies on their series designations along with the type, number, bonding arrangement, and position of sugar units within the glycan structure (Table 2). As an illustration, gangliosides are commonly abbreviated using a combination of two letters and a numeral, for instance, GM1, GD1, GT1, GQ. Here, G signifies the