Glycans hybrid

Fig. 15.1. The A/-glycan biosynthetic pathway, illustrating how the three classes of A/-glycans (high mannose, hybrid and complex glycans) are derived from a common biosynthetic precursor. The trimannosyl-chitobiose core, which is common to all A/-glycans, is outlined on the precursor A/-glycan. Glc, o Man, GIcNAc.

A fourth family of A-glycans are referred to as hybrid glycans. These glycans share structural features of the high mannose and complex-type families. They usually retain two mannoses on the 6-arm of the trimannosyl core whilst complex-type antennae are elaborated on the 3-arm. Hybrid structures are frequentiy bisected and may also be core fucosylated (Fig. 15.1). It should be noted that whilst core fucosylation is common in complex-type, hybrid and truncated glycans, it is rarely found in high mannose glycans. 

Two other structurally characterized transferases have the same or almost the same mode of Mn + coordination as Mn + in Bacillus subtilis glycosyltransferase SpsA described above. A-acetylglucosaminyltransferase I which serves as the gateway from oligomannose to hybrid and complex A-glycans and plays a critical role in mammalian development, has the same active site structure except the Mg + ion and the glycerol are not present. /3 1,3-Glucuronyltransferase I... 

Fig. 2. iV-glycans of the hybrid type. A triantennary structure with sialylated Af-acetyllactosamine (LacNAc) building blocks B biantennary structure with sialylated or fucosylated poly-LacNAc building blocks. The N-glycan core structure is written in bold letters... 

A diverse range of glycan forms may be produced in some recombinant proteins. For example, the HIV envelope glycoprotein (gpl20) produced from CHO cells contains 24 occupied sequons, of which 13 are complex glycans and 11 are high mannose or hybrid structural forms (Leonard etal., 1990). 

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