Polysaccharides and oligosaccharides are also known as glycans. Glycans usually possess O-glycosidic linkages between monosaccharides. Cellulose, for example, is a glycan with β-1,4-linked D-glucose. Chitin is another glycan with β-1,4-linked N-acetyl-D-glucosamine. Glycans can be homo or heteropolymers of monosaccharide residues. They can have linear or branched features. Glycan may also refer to carbohydrate portions of glycoprotein, glycolipid, or a proteoglycan. Glycans can also be modified by a variety of different substituents, such as sulfation and acetylation. A variety of modifications of glycan enhances their diversity in nature and often serves as mediators of specific biological functions. For example, because many glycan are on the outer surface of cellular and secreted macromolecules, they are in charge of a wide variety of events in cell-cell, cell-matrix, and cell-molecule interactions that are important in the development and functions of a cell. Another function of these glycan is to act as a mediator in the interactions between different organisms.
Human Milk Oligosaccharides
Human milk oligosaccharides are complex glycans that can be found in breast milk. One of the most important factors in infant’s diet is from breast milk, which pertains one of the most complex group of oligosaccharides known as Human milk oligosaccharides(HMOs). They are found in three, four, five, or even six chain sugars. For example, some of the HMOs include raffinose, 2’-fucosyl-lactose, 3’-fucosyl-lactose, 3’-sialyl-lactose, 6’-sialyl-lactose, and Lacto-N-tetraose. These HMOs differ in their size, structure, and specific linkages. There are more than 150 distinct Human Milk Oligosaccharide structures out there that are identified so far. Also, these HMOs are distinct in their structure, acidity, and functions. The backbone of Human Milk Oligosaccharides is the disaccharide lactose, which is formed by the linkage between galactose and glucose sugars. The final structure of HMO all depends on whether the backbone, lactose, is fucosylated or sialated, in either beta or alpha configurations or at a different carbon. For example, 2’-fucosyl-lactose has a fucose group at the alpha-1-3 position of the glucose monosaccharide of the lactose. Being sialated means the addition of a sialic acid group and formation of an acidic HMO.
Introduction to Glycobiology
Chemical glycobiology deals with how glycans are formed and broken down. It deals with what glycan's biological roles are when they are settled and how the roles can be modified. To understand these issues, scientists have used a cooperative strategy of interrogation and perturbation. The interrogation strategy main purpose is to study and understand endogenous reactions and interactions between natural glycans and their binding complements. To be able to possess naturally forming and special glycans will enable researches to study protein-glycan and enzyme-glycan interactions. Arrangements made up of glycoconjugates, also known as lectins are useful tools for finding out more about protein-binding specificity or cellular glycosylation patterns. Along with the perturbation approach, using inhibtors, analaogs and substrates that are not natural can find out more about biosynthesis and how glycans function biologically. Both different oligosaccharides that are not natural and synthetically produced glycoconjugates can discourages or promote certain biomolecular interactions within the cells and organisms. In addition, there are compounds that are found to close off important steps to the process of the glycan biosynthetic pathways.
Another important subjects to cover is carboyhdrate analogs, which are carbohydrates of similar structures with just different substituted groups. Carbohydrate analogs are used with glycans in many different ways. One example is imaging glycans. Another one is cross linking them to binding partners. Using these tools and chemical strategies, the molecular mechanisms that deal with glycan function can be better understood.
Pinpointing and defining oligosaccharides and glycoconjugates are important for understanding how glycans operate and function. To get these things from natural soruces is very hard because when these substances produce, it deals with the interaction of multiple transporters and enzymes. This makes the whole process very complex. It is very complex as shown by the pathway for eukaryotic glycoprotein synthesis. The materials that form saccharides need to be produced and then sent to the correct cellular place. This place is where they are utilized for glycosyltransfereases. The speediness and optimization of making certain glycans are dependent upon how concentrated the building are, the type of glycosyltransferases and different biosynthetic enzymes in addition to the Km value of the these building blocks that are responsible for the glycosyltransferases that utilize them. The pathways that create N-glyocporteins, O-glycoproteins, glycolipids, glycosylphosphatidlyinositol anchors, proteoglycans, and polysacchardies are affected how easily the nucleotide donors can be obtained. However, the mechanims that control the governance of these pathways are currently in the process of being figured out. Therefore, is extremely hard to get enough amounts of glycans to examine and study from their biological sources.
Chemical strategies are now being utilize to deal with this issue. The strategy for dealing with this is providing the means to produce growning amounts of a variety of glycans. Glycans that occur naturally can be synthesized just like derivatives. This is significant because important relationships between the structure and activity can be examined further. Two basic strategies for synthesizing oligosaccharides are chemical and enzymatic. 
- Laura L. Kiessling and Rebecca A. Splain (2010). . "PubMed", p. 3-6.