Glucides are best known as a source of energy.
For example, rice and potato contain starch, milk contains lactose, and sugarcane contains sucrose. All of these are enzymatically broken down in the mouth, stomach, and small intestine, ultimately into monosaccharides. A detailed description can be seen in Part 3: "The most accessible source of energy," focusing on the relationship between the brain and sucrose, but simply stated, glucides play a role in an organism that corresponds to gasoline for a car.
Another important role of glucides is the function to protect an organism. Famous examples of this are cellulose ,which is the main component of plant cell walls, and chitin, which is the main component of the insect external skeleton. There exists a substance called hyaluronic acid in the extracellular matrix of vertebrates, which plays the same role as cellulose and chitin to protect the cell from losing too much water. A detailed description can be seen in Part 4: "Protector of multifunctional cells."
Glucides are best known for the two roles described above. In actuality, however, they do much more.
It has become more and more clear from the accumulation of recent research results that sugars constituting glucides (a number of sugars are typically linked together, forming sugar chains) have numerous essential functions that are associated with the foundations of life.
Some topics will be introduced in later columns. Here, the depth of sugar chains is described using one example for which an association with a disease has been discovered. It is a disorder that you all know well, chronic rheumatoid arthritis.
In fact, the precise cause of chronic rheumatoid arthritis has not yet been discovered. Various theories exist, including one that holds that it is a kind of autoimmune disease. According to this theory, in the body of a chronic rheumatoid arthritis patient, an antibody (called rheumatoid factor) against the patient's antibody (in this case immunoglobulin G; IgG) is expressed and this attacks the patient's body. Why are antibodies that bind to the patient's own antibody expressed? The secret lies in the abnormalities in sugar chains that constitute IgG.
IgG is a type of glycoprotein that is a sugar chain bound to particular amino acids. In the IgG sugar chain of chronic rheumatoid arthritis, it was found that a significant amount of galactose that should be at the end is mutated into another sugar. Thus, the IgG of a chronic rheumatoid arthritis patient is taken to be a foreign object, and rheumatoid factors are produced. Chronic rheumatoid arthritis may be a disease caused merely by a single change of sugar in the sugar chain.
Specialists often refer to sugar chains as the "face of a cell." The surfaces of cells are covered with sugar chains, and different cell types have different sugar chain patterns. That is, sugar chains act to generate the identity and diversity of cells. It is now known that sugar chains take part in essential processes such as development, cell proliferation, immunity, as well as cancer and viral infection. When we think of life phenomenon mechanisms we tend to think only of genes and proteins. Only after the functions of sugar chains are elucidated, however, can we for the first time have a true glimpse of the mysterious phenomena of life.