Glycosaminoglycans (GAG) are long chains of sugar molecules used in the building of bones, cartilage, skin, tendons and many other tissues in the body. These sugar chains are submicroscopic and cannot be seen with the eye, but can be studied using special scientific instruments and analytical methods.

GAG form part of the structure of the body and also give the body some of the special features that make it work. For example, the slippery, gooey fluid that lubricates your joints contains GAG. The rubbery resilient cartilage in your joints is another example. All tissues have some of this substance as a normal part of their structure. However, individuals with MPS have too much GAG accumulation.

To understand how GAG accumulates and causes MPS II, it is important to understand that in the course of the normal life process, there is a continuous cycle of building new GAG and breaking down old ones—a recycling process. The breaking down of GAG occurs in a part of the cell called the lysosome. That is why MPS II is considered one of the approximately 40 different kinds
of LSDs. All of the LSDs are caused by the inherited deficiency of individual enzymes and are very rare. This ongoing recycling process is required to keep the human body healthy. The breakdown and recycling process requires a series of special biochemical tools called enzymes. To break down GAG, a series of enzymes or tools work in sequence one after another to
split the GAG into pieces. Each enzyme in the process has its special purpose in the body and does one very specific action—just like a screwdriver works on screws and a hammer works on nails.

Individuals with MPS II are missing one specific enzyme called iduronate sulfatase, which is essential in one step in the breakdown of certain GAG called dermatan sulfate and heparan sulfate. The incompletely broken down dermatan sulfate and heparan sulfate remain stored inside cells in the body and begin to build up, causing progressive damage. The GAG itself is not toxic but the amount of it and the effect of storing it in the body leads to many physical problems.