A glycosyl group is a univalent free radical or substituent

Substituent

In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

 structure obtained by removing the hemiacetal

Hemiacetal

Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...

 hydroxyl

Hydroxyl

A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 group from the cyclic form of a monosaccharide

Monosaccharide

Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...

 and, by extension, of a lower oligosaccharide

Oligosaccharide

An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...

.
Glycosyl also reacts with inorganic acids, such as phosphoric acid

Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

, forming an ester

Ester

Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 such as glucose 1-phosphate.

Examples

In cellulose, glycosyl groups link together 1,4-beta-D-glucosyl units to form chains of (1,4-beta-D-glucosyl)_n.
Another example is ribityl in 6,7-Dimethyl-8-ribityllumazine

6,7-Dimethyl-8-ribityllumazine

6,7-Dimethyl-8-ribityllumazine is a precursor for riboflavin. It is acted upon by riboflavin synthase....

.

Substituent groups other than glycosyl groups

Instead of the hemiacetal hydroxyl group, a hydrogen atom can be removed to form a substituent, for example the hydrogen off the hydroxyl on C3 of a glucose molecule. Then the substituent is called D-glucopyranos-3-O-yl as it appears in the name of the drug Mifamurtide

Mifamurtide

Mifamurtide is a drug against osteosarcoma, a kind of bone cancer mainly affecting children and young adults, which is lethal in about a third of cases. The drug was approved in Europe in March 2009.- History :...

.