Glycosidic Bonds
Let's take a closer look at glycosidic bonds (also called glycosidic linkages), using a simple glucose disaccharide:
Glycosidic linkages refer to ether linkages that join two sugar monomers/rings. Glycosidic linkages are categorized as either alpha or beta, depending on the orientation of carbon 1 (circled in red) in the left sugar monomer. If the bond between carbon 1 and the oxygen atom within the glycosidic linkage (highlighted in green) is oriented upward, it is a beta bond. If this bond is oriented downward, it is an alpha bond.
Before we continue, let's explore carbon numbering for a glucose ring:
The right-most carbon (highlighted in red or yellow) is designated to be carbon 1.
Numbering continues clockwise around the ring. Thus, the carbon bonded to the green CH2OH group is carbon 5.
The other end of our glycosidic linkage (highlighted in blue) connects to carbon 4 of the right sugar monomer. In our example, we have a disaccharide of glucose, which always has a bond facing downward at carbon 4. Other sugars, such as galactose, may have a bond that faces upward at carbon 4. Either way, the carbon-oxygen bond at carbon 4 in the right sugar monomer does not affect whether the glycosidic linkage is alpha or beta.
To summarize, only the directions of the carbon-oxygen bonds highlighted in green determine the alpha vs. beta orientation of the glycosidic linkage.
*Note: Carbon 1 in the right glucose monomer (circled in yellow) is not part of the glycosidic linkage. Therefore, its orientation does not affect whether the glycosidic linkage is alpha or beta.