Alternative-chirality Biomolecules
Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the form and sugars are of the form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used amino acids or sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. It is questionable, however, whether such a biochemistry would be truly alien; while it is certainly an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archea and other domains, making it an open topic whether an alternative stereochemistry is truly novel.
It has also been speculated that Earth itself could possess a shadow biosphere of microscopic life with unfamiliar chemistry.
Read more about this topic: Hypothetical Types Of Biochemistry