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Carbohydrate isomerism includes differences in connectivity and spatial arrangement. Stereoisomers have the same connectivity but differ in three-dimensional arrangement; enantiomers are mirror-image pairs, while diastereomers are not. Epimers differ at one non-anomeric chiral carbon, and anomers differ at the anomeric carbon formed during ring closure. In water, α and β forms can interconvert through an open-chain form, changing the measured optical rotation; this process is called mutarotation. It requires a free hemiacetal or hemiketal, so it occurs in sugars such as glucose but not sucrose. Ring opening also helps explain reducing-sugar reactions. Isomerases and epimerases mediate distinct kinds of sugar interconversion, with relevance to carbohydrate metabolism.