Disaccharides: glycosidic links compared template

Maltose (Glc α1→4 Glc), lactose (Gal β1→4 Glc) and sucrose (Glc α1↔β2 Fru) with the glycosidic oxygen picked out, and the digestive enzyme that hydrolyses each one.

What the template shows

• Maltose α(1→4), lactose β(1→4) and sucrose α1↔β2: which anomeric carbons are tied up decides whether the sugar is reducing
• Maltose: Glc-α(1→4)-Glc (reducing)
• One glucose is joined through its α-C1 to O4 of the other, which keeps a free hemiacetal (either anomer, drawn unassigned).
• Lactose: Gal-β(1→4)-Glc (reducing)
• Galactose (the C4 epimer of glucose) is joined through its β-C1 to O4 of glucose; the glucose half keeps a free hemiacetal.
• Sucrose: Glc-α1↔β2-Fru (non-reducing)
• The link joins both anomeric carbons (C1 of glucose, C2 of fructose), so neither ring can open.
• A glycosidic bond is an acetal: stable in base, hydrolysed by acid or by the specific glycosidase. α and β links need different enzymes.
• Maltose and lactose have one free anomeric carbon (reducing, mutarotate); sucrose has none (non-reducing). Starch and glycogen are α(1→4) chains like maltose; cellulose is β(1→4).
• Lactose intolerance: adults who lose intestinal lactase cannot hydrolyse the β(1→4) link. Sucrose hydrolysis inverts the sign of rotation (invert sugar).

Drawn with 144 atoms, 3 arrows, 3 shapes – every one editable in MolDraw.

Sources

• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 7 Textbook
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019 Textbook
• Voet & Voet, Biochemistry, 4th ed., Wiley 2011 Textbook