Panel A shows the structures of the disaccharide building blocks that make up heparin and heparan sulfate. Numbers represents positions. Panel B illustrates the scheme to prepare 3-O-sulfated octasaccharide using 3-OST-3 enzyme. For clarity, each sugar unit of the octasaccharide is labeled as U1, G2, etc, where U is uronic acid, G is glucosamine, and I is iduronic acid. Also, R can be either H or SO3H while R′ can be either H, Ac, or SO3H where Ac is an acetyl group.