Glucuronidases hydrolyze the a-1,3 bond between glucuronic acid and xylose in hemicelluloses. Most of them appear to be inverting enzymes.

For an example, we take the protein from P. cellulosa, given the acronym GlcA67A, which is dimeric in solution:

The Native Enzyme, a Homodimer (1gqi)	The Catalytic Site is a Pit (1gql)

• For this crystal structure, the enzyme was fed a xylotriose with a glucuronic acid on the terminal xylose

• Two of the xylose residues are too disordered to be seen; therefore they are not bound

• Thus, the catalytic site binds the glucuronic acid residue and one xylose

• Therefore it is suggested that this enzyme hydrolyzes only small xylose fragments bearing glucuronic acid, and connot hydrolyze an intact hemicellulose

The enzyme employs the standard strategy of using coplanar aromatic rings to help bind the sugars:

Tryptophans Coplanar with Sugar Rings	A Positive Pocket Binds the Glucuronate Carboxyl

Although humans do not know how to digest cellulose, we do ingest and hydrolyze diastereomers of cellulose:

Amylose (Starch)

We also can handle two branched isomers of amylose: pectin and glycogen.

The enzymes involved are amylases:

Human Pancreatic Amylase (1hny)	The Catalytic Site Is in a Trench

Big surprise: the catalytic residues are an Asp and a Glu. This particular enzyme is a retaining one, although inverting ones do exist. There likewise are exo and endo versions, and processive and non-processive types.