Enzymes

Because of the biochemical/metabolic similarity between the process of starch, cellulose and hemicellulose synthesis and degradation, there is also a similarity at the genetic level.

 

Thus all of the enzymes act on polymerized sugars and utilize similar enzyme domains. The key enzymes are glycosyltransferases (GT) that synthesize the α- or β-linked polymer backbones and glycosylhydrolases (GH) enzymes to bring about backbone chain branching. Starch Synthase enzymes are classified by CAZY as part of the GT-B superfamily, whilst the Cellulose Synthase enzymes are part of the GT-A superfamily.

 

 

DOMAIN STRUCTURE: As well as using the CAZY classification, another classification system PFAM uses the discrete domain structure of enzymes allowing a more refined comparison of the structures and origins of these important enzymes.

· Starch synthases (SS) catalyze addition of glucosyl units from ADPglucose to growing chains through new α(1→4) linkages (Fig. 3A).  SS isoforms are all members of the glycosyltransferase class of enzymes. SS is more formally named ADPglucose:1,4-α-D-glucan-4-α-D-glucosyltransferase (E.C. 2.4.1.21).  Like GBSSI, the SS enzymes are also associated with starch granules, but are also present in the stroma of the amyloplast, leading to their classification as “soluble” starch synthases.

· Branching enzymes (BE) introduce α(1→6) linkages into the polymers (Fig. 3B).  BE is a member of the glycosyltransferase class of enzymes.  The branching enzymes are more formally and collectively named 1,4-α-D-glucan:1,4-α-D-glucan 6-α-D-(1,4-α-D-glucano)-transferase (E.C. 2.4.1.18).  Like the SS enzymes, the BEs appear to have a weak association with the starch granule as evidenced by the fact that these enzymes may be found entrapped inside starch granules.

· Debranching enzymes (DBE) hydrolyse α(1→6) linkages in the polymers (Fig. 3C). DBE is a member of the glycosylhydrolase class of enzymes.  The debranching enzymes are more formally and collectively named glycogen α-1,6-glucanohydrolase (E.C. 3.2.1.68).  Unlike the SS and BE enzymes, the DBEs appear to have a poor association with the starch granule as evidenced by the fact that these enzymes may not be found entrapped inside starch granules.