Chain-length specificities of maize starch synthase I enzyme: studies of glucan affinity and catalytic properties.

摘要: It is widely known that some of the starch synthases and starch-branching enzymes are trapped inside granule matrix during course deposition in amyloplasts. The objective this study was to use maize SSI further our understanding protein domains involved entrapment identify chain-length specificities enzyme. Using affinity gel electrophoresis, we measured dissociation constants its truncated forms using various glucans. enzyme has a high degree specificity terms substrate-enzyme constant, but greatly elevated for increasing chain lengths alpha-1, 4 Deletion N-terminal arm did not affect Kd value. Further small deletions either N- or C-terminal resulted complete loss any measurable substrate, suggesting starch-affinity domain discrete from catalytic domain. Greater displayed amylopectin fraction as compared amylose, whereas glycogen revealed lowest affinity. However, when outer (OCL) were extended phosphorylase enzyme, found an increase between average OCL 7 14, then apparently exponential 21. On other hand, ability reduced several-fold these glucans with substrates, most label [14C]ADPG incorporated into shorter chains dp < 10. We conclude rate catalysis decreases glucan it very longer approximately 20, rendering catalytically incapable at lengths. Based on observations study, propose synthesis A B1 by up critical length soon becomes unsuitable hence cannot be elongated Instead, likely become entrapped relatively inactive within granule. extension continuation must require handover SS which can extend branching enzymes. If correct, proposal provides biochemical basis explain how determine set limitations A, B, C