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Titolo:
Chain-length specificities of maize starch synthase I enzyme: studies of glucan affinity and catalytic properties
Autore:
Commuri, PD; Keeling, PL;
Indirizzi:
Iowa State Univ, ExSeed Genet & Agron Dept, Ames, IA 50011 USA Iowa State Univ Ames IA USA 50011 Genet & Agron Dept, Ames, IA 50011 USA ExSeed Genet LLC, Ames, IA 50010 USA ExSeed Genet LLC Ames IA USA 50010ExSeed Genet LLC, Ames, IA 50010 USA
Titolo Testata:
PLANT JOURNAL
fascicolo: 5, volume: 25, anno: 2001,
pagine: 475 - 486
SICI:
0960-7412(200103)25:5<475:CSOMSS>2.0.ZU;2-E
Fonte:
ISI
Lingua:
ENG
Soggetto:
BETA-AMYLASE GENE; BRANCHING ENZYME; PEA EMBRYOS; BIOSYNTHETIC-ENZYMES; MOLECULAR-CLONING; ESCHERICHIA-COLI; BINDING DOMAIN; POTATO-TUBERS; ENDOSPERM; AMYLOPECTIN;
Keywords:
maize soluble starch synthase; starch synthase I; protein encapsulation; chain-length specificity; starch-binding domain; starch granule entrapment;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Agriculture,Biology & Environmental Sciences
Life Sciences
Citazioni:
47
Recensione:
Indirizzi per estratti:
Indirizzo: Keeling, PL Iowa State Univ, ExSeed Genet & Agron Dept, Ames, IA 50011 USAIowa State Univ Ames IA USA 50011 on Dept, Ames, IA 50011 USA
Citazione:
P.D. Commuri e P.L. Keeling, "Chain-length specificities of maize starch synthase I enzyme: studies of glucan affinity and catalytic properties", PLANT J, 25(5), 2001, pp. 475-486

Abstract

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

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Documento generato il 20/09/20 alle ore 01:02:50