Abstract
A comprehensive description of starch biosynthesis and granule assembly remains undefined despite the central nature of starch as an energy storage molecule in plants and as a fundamental calorie source for many animals. Multiple theories regarding the starch synthase (SS)-catalyzed assembly of (α1-4)-linked d-glucose molecules into maltodextrins generally agree that elongation occurs at the non-reducing terminus based on the degradation of radiolabeled maltodextrins, although recent reports challenge this hypothesis. Surprisingly, a direct analysis of the SS catalytic product has not been reported, to our knowledge. We expressed and characterized recombinant Zea mays SSIIa and prepared pure ADP-[13CU]glucose in a one-pot enzymatic synthesis to address the polarity of maltodextrin chain elongation. We synthesized maltoheptaose (degree of polymerization 7) using ADP-[13CU]glucose, maltohexaose (degree of polymerization 6), and SSIIa. Product analysis by ESI-MS revealed that the [13CU]glucose unit was added to the non-reducing end of the growing chain, and SSIIa demonstrated a >7,850-fold preference for addition to the non-reducing end versus the reducing end. Independent analysis of [13CU]glucose added to maltohexaose by SSIIa using solution NMR spectroscopy confirmed the polarity of maltodextrin chain elongation.
Highlights
A comprehensive description of starch biosynthesis and granule assembly remains undefined despite the central nature of starch as an energy storage molecule in plants and as a fundamental calorie source for many animals
SSI, SSIIa, and SSIII are all involved in producing the linear chains of amylopectin, and SSIV along with SSIII appear to be involved in the initiation of granule formation [7, 8]
NMR spectra proved less sensitive than the ESI-MS spectra and could not independently support the Ͼ7,850:1 addition ratio, observation of 1H correlation signals requiring two or three adjacent 13C atoms qualitatively demonstrated that the great majority of additions are at the non-reducing end
Summary
The majority of the protein expressed in an insoluble form, soluble S-tagged ZmSSIIa constituted the predominant protein (Ͼ80%) following purification with S proteinagarose beads.
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