Functional analysis of maize GBSSI and SSI: characterization of recombinant chimeric proteins

Huegel, Rachel
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Theses & dissertations (Interdisciplinary)

Starch is a semi-crystalline glucose polymer synthesized in higher plants for the efficient storage of a large amount of glucose in plastids without osmotic consequences. Amylose and amylopectin are two different glucose polymers that make up the starch granule which are created from adenosine 5' diphosphate glucose (ADPG) via starch synthases (SS's), branching enzymes (BE's), debranching enzymes (DBE's), starch phosphorylase (SP), and disproportionating enzyme (D-enzyme). This focus of this research is to learn more about SS's exact structure and function. There are five known SS's in maize including granule bound starch synthase I (GBSSI), starch synthase I (SSI), starch synthase IIa (SSIIa), starch synthase IIb (SSIIb), and starch synthase III (SSIII). GBSS is primarily involved in the synthesis of amylose and the remaining SS's are involved in the synthesis of amylopectin. The structure of each SS is similar to one another and consists of the glucan association domain (GLASS) and the glycosyl transferase domain (GLYTR) with a linker region connecting the two domains. The role of the GLASS domain is suggested to possess binding properties to glucan chains and the GLYTR domain is thought to be involved in the interaction with the glucan during the process of starch synthesis and glucan chain elongation. This report investigates the how the interaction between the GLASS and GLYTR domains effects the degree of functionality of the enzyme by combining the GLASS and linker domains of GBSS with the GLYTR domain of SSI. The results indicate that the relationship between these two domains is involved in how the enzyme functions. The susceptibility of the linker region is also investigated to determine the consequences of recombination in this region. The results indicate that this domain can not withstand minor alteration without changing its enzymatic properties indicating its importance in SS enzymes. However, further research is required to explain the precise function of each SS domain in order to understand exactly how each individual SS enzyme functions.