Potato starch synthases: Functions and relationships

Abstract

Starch, a very compact form of glucose units, is the most abundant form of storage polyglucan in nature. The starch synthesis pathway is among the central biochemical pathways, however, our understanding of this important pathway regarding genetic elements controlling this pathway, is still insufficient. Starch biosynthesis requires the action of several enzymes. Soluble starch synthases (SSs) are a group of key players in starch biosynthesis which have proven their impact on different aspects of the starch biosynthesis and functionalities. These enzymes have been studied in different plant species and organs in detail, however, there seem to be key differences among species regarding their contributions to the starch synthesis. In this review, we consider an update on various SSs with an emphasis on potato SSs as a model for storage organs. The genetics and regulatory mechanisms of potato starch synthases will be highlighted. Different aspects of various isoforms of SSs are also discussed.

Keywords: Bioinformatics; Glycosyltransferase; Potato; Starch biosynthesis; Starch synthases.

Fig. 1

Schematic representation of the structure of a starch granule, with alternating amorphous and semi-crystalline regions constituting the growth rings. The glucose residues are connected through α−1,4 and α−1,6 linkages. In potato starch one out of 200–300 glucose units of amylopectin is phosphorylated. Phosphate groups can be attached to the C-3 or the C-6 of a glucose residue. The position of the phosphate group with respect to the α−1,6 branch point is arbitrary (Figure reproduced with minor modifications from [98]).

Fig. 2

Illustration of the starch biosynthesis pathway in potato tubers. and represent putative transporters.

Fig. 3

Domain structure and comparison of various SSs. Conserved domains were identified through a Genbank conserved domain database search service. The putative transit peptide cleavage sites are identified using the ChloroP neural network analysis at the N-terminal of each particular SS. Glycos_transf_1 (PF00534; GT1) and Glyco_transf_5 (PF08323; GT5). GT; Glycosyltransferase, TP: Transit peptide, CBM: Carbohydrate binding module.

Fig. 4

A) The sequence alignment of different potato SSs together with AgtGS, OsGBSSI and HvSSI. The ClustalW program (Larkin et al., 2007) was used to align the protein sequences. B) A partial alignment of different potato SSs together with AgtGS, OsGBSSI and HvSSI. The logo of conserved motifs was generated by WebLogo (http://weblogo.berkeley.edu/logo.cgi). All the sequences were obtained from EMBL/DDBJ database. stSSI(P93568.1), StSSII(CAA61241.2), StSSIII(Q43846.1), StGBSSI(Q00775.1), OsGBSSI (AK070431) and HvSSI (AAF37876.1).

Fig. 5

Proposed reactions catalyzed by SSs. Two catalytic-site insertion mechanism for starch polymer biosynthesis. The synthesis of amylose or α−1,4 glucans by the action of SSs which have been reported to occur by the addition of glucose moieties to the non-reducing end of a growing α−1,4-glucan. Glucose number one with a slanted line inside designates the reducing end of α−1,4 glucan chain. : represent a glucose moiety with the reducing end.