Amylose in starch: towards an understanding of biosynthesis, structure and function

Abstract

Summary Starch granules are composed of two distinct glucose polymers – amylose and amylopectin. Amylose constitutes 5–35% of most natural starches and has a major influence over starch properties in foods. Its synthesis and storage occurs within the semicrystalline amylopectin matrix of starch granules, this poses a great challenge for biochemical and structural analyses. However, the last two decades have seen vast progress in understanding amylose synthesis, including new insights into the action of GRANULE BOUND STARCH SYNTHASE (GBSS), the major glucosyltransferase that synthesises amylose, and the discovery of PROTEIN TARGETING TO STARCH1 (PTST1) that targets GBSS to starch granules. Advances in analytical techniques have resolved the fine structure of amylose, raising new questions on how structure is determined during biosynthesis. Furthermore, the discovery of wild plants that do not produce amylose revives a long‐standing question of why starch granules contain amylose, rather than amylopectin alone. Overall, these findings contribute towards a full understanding of amylose biosynthesis, structure and function that will be essential for future approaches to improve starch quality in crops.

Structural breakdown of starch‐based foods during gastric digestion and its link to glycemic response: In vivo and in vitro considerations

“…Starch is a biopolymer that naturally consists of two complex carbohydrates, namely, amylose (15% to 35%) and amylopectin (65% to 85%), and some other minor components such as lipids, proteins, and minerals (Bates et al., 1943; Ledezma, 2018; Pérez & Bertoft, 2010). However, with genetic modifications, mutants with no amylose (known as waxy starch) or higher amylose content (50% to 80%) have been developed in various crops, as recently reviewed by Seung (2020). Both amylose and amylopectin contain α‐D‐glucose units linked by α‐1,4 and α‐1,6 glycosidic chains (Zobel, 1988).…”

“…The blocklets contain crystalline and amorphous lamellae that regularly repeat every 9 to 10 nm (Blazek & Gilbert, 2011; Wang, Li, et al., 2015). With the advancement of characterization techniques, understanding has increased about the biosynthesis and molecular and microstructural aspects of starch, which have been described in detail elsewhere (Ai & Jane, 2018; Alcázar‐Alay & Meireles, 2015; Bertoft, 2017; Biliaderis, 2009; Blazek & Gilbert, 2011; Dhital et al., 2017; Dona et al., 2010; Pfister & Zeeman, 2016; Seung, 2020; Wang & Copeland, 2013). These topics are outside the scope of the discussion of this review and readers are referred to those reviews for more details on starch chemistry and microstructure.…”

STARCH SYNTHASE 4 is required for normal starch granule initiation in amyloplasts of wheat endosperm

“…Granule-Bound Starch Synthase (GBSS) is required for amylose synthesis (Seung, 2020). In Arabidopsis leaves, SS4 is required for both normal granule initiation and morphogenesis, but does not make a major contribution to amylopectin structure (Roldán et al, 2007;Szydlowski et al, 2009;Crumpton-Taylor et al, 2012Seung et al, 2017;Lu et al, 2018).…”

“…Amylopectin synthesis also requires branching enzymes (BEs) and debranching enzymes (isoamylases – ISAs) (Delatte et al ., 2005; Dumez et al ., 2006; Sundberg et al ., 2013). GBSS is required for amylose synthesis (Seung, 2020). In Arabidopsis leaves, SS4 is required for both normal granule initiation and morphogenesis, but does not make a major contribution to amylopectin structure (Roldán et al ., 2007; Szydlowski et al ., 2009; Crumpton‐Taylor et al ., 2012, 2013; Seung et al ., 2017; Lu et al ., 2018).…”

STARCH SYNTHASE 4 is required for normal starch granule initiation in amyloplasts of wheat endosperm

“…Amylopectin synthesis also requires branching enzymes (BEs) and debranching enzymes (isoamylases - ISAs) (Delatte et al ., 2005; Dumez et al ., 2006; Sundberg et al ., 2013). Granule-Bound Starch Synthase (GBSS) is required for amylose synthesis (Seung, 2020). In Arabidopsis leaves, SS4 is required for both normal granule initiation and morphogenesis, but does not make a major contribution to amylopectin structure (Roldán et al ., 2007; Szydlowski et al ., 2009; Crumpton-Taylor et al ., 2012, 2013; Seung et al ., 2017; Lu et al ., 2018).…”