Starch Technologies Co., Ltd, Akita Prefectural University, Shimoshinjo-Nakano, Akita-City, Akita, 010-0195, Japan.
Faculty of Bioresource Sciences, Akita Prefectural University, Shimoshinjo-Nakano, Akita-City, Akita, 010-0195, Japan.
Plant Mol Biol. 2023 Jul;112(4-5):199-212. doi: 10.1007/s11103-023-01352-6. Epub 2023 Jun 9.
Amylopectin is a highly branched glucan which accounts for approximately 65-85% of starch in most plant tissues. It is crucially important to understand the biosynthetic process of this glucan in regulating the structure and functional properties of starch granules. Currently, the most accepted ideas of structural feature and biosynthesis of amylopectin are that amylopectin is composed of a branched element called "cluster" and that the essential process of amylopectin biosynthesis is to reproduce a new cluster from the existing cluster. The present paper proposes a model explaining the whole process of amylopectin biosynthesis as to how the new cluster is reproduced by concerted actions of multiple isoforms of starch biosynthetic enzymes, particularly by combinations of distinct roles of starch branching enzyme (BE) isoforms. This model proposes for the first time the molecular mechanism as to how the formation of a new cluster is initiated, and the reason why BEI can play a major role in this step. This is because BEI has a rather broad chain-length preference compared to BEIIb, because a low preference of BEI for the substrate chain-length is advantageous for branching a couple of elongated chains that are not synchronously formed and thus these chains having varied lengths could be safely attacked by this isoform. On the contrary, it is unlikely that BEIIb is involved in this reaction because it can react to only short chains having degree of polymerization of 12-14. BEIIa is possibly able to complement the role of BEI to some extent, because BEIIa can attack basically short chains but its chain-length preference is lower compared with BEIIb. The model implies that the first branches mainly formed by BEI to construct the amorphous lamellae whereas the second branches predominantly formed by BEIIb are located mainly in the crystalline lamellae. This paper provides new insights into the roles of BEI, BEIIb, and BEIIa in amylopectin biosynthesis in cereal endosperm.
支链淀粉是一种高度分支的葡聚糖，约占大多数植物组织中淀粉的 65-85%。了解这种葡聚糖的生物合成过程对于调节淀粉颗粒的结构和功能特性至关重要。目前，关于支链淀粉结构特征和生物合成的最被接受的观点是，支链淀粉由一个称为“簇”的分支单元组成，支链淀粉生物合成的基本过程是从现有簇中复制新簇。本文提出了一个模型，解释了支链淀粉生物合成的整个过程，即多个淀粉生物合成酶同工型如何协同作用，特别是通过淀粉分支酶（BE）同工型的不同作用的组合，复制新簇。该模型首次提出了新簇形成的分子机制，以及 BEI 能够在这一步中发挥主要作用的原因。这是因为与 BEIIb 相比，BEI 对底物链长的偏好范围较宽，因为 BEI 对底物链长的低偏好性有利于分支几个伸长链，这些伸长链不是同步形成的，因此这些具有不同长度的链可以由该同工型安全攻击。相反，BEIIb 不太可能参与此反应，因为它只能与聚合度为 12-14 的短链反应。BEIIa 可能在某种程度上能够补充 BEI 的作用，因为 BEIIa 基本上可以攻击短链，但与 BEIIb 相比，其链长偏好较低。该模型意味着由 BEI 主要形成的第一个分支主要用于构建无定形层，而主要由 BEIIb 形成的第二个分支主要位于结晶层中。本文为 BEI、BEIIb 和 BEIIa 在谷物胚乳支链淀粉生物合成中的作用提供了新的见解。
