Biochemistry

Amylose: structure, properties, synthesis and use

Amylose is a polysaccharide made up of α-D-glucose units linked by α-(1→4) glycosidic bonds, with few branches connected to the main chain by α-(1→6) glycosidic bonds.[17]
Together with amylopectin, it is one of the two main constituents of the starch, the major storage form of energy and carbohydrates in the Biosphere.[12]

Its synthesis is catalyzed by the enzyme granule-bound starch synthase or GBSS (EC 2.4.1.242), and requires the presence of a second protein, named protein targeting to starch 1 or PTST1, with no catalytic activity.[15]

Inside the starch granule, amylose is embedded within the semi-crystalline matrix formed by amylopectin.[13] Unlike amylopectin, amylose is not necessary for the formation of starch granules, is present in smaller amounts, but it has a great influence on the physicochemical properties of the starch.[7]

Plants have been selected whose starch granules hold negligible amounts or, conversely, very high amounts of amylose. These phenotypes have both industrial applications and potential health benefits.[4][20]

Contents

Structure

Amylose molecules have a molecular weight of about 106 daltons, are mostly linear and made up of α-D-glucose units, hereinafter referred to as glucose, linked by α-(1→4)-glycosidic bonds, namely, covalent bonds between C-1 of one unit and the hydroxyl group on the C-4 of the next unit.[1] The linear chains are made up of a number of monosaccharides ranging from a few hundred to several thousand; therefore, they are much longer than amylopectin chains.[17]

Structure of amylose and amylopectinThe few branches are connected to the linear chain by α-(1→6) glycosidic bonds, such as in amylopectin and glycogen, the storage form of carbohydrates in animals. An α-(1→6) glycosidic bond is a covalent bonds between C-1 of one unit and the hydroxyl group on the C-6 of another glucose unit. The number of branches is between 5 and 20, depending on the botanical origin of the starch, and the branches, compared to amylopectin, are not grouped.[6] Studies on the length of the amylose branches have shown a bimodal distribution, with the two fractions termed as:

  • AM1, which includes the shorter chains, with a degree of polymerization between 100 and 700 daltons;
  • AM2, which includes the longer chains, with a degree of polymerization between 700 and 40,000 daltons.[19]

A similar bimodal distribution of the length of the branches is also observed for amylopectin, whose fractions are indicated as AP1, shorter and more abundant, and AP2.
The intraspecies variation of the distribution of the AM1 and AM2 fractions is relatively small, whereas it is large between different species, variation that has a genetic basis.[19]

Amylose location in the starch granules

The precise location of amylose in the starch granule is not known, although it is believed that most are found in the amorphous regions. However, some studies have suggested that its localization is not restricted to the amorphous regions, but is also present between amylopectin chains and on the surface of the granules.[13] Hence, amylose could have several locations within the granule.

References

  1. ^ Bertoft E. Understanding starch structure: recent progress. Agronomy 2017;7:56. doi:10.3390/agronomy7030056
  2. ^ Birt D.F., Boylston T., Hendrich S., Jane J.L., Hollis J., Li L., McClelland J., Moore S., Phillips G.J., Rowling M., Schalinske K., Scott M.P., Whitley E.M. Resistant starch: promise for improving human health. Adv Nutr 2013;4(6):587-601. doi:10.3945/an.113.004325
  3. ^ Crofts N., Abe N., Oitome N.F., Matsushima R., Hayashi M., Tetlow I.J., Emes M.J., Nakamura Y., Fujita N. Amylopectin biosynthetic enzymes from developing rice seed form enzymatically active protein complexes. J Exp Bot 2015;66(15):4469-82. doi:10.1093/jxb/erv212
  4. ^ a b Funnell-Harris D.L., Sattler S.E., O’Neill P.M., Eskridge K.M., and Pedersen J.F. Effect of waxy (low amylose) on fungal infection of Sorghum grain. Phytopathology 2015;105(6):716-846. doi:10.1094/PHYTO-09-14-0255-R
  5. ^ a b Gous P.W., Fox G.P. Review: amylopectin synthesis and hydrolysis – Understanding isoamylase and limit dextrinase and their impact on starch structure on barley (Hordeum vulgare) quality. Trends Food Sci Technol 2016;62:23-32. doi:10.1016/j.tifs.2016.11.013
  6. ^ Hizukuri S., Takeda Y., Yasuda M., Suzuki A. Multi-branched nature of amylose and the action of debranching enzymes. Carbohydr Res 1981;94:205-213. doi:10.1016/S0008-6215(00)80718-1
  7. ^ a b Jobling S. Improving starch for food and industrial applications. Curr Opin Plant Biol 2004;7(2):210-8. doi:10.1016/j.pbi.2003.12.001
  8. ^ Kram A.M., Oostergetel G.T., Van Bruggen E. Localization of branching enzyme in potato tuber cells with the use of immunoelectron microscopy. Plant Physiol 1993;101(1):237-243. doi:10.1104/pp.101.1.237
  9. ^ Leloir L.F., de Fekete M.A., Cardini C.E. Starch and oligosaccharide synthesis from uridine diphosphate glucose. J Biol Chem 1961;236:636-41. doi:10.1016/S0021-9258(18)64280-29
  10. ^ a b Magallanes-Cruz P.A., Flores-Silva P.C., Bello-Perez L.A. Starch structure influences its digestibility: a review. J Food Sci 2017;82(9):2016-2023. doi:10.1111/1750-3841.13809
  11. ^ a b c Pfister B., Zeeman S.C. Formation of starch in plant cells. Cell Mol Life Sci 2016;73(14):2781-807. doi:10.1007/s00018-016-2250-x
  12. ^ Qu J., Xu S., Zhang Z., Chen G., Zhong Y., Liu L., Zhang R., Xue J., Guo D. Evolutionary, structural and expression analysis of core genes involved in starch synthesis. Sci Rep 2018;8(1):12736. doi:10.1038/s41598-018-30411-y
  13. ^ a b c d e Seung D. Amylose in starch: towards an understanding of biosynthesis, structure and function. New Phytol 2020;228:1490-1504. doi:10.1111/nph.16858
  14. ^ Seung D., Boudet J., Monroe J., Schreier T.B., David L.C., Abt M., Lu K.J., Zanella M., Zeeman S.C. Homologs of PROTEIN TARGETING TO STARCH control starch granule initiation in Arabidopsis leaves. Plant Cell 2017;29(7):1657-1677. doi:10.1105/tpc.17.00222
  15. ^ a b c d Seung D., Soyk S., Coiro M., Maier B.A., Eicke S., Zeeman S.C. PROTEIN TARGETING TO STARCH is required for localising GRANULE-BOUND STARCH SYNTHASE to starch granules and for normal amylose synthesis in Arabidopsis. PLOS Biol 2015;13(2):e1002080. doi:10.1371/journal.pbio.1002080
  16. ^ Szydlowski N., Ragel P., Raynaud S., Lucas M.M., Roldán I., Montero M., Muñoz F.J., Ovecka M., Bahaji A., Planchot V., Pozueta-Romero J., D’Hulst C., Mérida A. Starch granule initiation in Arabidopsis requires the presence of either class IV or class III starch synthases. Plant Cell 2009;21(8):2443-57. doi:10.1105/tpc.109.066522
  17. ^ a b c d e Tetlow I.J., Bertoft E. A review of starch biosynthesis in relation to the building block-backbone model. Int J Mol Sci 2020;21(19):7011. doi:10.3390/ijms21197011
  18. ^ Vrinten P.L., Nakamura T. Wheat granule-bound starch synthase I and II are encoded by separate genes that are expressed in different tissues. Plant Physiol 2000;122(1):255-64. doi:10.1104/pp.122.1.255
  19. ^ a b Wang K., Hasjim J., Wu A.C., Henry R.J., Gilbert R.G. Variation in amylose fine structure of starches from different botanical sources. J Agric Food Chem 2014;62(19):4443-53. doi:10.1021/jf5011676
  20. ^ a b c Wang J., Hu P., Chen Z., Liu Q., Wei C. Progress in high-amylose cereal crops through inactivation of starch branching enzymes. Front Plant Sci 2017;8:469. doi:10.3389/fpls.2017.00469