Molecular evolution of the granule-bound starch synthase in flowering plants

Abstract Starch is the main storage carbohydrate in plants and, as well as being a natural resource of global importance for human consumption, serves as a raw material for some industries. Granule-bound starch synthase (GBSS) is the key enzyme in starch synthesis. However, the evolution of structural diversity of this enzyme in flowering plants remains poorly understood. This study applied a comprehensive phylogenetic analysis of genes encoding the GBSS enzyme to better understand its evolutionary history and used homology modeling to determine the enzyme's structure by identifying important protein domains and motifs. The gene encoding the GBSS enzyme was highly conserved in angiosperms and phylogenetic analysis confirmed the evolution of two isoforms, GBSSI and GBSSII. This molecular diversification could be linked to duplication events in plant genomes. Although the two isoforms share similarities, they showed structural variances and amino acid substitutions. Furthermore, the occurrence of destabilizing mutations in key residues in binding sites and conserved domains were under greater influence of purifying selection, maintaining the stability of the protein. This study provides a more comprehensive understanding of the evolutionary and structural history of the main enzymes involved in starch synthesis, which could support future studies that aim to increase our understanding of starch biosynthesis and the evolutionary and functional divergence of GBSS in plants. We highlight that attention regarding isoforms is required when using GBSS to reconstruct phylogenetic trees.