GBSSI

GBSSI, or granule-bound starch synthase I, is a plastid-localized enzyme that catalyzes the elongation of α-1,4-glucan chains in starch using ADP-glucose as the glucosyl donor. It is primarily responsible for synthesizing amylose, a relatively linear component of starch, within the starch granule of chloroplasts and amyloplasts.

In higher plants, starch biosynthesis involves multiple starch synthases. GBSSI is characteristically bound to the starch granule and works in concert with other granule-bound or soluble synthases to form starch polymers, but its activity predominates in amylose synthesis. The amylose content and thereby the amylose/amylopectin ratio influence starch properties such as gelatinization, paste clarity, and digestibility.

Genetics and phenotype: The GBSSI gene is often referred to as waxy in crops; loss-of-function mutations reduce or abolish amylose synthesis, yielding waxy, amylose-deficient starch. Conversely, functional GBSSI confers standard amylose-containing starch. Expression can vary by tissue, developmental stage, and environmental conditions.

Structure and localization: The GBSSI protein is synthesized with a plastid-targeting sequence and resident within plastids where it associates with starch granules. It forms part of a network of starch biosynthetic enzymes that coordinate chain elongation.

Applications and importance: Variation in GBSSI activity affects starch properties important for food and industrial uses. Waxy starches are valued for their translucency and certain textural properties, while higher amylose starches have different rheological behavior. GBSSI is a target in crop breeding and genetic research to tailor starch composition.