The soluble starch synthases

The soluble starch synthases (SSS), together with the starch-bound starch synthase(s) (GBSS) and branching enzymes (SBE), form the complex structures of the amylose and amylopectin components of starch. The SSS synthesizes the linear (alpha-1,4-glucoside) portion of the amylopectin molecule, which is then branched by the branching enzyme. The GBSS, however, is responsible for producing the amylose exclusively. The waxy mutants, which lack GBSS activity, are deficient in amylose but have almost starch content due to increased amylopectin synthesis. The SSS, therefore, has a specific role in defining amylopectin structure and quantity. Starch synthases, both SSS and GBSS, are present in a multiple forms. Understanding the role of each form is starch biosynthesis is of major basic interest and practical importance. We have purified barley SSS from developing endosperm for the purpose of cloning the genes expressing this type of enzyme. We are now engaged in the analysis of sss clones.

Mutants are always helpful in defining the role of a given enzyme or its gene. The shx mutant of barley is so far unique in its primary effect on SSS I. This mutation is inherited as a single-locus Mendelian recessive trait, with the phenotype of the grain independent of the maternal phenotype or the direction of the cross (xenia). At m aturity, shx contains only 31% the normal weight of starch per grain, with A-granules reduced in size together but B-granules of normal dimensions. Developing grains of shx contain increased levels of sucrose, hexoses and hexose phosphates, and, critically, the substrate for starch synthesis, ADP-Glc. The structure of the amylose and amylopectins are, however, not affected by this mutation. We are continuing are analyses of shx to understand the role of the affected form of SSS.

Biochemical pathway of starch synthesis

1. Sucrose synthase
2. UDP glucose pyrophosphorylase
3. ADP glucose pyrophosphorylase
4. Granule-bound starch synthases
5. Soluble starch synthases
6. Starch branching enzymes