Starch synthase is a family of enzymes responsible for producing starch, a complex carbohydrate made of glucose units. These enzymes function by linking glucose molecules together, forming long chains that constitute the starch polymer. This process is fundamental for creating the primary energy storage molecule in most plants.
The Role of Starch Synthase in Plants
Starch is the main form of stored energy in plants, similar to fat in animals. This energy storage is required for plant growth, development, and reproduction. Starch synthesis occurs in different cellular locations depending on the plant’s needs.
In the leaves, starch synthesis occurs within chloroplasts, the sites of photosynthesis. Here, it serves as a temporary energy depot, called transitory starch. During the day, excess energy from sunlight is converted into starch, which is then broken down at night to fuel metabolic processes when photosynthesis stops. This daily cycle ensures a continuous energy supply.
For long-term energy reserves, starch is stored in specialized, non-photosynthetic plastids called amyloplasts, which are abundant in storage organs like seeds, roots, and tubers. The starch in these organs, known as storage starch, provides energy for life events like seed germination. These reserves support growth until the new plant can sustain itself through photosynthesis.
The Starch Synthesis Process
The creation of starch is a multi-step process that begins with glucose. Before starch synthase can use it, glucose must be chemically prepared. This involves converting glucose into an “activated” form known as adenosine diphosphate-glucose (ADP-glucose), a regulatory point in the synthesis pathway.
Once ADP-glucose is formed, starch synthase enzymes begin the elongation process. The enzyme takes an ADP-glucose molecule and transfers its glucose unit to the end of a pre-existing sugar chain, called a glucan. In this reaction, a new glycosidic bond is formed, extending the chain by one glucose unit, and ADP is released. This cycle is repeated numerous times to form long polysaccharide chains.
These chains form the two types of polymers found in starch: amylose and amylopectin. Amylose consists of long, linear chains of glucose. Amylopectin is a much larger molecule characterized by a highly branched structure. The distinct structures of these two polymers give starch its unique properties and are determined by the specific enzymes involved in their synthesis.
Different Forms of Starch Synthase
The term ‘starch synthase’ refers to a diverse family of enzymes. These enzymes are categorized into two main classes: Granule-Bound Starch Synthase (GBSS) and Soluble Starch Synthases (SSs). This division allows for the creation of both the linear amylose and the branched amylopectin components of starch.
Granule-Bound Starch Synthase is physically embedded within the structure of the starch granule. Its primary role is the synthesis of amylose. By operating within the granule’s matrix, GBSS elongates long, linear glucan chains that become intertwined with the amylopectin framework. Plants genetically modified to lack functional GBSS produce starch that is virtually free of amylose, called ‘waxy’ starch.
In contrast, Soluble Starch Synthases are found in the stroma, the fluid-filled space within the plastid that surrounds the starch granule. This class includes several forms, such as SSI, SSII, and SSIII, each responsible for producing chains of a particular length. These soluble enzymes work with starch branching enzymes to construct the complex, branched architecture of amylopectin. While the SSs extend the chains, the branching enzymes cleave a segment and reattach it to form a branch point.
Industrial and Agricultural Significance
Understanding the different starch synthase enzymes has practical implications for agriculture and industry. Using genetic modification, scientists can alter specific synthase genes in crops to produce starches with customized properties. This ability creates ‘designer starches’ for applications in food and non-food sectors.
An example is the development of waxy corn. By inactivating the gene for GBSS, researchers have created corn varieties that produce starch composed entirely of amylopectin. This waxy starch has superior freeze-thaw stability and is used as a thickener in processed foods. Conversely, by manipulating other enzymes, it is possible to create high-amylose starches, which are valued as a source of resistant starch, a dietary fiber with health benefits.
The applications extend beyond the food industry. Modified starches are used for:
- Producing biodegradable plastics as an environmentally friendly alternative to petroleum-based polymers.
- The paper industry as coatings to improve print quality and strength.
- Adhesives in products from cardboard to bookbinding.
- Fermenting to produce biofuels like ethanol, making starch composition a target for improving energy production efficiency.