BIOSYNTHESIS OF STARCH AND SUCROSE 

 

Ghulam Mustafa7th Semester Botany GGC Jhang

Photosynthesis and carbohydrate production

During active photosynthesis in bright light, a leaf produces an excess of triose phosphate as carbohydrates, surpassing its immediate energy needs.

II. Utilization and Transport of Excess Carbohydrates

The surplus triose phosphate is converted to sucrose and transported to other plant parts, serving as fuel or for storage.

III. Storage Forms of Carbohydrates

In most plants, starch is the primary storage form of carbohydrates, while in specific plants like sugar beet and sugar cane, sucrose takes on this role.

IV. Cellular Compartmentalization of Synthesis Processes

The synthesis of sucrose and starch occurs in distinct cellular compartments, ensuring the proper allocation of resources.

V. Regulatory Mechanisms for Coordination

Various regulatory mechanisms coordinate the synthesis processes, responding to changes in light levels and photosynthetic rates.

VI. Significance for Plant Metabolism

The synthesis of sucrose and starch is crucial for meeting the metabolic needs of the plant.

VII. Importance for Human Nutrition

Starch, as the predominant storage form in most plants, contributes significantly to human nutrition by providing over 80% of dietary calories worldwide.

Biosynthesis of Sucrose

1. UDP-Glucose as the Substrate for Sucrose Synthesis

UDP-glucose serves as the substrate for sucrose synthesis in the cytosol of leaf cells.

2. Utilization of Triose Phosphates in Carbon Dioxide Fixation

Most of the triose phosphates generated through carbon dioxide fixation in plants are converted to sucrose.

3. Why Sucrose as the Transport Material?

The choice of sucrose as a transporting material by plants may be attributed to its unique linkage between the anomeric carbon 1 of glucose and the anomeric carbon 2 of fructose. This particular bond remains resistant to hydrolysis by common carbohydrate-cleaving enzymes like amylase. The unavailability of sucrose molecules to react non-enzymatically with amino acids and proteins is another advantage.

4. Evolutionary Perspective on Sucrose as a Transport Form

In the course of evolution, sucrose may have been selected as the preferred transport form of carbon due to its distinctive linkage characteristics, providing stability against non-enzymatic reactions.

5. Sucrose Synthesis in the Cytosol

Sucrose is synthesized in the cytosol, initiating with dihydroxyacetone phosphate and glyceraldehyde 3-phosphate exported from the chloroplast. The condensation of two triose phosphates, catalyzed by aldolase, forms fructose 1,6-bisphosphates. Subsequent hydrolysis by fructose 1,6-bisphosphatase yields fructose 6-phosphate.

6. Catalysis by Sucrose 6-Phosphate Synthase

Sucrose 6-phosphate synthase then catalyzes the reaction between fructose 6-phosphate and UDP-glucose, forming sucrose 6-phosphate.

7. Final Step: Phosphate Removal by Sucrose 6-Phosphate Phosphatase

The last step involves sucrose 6-phosphate phosphatase removing the phosphate group, making sucrose available for export to other tissues

Diagram

 

Energetics of Sucrose Synthesis

The reaction catalyzed by sucrose 6-phosphate synthase is a low-energy process (∆G°= -5.7 kJ/mol). However, the hydrolysis of sucrose 6-phosphate to sucrose is significantly exergonic (∆G’°=-16.5 kJ/mol), making the overall synthesis of sucrose thermodynamically favorable.

Movement of Sucrose between Source and Sink

In daylight, photosynthetic leaves fix carbon dioxide into triose phosphate via the Calvin cycle in chloroplasts. Some of the triose phosphate is utilized within the chloroplasts to synthesize starch, while the remainder is exported to the cytosol.

In the cytosol, triose phosphate can undergo gluconeogenesis to form fructose 6-phosphate and glucose 1-phosphate. The synthesis of sucrose from UDP-glucose and fructose follows, and the produced sucrose is exported from leaf mesophyll cells to the plant phloem.

The high sucrose content in the phloem induces water influx through osmosis, resulting in increased turgor pressure. This pressure facilitates the movement of the sucrose solution in the phloem towards sink tissues.

Sucrose then moves from the phloem into the sink tissue, where it undergoes various transformations. It may be converted to starch, incorporated into the cell wall, or utilized as fuel for glycolysis, the citric cycle, and oxidative phosphorylation, providing ATP for non-photosynthetic tissue synthesis.

Sugar Transport Mechanisms

The transport of sugar across the plasma membrane and between intracellular compartments is facilitated by several symporters and antiporters. These transporters are coupled to a proton gradient, ensuring the efficient movement of sucrose within the plant, supporting various metabolic processes, and fulfilling the energy needs of non-photosynthetic tissues.

 

 

Starch Synthesis in Plants

Starch synthesis in plants is a complex process involving the transformation of simple sugars generated through photosynthesis into starch, a polysaccharide serving as a crucial storage form for energy. This intricate process primarily takes place within the chloroplasts of plant cells, particularly in non-photosynthetic tissues where starch acts as a stable end product of photosynthesis and a long-term energy storage reservoir.

Substrate for Starch Synthesis: ADP -Glucose

The substrate for starch synthesis in plant plastids is ADP-glucose. This high molecular weight polymer of D-glucose forms in (1→4) linkage, similar to glycogen. Starch is synthesized in chloroplasts for temporary storage as an end product of photosynthesis, while long-term storage occurs in the amyloplasts of non-photosynthetic plant parts such as seeds, roots, and tubers.

Mechanisms of Glucose Activation and Starch Synthesis

The mechanism of glucose activation in starch synthesis mirrors that of glycogen synthesis. ADP-glucose, an activated nucleotide sugar, is formed by the condensation of glucose 1-phosphate with ATP. This reaction is made irreversible by the presence of inorganic pyrophosphatase in plastids. Starch synthase then transfers glucose residues from ADP-glucose to preexisting starch molecules.

Contrary to the traditional assumption that glucose is added to the nonreducing end of starch, recent evidence suggests that starch synthase has two equivalent active sites. These sites alternately insert a glucosyl residue onto the reducing end of the growing chain. The amylose of starch is unbranched, while amylopectin contains numerous (1→6)-linked branches. A branching enzyme in chloroplasts, similar to the glycogen-branching enzyme, introduces these branches.

Factors Influencing Starch Synthesis

Several factors influence starch synthesis:

1. Light: Starch synthesis is generally more active in the presence of light.
2. Temperature: Optimal temperatures support enzymatic activity related to starch synthesis.
3. Plant Developmental Stage: Starch accumulation varies during different stages of plant development.

 

 

Tight Regulation of Triose Phosphate Conversion to Sucrose and Starch

The conversion of triose phosphate to sucrose and starch is a tightly regulated process in plants. Triose phosphates, produced by the Calvin cycle in bright sunlight, can be temporarily stored in the chloroplast as starch or converted to sucrose and exported to non-photosynthetic parts of the plant, or both. This delicate balance between starch synthesis and sucrose export must be tightly coordinated with the rate of carbon fixation.

1. Coordination with the Carbon Fixation Rate

In bright sunlight, the triose phosphates generated by the Calvin cycle play a crucial role. Five-sixths of these triose phosphates must be efficiently recycled to ribulose 1,5-bisphosphate to sustain the continuous operation of the Calvin cycle. If more than one-sixth of the triose phosphate is redirected out of the cycle to produce sucrose and starch, it can disrupt the cycle, leading to a slowdown or even a halt in the overall process.

2. Importance of Phosphate Recycling

Effective recycling of phosphate to ribulose 1,5-bisphosphate is essential for maintaining the optimal functioning of the Calvin cycle. If insufficient triose phosphate is converted to starch or sucrose, it can result in the sequestration of phosphate, leading to a deficiency of Pi (phosphate ions) within the chloroplast. This Pi deficiency is critical for the proper operation of the Calvin cycle.

In summary, the tight regulation of the conversion of triose phosphate to sucrose and starch ensures a delicate balance between energy storage and carbon fixation, preventing disruptions in the Calvin cycle and maintaining the essential availability of phosphate for sustained photosynthetic activity.

Fructose 2,6-Bisphosphate as a Regulator of Sucrose Synthesis

The regulation of the flow of triose phosphates into sucrose involves the critical interplay between fructose 1,6-bisphosphatase (FBPase-1) and its counterpart, PPi-dependent phosphofructokinase (PP-PFK-1). These enzymes play pivotal roles in determining the destiny of triose phosphates generated through photosynthesis. Fructose 2,6-bisphosphate (F2,6BP) emerges as a key regulator, inhibiting FBPase-1 and stimulating PP-PFK-1. Notably, in vascular plants, the concentration of F2,6BP fluctuates inversely with the rate of photosynthesis.

Regulation of Phosphofructokinase 2 (PFK-2)

The responsibility for synthesizing F2,6BP lies with Phosphofructokinase 2 (PFK-2). This enzyme encounters inhibition from dihydroxyacetone phosphate or 3-phosphoglycerate but is stimulated by fructose 6-phosphate and Pi. During active photosynthesis, the production of dihydroxyacetone phosphate and the consumption of Pi lead to the inhibition of PFK-2, resulting in decreased concentrations of F2,6BP.

Impact on Sucrose Synthesis

The regulatory system is finely tuned: lowered concentrations of F2,6BP favor a heightened flux of triose phosphate into fructose 6-phosphate formation and subsequently, sucrose synthesis. This elegant mechanism ensures that sucrose synthesis occurs precisely when the level of triose phosphate produced by the Calvin cycle surpasses the requirements to sustain the cycle's operation.

In essence, the intricate interplay of F2,6BP, FBPase-1, and PP-PFK-1 serves as a sophisticated regulatory system, finely orchestrating the balance between photosynthetic carbon utilization and sucrose synthesis in response to the dynamic needs of the plant

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Regulation of Sucrose Phosphate Synthase by Phosphorylation

Sucrose synthesis undergoes regulation at the level of sucrose 6-phosphate synthase. This enzyme is allosterically activated by glucose 6-phosphate and inhibited by Pi (phosphate ions). Moreover, the activity of sucrose 6-phosphate synthase is modulated through the processes of phosphorylation and dephosphorylation.

A protein kinase phosphorylates the enzyme on a specific Ser (serine) residue, rendering it less active. Conversely, a phosphatase reverses this inactivation by removing the phosphate group (Fig. 20–27). The interplay between phosphorylation and dephosphorylation serves as a dynamic regulatory mechanism for the activity of sucrose 6-phosphate synthase.

Influence of Allosteric Regulators

The allosteric regulation of sucrose 6-phosphate synthase is notable. Activation by glucose 6-phosphate and inhibition by Pi are key factors in determining the enzyme's activity. The inhibitory effect of glucose 6-phosphate on the kinase, along with the inhibitory impact of Pi on the phosphatase, intensifies the regulatory effects of these two compounds on sucrose synthesis.

Response to Cellular Conditions

Sucrose 6-phosphate synthase responds to the cellular environment. When hexose phosphates are abundant, the enzyme is activated by the presence of glucose 6-phosphate. Conversely, when Pi levels are elevated, indicating a slowdown in photosynthesis, sucrose synthesis is impeded.

During active photosynthesis, triose phosphates are efficiently converted to fructose 6-phosphate, which readily equilibrates with glucose 6-phosphate through the action of phosphohexose isomerase. Given the equilibrium strongly favoring glucose 6-phosphate, the accumulation of fructose 6-phosphate promptly leads to a rise in glucose 6-phosphate levels, thereby stimulating sucrose synthesis.

In summary, the intricate regulation of sucrose 6-phosphate synthase through allosteric activation, inhibition, and phosphorylation dynamics ensures that sucrose synthesis is finely tuned to cellular conditions, responding to the availability of substrates and the energy status of the cell

Regulation of ADP-Glucose Pyrophosphorylase by 3-Phosphoglycerate and Pi

The central regulatory enzyme in starch synthesis is ADP-glucose pyrophosphorylase. Its activity is finely tuned, being activated by 3-phosphoglycerate (which accumulates during active photosynthesis) and inhibited by Pi (phosphate ions) (which accumulates when the light-driven condensation of ADP and Pi slows).

Activation by 3-Phosphoglycerate

During active photosynthesis, when sucrose synthesis slows down, 3-phosphoglycerate formed by CO2 fixation begins to accumulate. This accumulation serves as an activator for ADP-glucose pyrophosphorylase. The heightened levels of 3-phosphoglycerate stimulate the activity of the enzyme, initiating a cascade that promotes the synthesis of ADP-glucose, a critical precursor for starch formation.

Inhibition by Pi

Conversely, the enzyme is subjected to inhibition by Pi, especially when the light-driven condensation of ADP and Pi slows down. As Pi levels increase, it acts as a suppressor of ADP-glucose pyrophosphorylase activity, acting as a safeguard mechanism to prevent excessive starch synthesis under conditions where energy availability might be compromised.

Dynamic Regulation in Starch Synthesis

The intricate regulation of ADP-glucose pyrophosphorylase by 3-phosphoglycerate and Pi ensures dynamic control over starch synthesis. This regulatory system allows the plant to modulate starch production in response to changing environmental conditions and metabolic demands. It highlights the intricate balance required for efficient carbohydrate metabolism in plants, linking starch synthesis to the availability of substrates and the energy status of the cell.

 

Metabolism Demystified: Exploring Types, Examples, Primary Metabolites, and Secondary Metabolites

References 

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