Biodegradation of Starch and Sucrose in Plants

1. Biodegradation of Starch

Starch is the main storage polysaccharide in plants, composed of two components: amylose (linear chains of α-1,4 linked glucose units) and amylopectin (branched chains with both α-1,4 and α-1,6 linkages). The enzymatic breakdown of starch is essential for releasing sugars that can be used in metabolism, growth, and survival, particularly at night.

Enzymes and Their Action

α-Amylase (Endoamylase):

Breaks internal α-1,4 glycosidic linkages randomly within the starch chain.
Produces shorter chains like maltose, maltotriose, and dextrins.
Found in plants, animals (saliva, pancreas), and microbes.

β-Amylase (Exoamylase):

Cleaves α-1,4 linkages from the non-reducing ends of starch.
Releases maltose units (disaccharide of glucose) sequentially.
Abundant in germinating seeds and fruits where stored starch must be rapidly mobilized.

γ-Amylase (Glucoamylase):

Hydrolyzes both α-1,4 and α-1,6 glycosidic linkages.
Capable of completely breaking starch down into free glucose molecules.

Debranching Enzymes (Isoamylase and Pullulanase):

Specifically target α-1,6 branch points in amylopectin.
Remove side chains so that α-amylase and β-amylase can act further.

Cellular Utilization of Starch Degradation Products

The maltose, glucose, and dextrins released are exported from the chloroplast to the cytosol.
Maltose transporter proteins move maltose across membranes.
Once in the cytosol, maltose and glucose enter glycolysis or are converted into sucrose, which is transported to other tissues through the phloem.
Thus, starch degradation ensures that plants have a night-time energy supply for respiration, growth, and biosynthesis.

2. Degradation of Sucrose

Sucrose is a disaccharide composed of glucose and fructose linked by an α-1,β-2 glycosidic bond (between C1 of glucose and C2 of fructose). It is the main transport sugar in plants, moving through the phloem to growing tissues, storage organs, and roots.

Enzyme Involved

Invertase (β-Fructofuranosidase, also called Sucrase):

Hydrolyzes the α-1,β-2 glycosidic bond in sucrose.
Splits sucrose into glucose + fructose.
Found in the plant cell wall (acid invertase) and cytosol (neutral invertase).

Sucrose Synthase (SuSy):

Catalyzes a reversible cleavage of sucrose into UDP-glucose + fructose (using UDP as cofactor).
Plays a central role in sucrose metabolism for biosynthesis of starch, cellulose, and cell wall components.

Cellular Utilization of Sucrose Degradation Products

Glucose: Quickly phosphorylated by hexokinase to glucose-6-phosphate, entering glycolysis for ATP production or converted into starch/cellulose for storage and structure.
Fructose: Phosphorylated by fructokinase to fructose-6-phosphate, which also enters glycolysis or the pentose phosphate pathway for biosynthesis of nucleotides and amino acids.
This ensures that sucrose, the transport sugar, is converted into forms directly usable for energy or structural building blocks in target tissues.

When Starch and Sucrose are Degraded

Starch: Degraded mainly at night when photosynthesis ceases. Provides a steady sucrose supply to sustain metabolism until dawn.
Sucrose: Degraded anytime a cell requires energy or biosynthetic precursors (e.g., in roots, seeds, or rapidly dividing cells).

Internal and External Control

Internal: High energy demand, hormone signals (auxins, gibberellins), and enzyme activation regulate starch and sucrose degradation.
External: Light/dark cycles, temperature, drought, and nutrient availability determine the rate of carbohydrate breakdown.
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Biodegradation of Starch and Sucrose in Plants: Enzymes, Linkages, and Cellular Utilization

https://rseco.org/content/241-starch-and-sucrose-degradation.html

🔑 Full Forms of Enzyme Abbreviations in the Image

GWD → Glucan Water Dikinase
- Adds phosphate groups to starch granules to initiate degradation.
PWD → Phosphoglucan Water Dikinase
- Works with GWD to phosphorylate starch, increasing solubility.
SEX4 → Starch Excess 4 (a Phosphoglucan Phosphatase)
- Removes phosphate groups from starch chains during turnover.
LSF2 → Like Sex Four 2 (another Phosphoglucan Phosphatase)
- Also removes phosphate groups to recycle starch efficiently.
ISA3 → Isoamylase 3
- A Debranching Enzyme that removes α-1,6 branch linkages in amylopectin.
LDA → Limit Dextrinase
- Another Debranching Enzyme that hydrolyzes branch points (α-1,6).
DBE → Debranching Enzyme (General Term)
- Includes Isoamylase and Limit Dextrinase, which clear branch points.
BAM1 / BAM3 → β-Amylase 1 and 3
- Exo-enzymes that cleave α-1,4 linkages from the non-reducing ends, releasing maltose.
D-Enzyme 1 & 2 → Disproportionating Enzyme (DPE1, DPE2)
- Transfer glucosyl units between maltodextrins, balancing sugar pools.
α-Glucan Phosphorylase
- Converts linear glucans into glucose-1-phosphate.
pGlcT → Plastidic Glucose Transporter
- Transports glucose out of the chloroplast.
MEX1 → Maltose Transporter 1
- Exports maltose from the chloroplast to the cytosol.
PGM → Phosphoglucomutase
- Converts glucose-1-phosphate ↔ glucose-6-phosphate.
HK → Hexokinase
- Phosphorylates glucose to form glucose-6-phosphate (entry into glycolysis).
OPP → Oxidative Pentose Phosphate Pathway
- Alternate route of glucose metabolism for NADPH and biosynthesis.