Bio-degradation of Starch and Sucrose

 INTRODUCTION

Biodegradation is the chemical dissolution of organic materials by biological means (mainly by microorganisms). It is nature's way of recycling waste or breaking down organic matter into nutrients that can be utilized by other organisms. For living organisms, this phenomenon is very important and very helpful for the survival of the biological world.

Mostly, it involves a series of enzymatic reactions and events which break complex molecules into simpler, and the products of these reactions are mostly useful in providing energy. Also, some products are the precursors of some other anabolic reactions.

Bio-degradation of Starch

Starch Storage in Plants

Starch is stored as water-insoluble granules (grains) consisting of highly branched amylopectin molecules and largely non-branched amyloses. Accumulated in chloroplasts during photosynthesis, it serves as the most abundant carbohydrate reserve in leaves of most species.

Starch Utilization in Storage Organs

Starch formed in amyloplasts of storage organs from translocated sucrose or other non-reducing sugars is a principal respiratory substrate for these storage organs. Parenchyma cells in roots and stems commonly store starch, and in perennial species, starch is stored during winter months for use in new growth the following spring. Potato tubers, rich in starch-containing amyloplasts, experience the disappearance of much of this starch due to respiration and translocation of sugars.

Biodegradation of Starch and Sucrose in Seeds

From tuber sections planted for a new crop to the endosperm or cotyledon storage tissues of seeds, abundant starch disappears during seedling development. Starch storage in various plant parts was extensively reviewed by Jenner in 1982.

Starch Degradation Process

During starch degradation, glucose polymers are converted to sugars that can either enter the metabolism of the cell storing the starch or be exported for metabolism in some other part of the plant. In most cases, starch degradation occurs in living cells. The initial stages occur in the plastid; then, a product is exported to the cytosol for metabolism to hexose phosphate and then to sucrose.

Differential Process in Germinating Cereal Seeds

In germinating cereal seeds, the degradation process differs as cell integrity is lost during seed maturation. Starch degradation occurs in nonliving tissue, and the product, in this case, is glucose. This glucose is taken up into living cells in the scutellum for transfer to the developing shoot and root axis.

Importance in Brewing and Distilling Industries

Starch degradation is well understood in cereal seeds due to its significance in the brewing and distilling industries. Glucose and oligosaccharides (short chains of glucoses) produced from starch during germination in these seeds serve as substrates for the synthesis of alcohol by yeast during beer and whisky production.

 

Important Enzymes and Their Functions

1. Chloroplastic α-Amylase:

   • Function: An endoamylase that hydrolyzes internal α-1,4 linkages of linear or branched glucans, producing a mixture of linear and branched malto-oligosaccharides from amylopectin.

2. Glucan, Water Dikinase (GWD):

   • Function: Adds the β-phosphate group of ATP to either the 3- or the 6-carbon of a glucosyl residue of amylopectin, resulting in the formation of Glucan-P, AMP, and inorganic phosphate.

3. Chloroplastic β-Amylase:

   • Function: An exoamylase that hydrolyzes alternate α-1,4 linkages sequentially from the non-reducing end of a glucan chain, producing maltose. It cannot pass α-1,6 linkages, leading to the formation of a dextrin with outer chains of two or three glucosyl units (β-limit dextrin).

4. Phosphoglucan, Water Dikinase (PWD):

   • Function: Catalyzes the same reaction as GWD, but amylopectin must already be phosphorylated. The phosphate group is added to the 3-position of glucosyl residues of amylopectin.

5. Limit Dextrinase:

   • Function: Hydrolyzes α-1,6 linkages of amylopectin and β-limit dextrin. Effective on yeast α-1,4, α-1,6-linked glucan pullulan as a substrate, but not on glycogen.

6. Chloroplastic Starch Phosphorylase:

   • Function: Catalyzes the conversion of the terminal glucosyl unit at the non-reducing end of glucan chains to glucose 1-phosphate, using inorganic phosphate. Cannot pass α-1,6 linkages.

7. γ-Amylase (Gamma Amylase):

   • Function: Cleaves both (1-6) glycosidic linkages and the last (1-4) glycosidic linkages at the non-reducing end of amylose and amylopectin, yielding glucose.

   Bio degradation of Starch in Cereals

   The pathway by which starch is converted to glucose in the endosperm of germinating cereal seeds is relatively simple. The pathway of starch degradation in the endosperm of a germinating cereal seed involves the following steps:

   1. Starch Granule Breakdown:

      • The starch granule is attacked by the endoamylase α-amylase, which releases soluble linear and branched glucans.

   2. Debranching Enzyme Action:

      • These glucans are acted on by the debranching enzyme limit dextrinase.

   3. Exoamylase Activity:

      • Simultaneously, the exoamylase (Amylase) works to produce maltose from the glucans.

   4. Maltose Hydrolysis:

      • Maltose is then hydrolyzed to glucose by an α-glucosidase (maltase).

   5. Glucose Uptake:

      • The resulting glucose is taken up into the growing embryo for further utilization.

   This process ensures the efficient breakdown of starch into glucose, providing the necessary energy and nutrients for the developing cereal seed during germination.

   
   

   

       https://images.app.goo.gl/YnpdnsRxnBQTnHNV6

    

   Biodegradation of Starch in Living Plants

   In these instances, only some of the glucose molecules derived from starch are totally oxidized to CO₂ and H₂O. Other glucose molecules are converted into sucrose molecules in the scutellum and are then moved into the growing root and shoot, where some are totally respired, and others are diverted into cell wall materials, proteins, and other substances needed for the growth of the seedling.

   Enzymatic Processes in Starch Degradation:

   Most steps in the degradation of starch to glucose can be catalyzed by three different enzymes, although other enzymes are needed to complete the process. The first three enzymes include:

   1. Alpha Amylase (α-Amylase):

      • Can attack intact starch granules, initiating the breakdown process.

   2. Beta Amylase (β-Amylase):

      • Works in conjunction with alpha amylase, presumably acting on the first products released by alpha amylase.

   3. Starch Phosphorylase:

      • Plays a role in catalyzing specific steps in the degradation process.

   Enzymatic Attack on Amylopectin:

   Some points of attack of these enzymes on amylopectin are shown in Figure, illustrating the intricate enzymatic processes involved in breaking down starch into glucose within living plants. These processes are crucial for providing energy and building blocks for various essential components needed for the growth and development of the seedling.

    

    https://images.app.goo.gl/dFCXebV2mG1hjPz39

    

    

   Alpha Amylase (α-Amylase):

   • Alpha amylase randomly attacks 1,4 bonds throughout both amylose and amylopectin, causing random pits in starch grains and releasing large products.
   • Repeated attacks on non-branched amylose chains lead to maltose, a disaccharide containing two glucose units.
   • Alpha amylase cannot attack the 1,6 bonds at the branch points in amylopectin, leading to incomplete digestion with branched dextrins of short chain lengths remaining.
   • Activation of many alpha amylases involves Ca², making calcium an essential element.

   Beta Amylase (β-Amylase):

   • Beta amylase hydrolyzes starch into ẞ-maltose, primarily acting on non-reducing ends.
   • B-maltose is rapidly changed by mutarotation into the natural mixtures of α- and β-isomers.
   • Hydrolysis of amylose by β-amylase is nearly complete, but amylopectin breakdown is incomplete due to untouched branch linkages, leaving branched dextrins.

   Enzymatic Activity and Principles:

   • Both amylases involve the uptake of one H₂O for each bond cleaved, making them hydrolase enzymes.
   • Hydrolytic reactions are not reversible, indicating no detectable starch synthesis by amylases.
   • The general principle is that large molecules are synthesized by one pathway and broken down by another.

   Distribution and Importance:

   • Amylases are widespread in various tissues but are most active in germinating seeds high in starch.
   • In leaves, α-amylase is likely more important than ß-amylase for starch hydrolysis, functioning during both day and night inside chloroplasts, often bound to starch grains.

   Action of Starch Phosphorylase:

   • Starch phosphorylase degrades starch starting at a non-reducing end by incorporating phosphate, making it a phosphorolytic enzyme.
   • The reaction catalyzed by starch phosphorylase is reversible in vitro but is mainly involved in starch degradation in vivo.
   • Amylopectin is only partially degraded by starch phosphorylase, leaving dextrins due to consecutive action from the non-reducing end to within a few glucose residues of the α-1,6 branch linkages.
   • Glucose-1-phosphate formation by starch phosphorylase avoids the need for ATP in glucose conversion during respiration.
   • Starch phosphorylase is widespread in plants, contributing to starch degradation, especially after partial hydrolysis by amylases.

    

    

   https://images.app.goo.gl/tdvAPH442JHAE9Vj6

    

   Action of Debranching Enzymes: The 1,6 branch linkages in amylopectin or branched dextrins not attacked by the mentioned enzymes are hydrolyzed by various debranching enzymes. Three main types in plants include pullulanase, isoamylase, and limit dextrinase. These enzymes act on branched starch chains, providing additional end groups for subsequent action by amylases or starch phosphorylase. The action of limit dextrinases on dextrins allows complete digestion of amylopectin into maltose, glucose, or glucose-1-phosphate.

   Degradation of Maltose: Maltose, rarely accumulating during starch digestion, is hydrolyzed to glucose by maltase enzyme: $\text{Maltose}+{\text{H}}_2\text{O}\to 2\alpha \text{-D-glucose}$ The resulting glucose units become available for conversion into other polysaccharides for degradation by respiration.

   In Cytosol: While all starch degradation to hexoses likely occurs within chloroplasts or amyloplasts, true respiration of these hexoses begins in the cytosol. Hexoses have limited movement out of chloroplasts or amyloplasts, necessitating their conversion to triose phosphate (3-PGaldehyde and dihydroxyacetone-P) in plastids. These molecules are then transported by the phosphate carrier into the cytosol, where they can either be reassembled into hexose phosphate or enter other metabolic pathways.

    

   

    

   Plant Physiology and Development(SIXTH EDITION) by Lincoln Taiz, Eduardo Zeiger, Lan Max Moller and Angus Murphy.

    

    

    https://images.app.goo.gl/WNxypkhrfHRepU2W9

    

   Biodegradation of Sucrose

   One crucial reaction in sucrose degradation is the irreversible hydrolysis by invertases to produce free glucose and fructose:

   $\text{Sucrose}+{\text{H}}_2\text{O}\to \text{Glucose}+\text{Fructose}$

   Invertases are present in the cytosol, vacuole, and sometimes even in cell walls. The cytosol invertase is alkaline, with a pH optimum near 7.5, while the vacuolar and cell-wall invertases are acidic, with pH optima of 5 or less. The cell-wall invertase, when present, hydrolyzes incoming translocated sucrose into glucose and fructose molecules, which are then absorbed by sink cells.

   Another enzyme involved in sucrose degradation is sucrose synthase. Despite its name, sucrose synthase catalyzes a reversible reaction and was initially thought to be mainly involved in sucrose synthesis. The reaction it catalyzes is:

   $\text{Sucrose}+\text{UDP}\to \text{Fructose}+\text{UDP-glucose}$

   Fructose generated can be used for respiration, and glucose in UDP-glucose can be released through other pathways. Evidence suggests that sucrose synthase is the primary enzyme degrading sucrose in starch-storage organs (e.g., developing seeds and potato tubers) or rapidly growing tissues converting translocated sucrose to cell-wall polysaccharides. In slow-growing and mature cells, invertase may be more critical in sucrose degradation, providing glucose and fructose for respiration.

    

    

    https://images.app.goo.gl/Ej4eM7JxYgEfh8cc6 

    

    

   • Decoding Monosaccharides and Carbohydrates: A Comprehensive Guide with 40 Solved MCQs and Detailed Explanations

     Carbohydrate Complexities: Heteropolysaccharide Mastery - Test Your Knowledge