In this article we will discuss about the breakdown and synthesis of: 1. Sucrose 2. Starch and 3. Cellulose in Plants.
Contents
Breakdown of Sucrose:
Sucrose is broken down or hydrolysed to yield glucose and fructose in the presence of the enzyme invertase or sucrase. The reaction is irreversible.
Synthesis of Sucrose:
Synthesis of sucrose in plants may take place by 3 different ways:
(1) From Glucose-1-Phosphate and Fructose in the presence of the enzyme sucrose phosphorylase e.g., in bacteria.
(2) From UDPG (Uridine Di-Phosphate Glucose) and Fructose in the presence of the enzyme sucrose synthetase e.g., in higher plants.
(3) From UDPG and Fructose-6-phosphate in the presence of the enzyme sucrose phosphate synthetase e.g., in higher plants.
Sucrose-phosphate thus produced is hydrolysed in the presence of the enzyme phosphatase to yield sucrose.
Breakdown of Starch:
Breakdown or the hydrolysis of starch to yield its constituent a-D-Glucose units may take place in two ways:
(1) By the enzyme diastase:
In fact diastase is not a single enzyme but a complex of many enzymes which are as follows:
(i) α-amylase,
(ii) β-amylase,
(iii) R-Enzyme,
(iv) Maltase
α-amylase and β-amylase attack 1 : 4 linkages of amylose and amylopectin (which constitute the starch) while R-Enzyme attacks 1 : 6 linkages of amylopectin, so that starch is hydrolysed to yield disaccharide units i.e., maltose. Finally, the enzyme maltase converts maltose into glucose molecules.
(2) By the enzyme starch phosphorylase.
Glucose-1-Phosphate may be converted into glucose by the enzyme phosphatase.
Synthesis of Starch:
Synthesis of starch involves the simultaneous synthesis of amylose (with α-(1: 4) glycosidic linkages) and amylopectin (with α-(1: 6) glycosidic linkages), the two important constituents of starch.
(A) Synthesis of Amylose (Or α-(1: 4) Glycosidic Linkages):
Synthesis of amylose may take place by any of the following ways:-
(1) According to Hanes (1940) amylose can be synthesised in the presence of the enzyme starch phosphorylase from glucose-1-phosphate and an acceptor molecule consisting of about 3 to 20 glucose units joined together by α-(1: 4) glycosidic linkages.
(2) Formation of α-(1 : 4) glycosidic linkages may also take place in the presence of the enzyme UDPG-transglycosylase (amylose synthetase) by the transfer of glucose from UDPG (Uridine Di Phosphate Glucose) to an acceptor molecule consisting of 2 to 4 or more glucose units joined together by α-(1 : 4) glycosidic linkages or even a starch molecule.
The structure of UDPG is given below:
UDPG (Uridine Diphosphate Glucose)
(3) According to Akazawa et al (1964) glucose molecule obtained as a result of the hydrolysis of sucrose in the presence of enzyme sucrase is transferred to UDP (Uridine Di Phosphate) molecule to form UDPG. Form UDPG the glucose molecule is transferred to starch (Fig. 13.2)
(4) Formation of α-(1: 4) glycosidic linkages leading to the synthesis of; amylose may also take place in the presence of D-Enzyme by the transfer of two or more glucose units from maltodextrins (consisting of more than two glucose units) to a variety of acceptors such as maltotroise, maltotetrose molecules.
(B) Synthesis of Amylopectin (Or α-(1: 6) Glycosidic Linkages):
It takes place in the presence of Q-Enzyme by the transfer of small chains of glucose units joined together by α-(1: 4) glycosidic linkages to an acceptor molecule consisting of at least four α (1:4) linked glucose units. The α-(1: 6) glycosidic bond is established between C-1 of the terminal glucose unit of donor molecule and C-6 of one of the glucose units of the acceptor molecule (Fig. 13.3).
Breakdown of Cellulose:
Cellulose is a straight chain polymeric carbohydrate molecule (a glucan), composed of a large number of D-glucopyranose units joined together by β(1 → 4) glycosidic linkages. In nature, cellulose is broken down by enzymatic hydrolysis through the enzymes called celluloses. These enzymes which are often grouped under generic name cellulase, randomly attack β(1 → 4) glycosidic linkages of the cellulose chain first forming cellodextrins and then disaccharides called as cellobiose. Cellobiose is then hydrolyzed to glucose by the enzyme cellobiose.
Cellulose degrading enzymes are not found in plants or humans. These are found only in certain organisms such as ruminants, termites, some bacteria and certain protozoa.
(Division Ruminantia of even-toed ungulates such as a deer, antelope, sheep, goat or cow).
Synthesis of Cellulose:
Long un-branched chains of cellulose (consisting of β(1→4) linked glucose residues) are synthesized in plants by the enzymes called cellulose synthases. The enzyme cellulose synthase is a multi-submit complex that is situated on plasma membrane and transfers a glucose residue from a sugar nucleotide donor called uridine diphosphate glucose (UDPG) to an acceptor molecule forming β (1 → 4) glucosyl acceptor.
UDPG + Acceptor → UDP + β (1→4) glucosyl-acceptor
It is believed that sterol-glycosides (i.e., sterols joined to a chain of one or more glucose units) such as β-sitosterol glucoside (Fig. 13.4), probably act as initial acceptors that start the elongation of cellulose chain. The process continues, and after the cellulose chain has attained desired length, the sterol is cut off from the glucan (Cellulose Chain) by the enzyme endoglucanase present in the plasma membrane. The separated cellulose chains are then extruded on the outer side of the plasma membrane (Fig. 13.5).
There are evidences to suggest that glucose in UDPG comes from sucrose, by the action of the reversible enzyme sucrose synthetase (Fig. 13.5). Alternatively, UDP-glucose may be directly obtained from cytoplasm.
