BIOSYNTHESIS OF POLYSACCHARIDES

SONU SAINI

ARUN PATELMSC ABT

Polysaccharides

Simply known as glycans.Consists of repeating units of monosaccharides(hundreds

and thousands) held together by glycosidic bonds.Linear and branched polymers.They are primarliy concerned with two important functions-

structural and storage of energy.

Types of polysaccharides Mainly two types :Homopolysaccharides : which on hydrolysis yield only a single type of

monosaccharides. They are named based on the nature of the monosaccharide unit. some are serve as the storage forms of monosaccharides that are

used as fuel; starch and glycogen. some are serve structural element in plant cell walls and animal

exoskeleton ; cellulose and chitin Heteroplysaccharides : Which on hydrolysis yield a mixture of a few monosaccharides

or their derivates. E.g Mucopolysaccharides Hyluronan

Multiple monomer types, branched

Homopolysaccharides

Unbranched Branched

Heteropolysaccharides

Two monomer types, unbranched

Example of homopolysaccharidesStarch: used for energy storage in plantscomplete hydrolysis yields only D-Glucose

Amylose –

composed of continuous, unbranched chains of as many as 4000 D-glucose units joined by alpha-1,4-glycosidic bonds

Amylopectin –contains chains of as many as 10,000 D-glucose units joined by alpha-1,4-glycosidic bondsnew chains of 24-30 units are started by alpha-1,6-glycosidic bonds

Glycogenacts as the energy-reserve carbohydrate for animalsit is branched polysaccharide containing approximately 106 glucose units joined by alpha-1,4 and alpha-1,6-glycosidic bonds

Cellulose most widely distributed plant skeletal polysaccharide is a linear polysaccharide of D-glucose units joined by beta-1,4-glycosidic bonds

structure

GLUCONEOGENESISSynthesis of glucose from noncarbohydrate precursors.This pathway shares six enzymes with the Embden-Meyerhof

pathway.Four reactions are catalyzed by enzymes that are specific for

gluconeogenesis.Gluconeogenic pathway used in many microorganisms.Four enzymes catalyzing irrversible reactions that are different

from those found in the Embden-Meyerhof pathway (EMP).

Glucose-6-phosphatase

Fructose-1,6-bisphosphatase

glucose Gluconeogenesis

Pi

H2O glucose-6-phosphate

Phosphoglucose Isomerase

fructose-6-phosphate

Pi

H2O fructose-1,6-bisphosphate

Aldolase

glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate

Triosephosphate Isomerase (continued)

Glyceraldehyde-3-phosphate Dehydrogenase

Phosphoglycerate Kinase

Enolase

PEP Carboxykinase

glyceraldehyde-3-phosphate

NAD+ + Pi

NADH + H+

1,3-bisphosphoglycerate

ADP

ATP

3-phosphoglycerate

Phosphoglycerate Mutase

2-phosphoglycerate H2O

phosphoenolpyruvate

CO2 + GDP

GTP oxaloacetate

Pi + ADP

HCO3 + ATP

pyruvate

Pyruvate Carboxylase

Gluconeogenesis

Summary of Gluconeogenesis Pathway:

Gluconeogenesis enzyme names in red.

Glycolysis enzyme names in blue.

Gluconeogenesis:

2 pyruvate + 2 NADH + 4 ATP + 2 GTP glucose+ 2 NAD+ + 4 ADP + 2 GDP + 6 Pi

Biosynthesis of polysaccharides

Nucleoside diphosphate sugars play a central role in the synthesis of polysaccharides such as starch and glycogen..

Biosynthesis is not simply a direct reversal of catabolism.During the synthesis of glycogen and starch in bacteria and

protist, ADP-glucose is formed from glucose-1 phosphate and ATP.

It then donate glucose to the end of growing glycogen and starch

ATP + glucose 1-phosphate ADP-glucose + PPi

(glucose)n + ADP -glucose (glucose)n+1 + ADP

Starch biosynthesis is growing from reducing end

Plant cell wall biosynthesisPlant cell wall is

made of cellulose microfibrils, which is consisted of about 36 chains of cellulose, a polymer of b(14)glucose.

Cellulose biosynthesisCellulose is

synthesized by terminal complexes or rosettes, consisting of cellulose synthase and associated enzymes.

Terminal complex (rosette)

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Cellulose synthaseCellulose synthase has not been isolated in its

active form, but from the hydropathy plots deduced from its amino acid sequence it was predicted to have eight transmembrane segments, connected by short loops on the outside, and several longer loops exposed to the cytosol.

Initiation of new cellulose chain synthesis

Glucose is transferred from UDP-glucose to a membrane lipid (probably sitosterol) on the inner face of the plasma membrane.

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New cellulose chain synthesis (1)

Intracellular cellulose synthase adds several more glucose residues to the first one, in (b14) linkage, forming a short oligosacchairde chain attached to the sitosterol (sitosterol dextrin).

New cellulose chain synthesis (2)

Next, the whole sitosterol dextrin flips across to the outer face of the plasma membrane, where most of the polysaccharide chain is removed by endo-1,4-b-glucanase.

New cellulose chain synthesis (3)

The dextrin primer (removed from sitosterol by endo-1,4-b-glucanase) is now (covalently) attached to another form of cellulose synthase.

New cellulose chain synthesis (4)

The UDP-glucose used for cellulose synthesis is generated from sucrose produced from photosynthesis, by the reaction catalyzed by sucrose synthase (this enzyme is wrongly named).

New cellulose chain synthesis (5)

The glucose associated with UDP is a-linked.

Its configuration will be converted by glycosyltransferases so the product (cellulose) is b-linked.

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